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FC-handset-spec: beginning of massive document
author Mychaela Falconia <falcon@freecalypso.org>
date Thu, 10 Jun 2021 07:16:17 +0000
parents
children 6b61d77f7700
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41
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Mychaela Falconia <falcon@freecalypso.org>
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1 FreeCalypso Handset Specification
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2 =================================
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3
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4 The purpose of this document is two-fold:
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5
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6 1) This document serves as the principal design specification for the
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7 FreeCalypso Libre Dumbphone handset hardware which I, Mother Mychaela,
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8 seek to build.
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9
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10 2) This document also defines the scope of functionality to be supported in
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11 FreeCalypso handset firmware, including support for additional hardware
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12 targets beyond the primary FC handset hw target.
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13
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14 1. FC handset hardware specification
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15
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16 1.1. Basic features
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17
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18 The Mother's goal is to produce a replacement for the proprietary Pirelli DP-L10
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19 phone, or more specifically, for the GSM-only subset of this Pirelli phone which
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20 the Mother actually uses, *without* Pirelli's key differentiating feature of
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21 non-GSM WiFi operation, and without Pirelli's camera. The following hardware
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22 features are to be included:
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23
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24 * 176x220 pixel color display (no touch)
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25 * 21-button main keypad
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26 * 3 side buttons for volume control and an auxiliary function
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27 * USB port that combines charging and computer interface
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28 * wired headset jack
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29 * single SIM slot
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30
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31 The following features which are commonly found in mainstream proprietary
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32 phones, particularly more recent ones, will NOT be included:
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33
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34 * camera
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35 * Bluetooth
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36 * FM radio
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37 * TV receiver
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38 * GPS receiver
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39 * dual SIM slot
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40 * torch light beyond LCD and keypad backlights
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41
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42 1.2. RF band capability
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43
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44 Our FC handset needs to be quadband GSM; this quadband capability will be
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45 achieved by copying the RF section and the core PCB layout around it from the
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46 reverse-engineered iWOW TR-800 modem module, which is itself a very direct
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47 (almost verbatim) derivative of TI's Leonardo+ quadband reference design.
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48
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49 1.3. RAM and flash
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50
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51 The Mother's intent is to use Spansion S71PL064JA0 flash+RAM MCP on the final
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52 handset motherboard, providing 8 MiB of flash and 2 MiB of XRAM in a 7x9 mm
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53 footprint. This flash and RAM capacity is already known to be fully sufficient
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54 for our FreeCalypso handset firmware in maximal feature configuration, hence
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55 any larger capacity would be excessive. However, on our FC Venus development
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56 board we may use the larger S71PL129NC0 MCP, same as used on FCDEV3B V2.
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57
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58 1.4. Liquid crystal display
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59
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60 1.4.1. Display size
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61
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62 The size of the display for our FC Libre Dumbphone handset design is fixed at
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63 176x220 pixels, 16-bit color, following TI's D-Sample platform and the starting
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64 point UI code that was developed for it. Thoughts of changing to a different
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65 display size have been considered and rejected:
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66
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67 * If we were to change to a smaller display size, we would have to do extra work
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68 on the firmware to shrink the UI to the smaller size, and we would reduce the
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69 amount of information that can be displayed at once. We would incur extra
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70 work and a functional loss, but gain absolutely nothing in return.
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71
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72 * If we were to change to a larger display size (240x320 pixels seems to be the
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73 largest reasonable size for dumbphones, used in high-end Nokia models), we
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74 would be venturing into uncertain territory - the greatest uncertainty would
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75 be the extra CPU load on Calypso to draw the larger UI and to refresh the
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76 larger framebuffer, which is done with PIO on Calypso, without any DMA
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77 assistance. The D-Sample LCD size of 176x220 pixels already appears to be a
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78 strain in some drawing code paths, hence the Mother's decision is to play it
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79 safe and stick with the known working display size. Expanding the UI to make
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80 sensible use of larger screen real estate would also entail additional work.
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81
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82 176x220 is the display size in pixels, and this resolution number by itself says
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83 nothing about the physical display size in inches or mm. However, most readily
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84 available LCDs that are made for this pixel resolution are made in 2.0" diagonal
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85 physical size, with 31.68x39.60 mm active area and 0.180 mm dot pitch, hence
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86 this physical size is the one we are going to use.
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87
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88 1.4.2. Specific LCD module selection
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89
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90 As of this writing, the specific LCD module to be used has not been firmly
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91 selected yet. We are actively looking for an LCD module that fits all of the
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92 following requirements:
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93
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94 * TFT color LCD, 2.0" diagonal, 176x220 pixel resolution;
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95
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96 * 16-bit microprocessor bus interface;
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97
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98 * 6:00 viewing direction as appropriate for cellular handsets;
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99
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100 * backlight consisting of 3 white LEDs in parallel, joined at the anode,
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101 with separately brought-out cathodes;
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102
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103 * mechanical design that supports mounting with the FPC tail folded under the
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104 module, either by way of direct solder termination (no connector) or by way
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105 of raised sides that create sufficient vertical space to accommodate the FPC
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106 connector.
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107
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108 The requirement of 16-bit microprocessor bus interface stems from the desire to
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109 interface this LCD to the Calypso in exactly the same way how TI did it on the
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110 D-Sample, the 6:00 viewing direction and mechanical mounting requirements stem
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111 naturally from the target application (cellular phone handset), and the
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112 backlight LED wiring requirement stems from the constraints of our chosen
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113 MAX1916 backlight LED driver chip - see section 1.4.4.
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114
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115 1.4.3. Backlight and readability considerations
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116
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117 Out of the various pre-existing mobile phones which I (Mychaela) have
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118 experienced, there have been 3 different kinds of LCDs in terms of how display
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119 operation and readability interacts with the backlight:
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120
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121 1) older phones with black&white LCDs: on all phones of this type which I've
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122 ever used, the display is perfectly readable without the backlight given
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123 ordinary ambient lighting, be it natural daylight or room lighting. Such
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124 LCDs are called reflective. With these B&W displays, you only need to turn
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125 on the backlight if you need to operate the phone in darkness, such as
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126 outdoors at night or inside with all lights off. The firmware in such phones
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127 is typically designed to leave the actual display functional and updated at
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128 all times, with only the backlight subject to on/off control.
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129
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130 2) most newer phones with color displays, of which Pirelli DP-L10 is a
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131 representative case, have transmissive LCDs that are not designed to be
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parents:
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132 readable without the backlight at all - backlight required for readability
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
133 (BLRR) is another way to describe such LCDs. Because the display is not
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
134 readable at all without the backlight, phone firmware is typically designed
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
135 to turn off the entire display (not just the backlight) when the screen goes
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
136 dark, and operation visible to the user is display on/off, rather than
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
137 backlight on/off. It is a good firmware design practice to "swallow" the
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
138 initial keypress that turns on the display from dark state, i.e., to block
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
139 the regular action of whatever button was pressed to "wake up" the display.
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
140
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
141 3) The color display on Motorola C139 phones is an odd intermediate case: this
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
142 display is NOT practically readable with the backlight off, yet the firmware
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
143 is designed as if the display were readable in this condition: the actual
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
144 display (unsure if it is CSTN or TFT) remains on and updated, and when you
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
145 press some button to "wake up" the display, that button still takes its
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
146 regular action, which is really bad for usability. How do we know that the
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
147 actual CSTN or TFT display remains on and actively updated when it is not
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
148 readable with the backlight off? Answer: the non-backlit display can be made
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
149 readable by shining a flashlight directly at it - but this trick requires a
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
150 directly pointed flashlight; no amount of ordinary ambient light is enough
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
151 to make the display readable.
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
152
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
153 Because our FC Libre Dumbphone handset will have a color display (contemporary
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
154 TFT) and because we are sane, not copying the monumental design mistake of Mot
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
155 C139, our display will fall into class 2 by the above classification: backlight
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
156 required for readability, full display on/off rather than just backlight on/off,
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
157 firmware operating like Pirelli's in terms of wake-up keypress swallowing.
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
158
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
159 1.4.3.1. Backlight dimming mode
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
160
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
161 Because our LCD is of BLRR type and because we seek to fully replicate Pirelli's
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
162 logic in terms of when keypresses are swallowed and when they are not, we need
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
163 to implement a dimming mode for our LCD backlight. In Pirelli's design which we
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
164 are copying, when you are playing with phone menus or composing SMS etc, but are
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
165 not in an active call, the display switches between full brightness and totally
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
166 off - it goes fully off on timeout, and when you press a button to wake it up,
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
167 it switches on at full brightness, together with the keypad backlight. But when
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
168 you are in a call, when the timer expires (and it's a shorter timer, 10 s
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
169 instead of 30 s), the display goes dim instead of fully off, and in this dimmed
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
170 (but still readable) state keypresses are NOT swallowed.
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
171
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
172 We only need to implement two different intensity levels for the LCD backlight:
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
173 full brightness and in-call dimmed. The backlight intensity level in the dimmed
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
174 state will need to chosen on this principle: use the lowest backlight LED
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
175 current (to conserve battery power and allow longest talk time on one charge) at
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
176 which the display is still readable, similarly to Pirelli's in-call dimmed
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
177 state.
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
178
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
179 In the user-actively-poking state, as opposed to the long-call dimmed state,
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
180 there is no need to provide different configurable backlight levels - see
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
181 section 1.4.5.
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
182
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
183 1.4.4. Backlight circuit implementation
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
184
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
185 In all candidate TFT LCD modules that are being considered (see section 1.4.2),
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
186 the backlight consists of 3 white LEDs wired in parallel, joined either at the
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
187 anode or at the cathode - although as we shall see momentarily, we require an
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
188 LCD module where the 3 LEDs are joined at the anode, with the 3 cathodes brought
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
189 out separately. LCD module datasheets call for 15 mA current through each LED
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
190 at maximum intensity, for 45 mA total, and the LED forward drop voltage (Vf) at
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
191 this rated current seems to range between 2.9 V (what I actually measured on one
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
192 candidate LCD module) to 3.2 V (what the datasheets list as typical) to perhaps
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
193 as high as 3.4 V (what one datasheet lists as the maximum).
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
194
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
195 Given the parallel (as opposed to series) wiring of the 3 LEDs and the
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
196 relatively low Vf, there is no need to use any kind of boost converter as part
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
197 of the LED driver circuit for this backlight - any boost converter will only add
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
198 inefficiency (more current will be drawn from the battery for the same LED
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
199 current), hence we need to avoid using such.
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
200
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
201 Regardless of whether a given phone design uses a boost converter or not (it
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
202 seems that older designs do use boost converters, either because older white
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
203 LEDs have higher Vf or because 2 or 3 LEDs are wired in series), all traditional
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
204 phone designs seem to share the quality where the display backlight brightness
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
205 remains the same as the battery discharges and as Vbat goes down - this quality
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
206 was directly observed on the Pirelli DP-L10 (unknown circuit design) and
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
207 inferred from the available schematics for Mot C139 and C155, with both of the
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
208 latter boost-converting to fixed 5.0 V. In our case, even though we choose to
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
209 not use a boost converter for efficiency reasons, we still need to achieve the
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
210 quality of the display brightness remaining the same through the discharge range
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
211 of our Li-ion battery - having the display dim in half as the battery discharges
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
212 from 4.2 V peak to 3.6-3.7 V plateau is simply not acceptable.
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
213
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
214 The simplest possible LED driving circuit would be one where a current limiting
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
215 resistor is inserted in series with each LED, and then the 3 parallel
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
216 LED+resistor sets are connected across battery terminals, with a transistor
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
217 inserted somewhere to act as the on/off switch. However, this trivial circuit
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
218 is not suitable in our application because it would produce unacceptably large
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
219 variation in display brightness as the battery discharges - hence we need a more
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
220 intelligent LED driving circuit. Our Luna LCD carrier board from the spring of
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
221 2020 features an LDO bringing Vbat down to fixed 3.5 V, followed by very low-
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
222 value resistors in series with each LED - but this approach is not good for
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
223 production either, as it makes the LED current extremely sensitive to any slight
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
224 variations in Vf.
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
225
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
226 Fortunately, I was able to find a specialized white LED driver chip that is just
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
227 perfect for our application, or more precisely, a specialized chip that acts as
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
228 a constant current sink for such LEDs - Maxim MAX1916, design from 2001, just
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
229 the right time frame for the kind of phone we are seeking to build. This
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
230 special chip takes the place of "dumb" ballast resistors: connect Vbat (battery
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
231 positive terminal) directly to the common anode of the 3 LEDs, but instead of
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
232 series resistors, connect each cathode to the corresponding LEDn pin of MAX1916
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
233 - *without* any resistors or transistors! FETs inside the MAX1916 take the
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
234 place of resistors as current-limiting elements, and the chip's global on/off
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
235 control takes the place of a separate switching transistor.
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
236
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
237 The special quality of MAX1916 is that it produces constant current through each
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
238 LED (based on a set reference current and 230x current multiplication circuit
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
239 inside the chip) regardless of variations in both Vbat and Vf! Of course the
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
240 requested current can only be sustained as long as Vbat >= Vf + Vds, where Vds
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
241 is the lowest drop voltage of the FETs inside MAX1916, and once Vbat falls below
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
242 this point, the LED current will begin to decline. However, the beauty of this
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
243 design is that no arbitrary artificial turnover points (like the 3.5 V point in
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
244 our hacky design from the spring of 2020) need to be set: the battery discharge
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
245 point at which the LED current begins to decline will be whatever it comes to be
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
246 naturally, based on Vf (perhaps depending on temperature) and MAX1916 Vds, and
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
247 the decline is expected to be gradual.
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
248
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
249 1.4.4.1. Backlight current selection and dimming
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
250
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
251 In the simplest MAX1916-based design, a fixed LED current is set by connecting
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
252 a resistor of appropriately computed value between MAX1916 SET pin and whatever
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
253 regulated fixed voltage rail happens to be available in the system. However,
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
254 in our application (see section 1.4.3.1) we need at least two different display
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
255 brightness levels, and thus at least two switchable LED currents. At first the
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
256 problem seems difficult, but an elegant solution has been found.
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
257
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
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258 LCD backlight LED current will be selected by way of two Calypso GPIO pins
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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259 configured as outputs, and a 74LVC2G125 dual tristate buffer. Each tristate
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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260 buffer's A input will be tied high, and the two Calypso GPIO outputs will be
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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261 connected to buffer output enable inputs. There will be two resistors with
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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262 different carefully computed values, each connected between one of the two
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
263 tristate buffer outputs and MAX1916 SET pin. One resistor will provide a small
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
264 current, the other will provide a large current, and each of these two currents
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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265 will be switchable on/off by Calypso GPIO signals switching the buffer outputs
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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266 between driving high (2.7-2.8 V) and Hi-Z. Resistor values will be chosen such
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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267 that the sum of both currents will be the 15 mA limit (the current is reckoned
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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268 per LED), whereas the small current alone will be whatever we need for the
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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269 battery-saving long-call dimmed mode.
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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270
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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271 1.4.5. Slight regression relative to Pirelli DP-L10
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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272
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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273 The actual LCD backlight LED driving circuit inside the Pirelli phone is not
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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274 known, but reverse engineering of Pirelli's firmware followed by experimentation
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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275 reveals that backlight intensity variation is achieved via a form of PWM, using
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
276 Calypso PWL output - although PWL is used in an inverted sense, such that the
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
277 backlight intensity increases with more 0s being put out on PWL, as opposed to
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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278 more 1s. Thus regardless of the unknown actual circuit implementation, the
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
279 backlight intensity appears to be continuously variable from 1/255 to 255/255,
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
280 which is certainly a much richer control than our crude selection of just 3
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
281 possible LED currents.
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
282
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
283 In terms of what Pirelli's fw offers to end users, the backlight intensity in
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
284 the dimmed in-call state is always set to 1/255, without any way to change it,
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
285 whereas the backlight intensity in the active interaction state is selectable
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
286 via a menu among 5 levels; the 5 offered levels turn into 1/255, 64/255,
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
287 128/255, 192/255 and 255/255 in the resulting PWL programming.
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
288
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
289 So in terms of both hardware capabilities and end user offering via the
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
290 firmware, Pirelli's LCD backlight level control is richer than what we are
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
291 proposing for our FC Libre Dumbphone. However, engineering is all about
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
292 trade-offs and compromises, and in the Mother's opinion, this slight reduction
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
293 in the richness of functionality is sufficiently offset by the efficiency of
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
294 our MAX1916-based approach: aside from the theoretical possibility of a
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
295 switching buck converter, which I've never seen used for LED driving
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
296 applications, our choice of MAX1916 is the most battery-efficient way to drive
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
297 our backlight LEDs. Furthermore, when dimming is effected by switching the
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
298 actual regulated LED current, as in our case, as opposed to applying PWM, our
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
299 backlight becomes more resilient to even lower battery voltages.
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
300
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
301 Consider what happens when Vbat falls below the point at which the design-
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
302 intended LED current can be maintained - what happens then? If no PWM is
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
303 applied, or if PWM is set to maximum, then display brightness will be whatever
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
304 maximum is possible at this low battery voltage. But if PWM is applied,
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
305 especially very low duty cycles as in the case of Pirelli's dimmed state, then
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
306 the display that has already been dimmed by low Vbat will be *further* dimmed
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
307 by this aggressive PWM, likely producing an unreadable display at this point.
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
308 It may be possible to compensate via extra complexity in the firmware, by
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
309 turning PWM up when Vbat (as measured via Iota MADC) falls too low - but then
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
310 we would be getting really messy, whereas switching the regulated current is so
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
311 much more elegant. With our approach, low-battery-induced dimming in the "full
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
312 brightness" mode will happen at the same discharge point as it would if we had
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
313 used PWM (and set PWM to maximum in this "full brightness" mode), but in the
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
314 in-call dimmed state, further dimming due to low Vbat will probably happen at a
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
315 lower discharge point (if Vf decreases with decreasing current), and when it
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
316 does happen, there won't be a combination of both natural and artificially-
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
317 induced reductions, just the natural one.
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
318
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
319 Thus based on all of the above considerations, I feel justified in my design
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
320 choice of foregoing PWM control of backlight intensity in favor of fixed current
7d77aa76bcaa FC-handset-spec: beginning of massive document
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
321 switching with much more limited selection.