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FC-handset-spec: LCD and backlight specification changes to ungate Venus board design
author Mychaela Falconia <falcon@freecalypso.org>
date Sat, 03 Jul 2021 07:01:39 +0000
parents a2857b6c3d87
children bdc5d8d9fa1f
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66:a2857b6c3d87 67:ed686a290f2d
89 physical size, with 31.68x39.60 mm active area and 0.180 mm dot pitch, hence 89 physical size, with 31.68x39.60 mm active area and 0.180 mm dot pitch, hence
90 this physical size is the one we are going to use. 90 this physical size is the one we are going to use.
91 91
92 1.4.2. Specific LCD module selection 92 1.4.2. Specific LCD module selection
93 93
94 As of this writing, the specific LCD module to be used has not been firmly 94 As of 2021-07, the frontrunner candidate LCD module for our FC handset is
95 selected yet. We are actively looking for an LCD module that fits all of the 95 Formike KWH020ST23-F01. Our selection criteria are as follows:
96 following requirements:
97 96
98 * TFT color LCD, 2.0" diagonal, 176x220 pixel resolution; 97 * TFT color LCD, 2.0" diagonal, 176x220 pixel resolution;
99 98
100 * 16-bit microprocessor bus interface; 99 * 16-bit microprocessor bus interface;
101 100
113 interface this LCD to the Calypso in exactly the same way how TI did it on the 112 interface this LCD to the Calypso in exactly the same way how TI did it on the
114 D-Sample, the 6:00 viewing direction and mechanical mounting requirements stem 113 D-Sample, the 6:00 viewing direction and mechanical mounting requirements stem
115 naturally from the target application (cellular phone handset), and the 114 naturally from the target application (cellular phone handset), and the
116 backlight LED wiring requirement stems from the constraints of our chosen 115 backlight LED wiring requirement stems from the constraints of our chosen
117 MAX1916 backlight LED driver chip - see section 1.4.4. 116 MAX1916 backlight LED driver chip - see section 1.4.4.
117
118 Formike KWH020ST23-F01 is an LCD module that was discovered by Mother Mychaela
119 in May-June of 2021, and it appears to satisfy all of our requirements. We have
120 designed a test board for the purpose of evaluating this LCD (it will connect
121 the new LCD to our Luna interface from 2020), but because of Digi-Key part
122 backorder issues, the assembly of this board and thus the actual test are being
123 delayed until Sep-Oct of 2021.
118 124
119 1.4.3. Backlight and readability considerations 125 1.4.3. Backlight and readability considerations
120 126
121 Out of the various pre-existing mobile phones which I (Mychaela) have 127 Out of the various pre-existing mobile phones which I (Mychaela) have
122 experienced, there have been 3 different kinds of LCDs in terms of how display 128 experienced, there have been 3 different kinds of LCDs in terms of how display
173 it switches on at full brightness, together with the keypad backlight. But when 179 it switches on at full brightness, together with the keypad backlight. But when
174 you are in a call, when the timer expires (and it's a shorter timer, 10 s 180 you are in a call, when the timer expires (and it's a shorter timer, 10 s
175 instead of 30 s), the display goes dim instead of fully off, and in this dimmed 181 instead of 30 s), the display goes dim instead of fully off, and in this dimmed
176 (but still readable) state keypresses are NOT swallowed. 182 (but still readable) state keypresses are NOT swallowed.
177 183
178 We only need to implement two different intensity levels for the LCD backlight: 184 In terms of absolute requirements, we only need to implement two different
179 full brightness and in-call dimmed. The backlight intensity level in the dimmed 185 intensity levels for the LCD backlight: full brightness and in-call dimmed.
180 state will need to be chosen on this principle: use the lowest backlight LED 186 The backlight intensity level in the dimmed state will need to be chosen on
181 current (to conserve battery power and allow longest talk time on one charge) at 187 this principle: use the lowest backlight LED current (to conserve battery power
182 which the display is still readable, similarly to Pirelli's in-call dimmed 188 and allow longest talk time on one charge) at which the display is still
183 state. 189 readable, similarly to Pirelli's in-call dimmed state. In the user-actively-
184 190 poking state, as opposed to the long-call dimmed state, there is no strict need
185 In the user-actively-poking state, as opposed to the long-call dimmed state, 191 to provide different configurable backlight levels - but it so happens that our
186 there is no need to provide different configurable backlight levels - see 192 implementation will allow a total of 4 different backlight intensity levels -
187 section 1.4.5. 193 see section 1.4.4.1.
188 194
189 1.4.4. Backlight circuit implementation 195 1.4.4. Backlight circuit implementation
190 196
191 In all candidate TFT LCD modules that are being considered (see section 1.4.2), 197 In all candidate TFT LCD modules that are being considered (see section 1.4.2),
192 the backlight consists of 3 white LEDs wired in parallel, joined either at the 198 the backlight consists of 3 white LEDs wired in parallel, joined either at the
242 switching transistor. 248 switching transistor.
243 249
244 The special quality of MAX1916 is that it produces constant current through each 250 The special quality of MAX1916 is that it produces constant current through each
245 LED (based on a set reference current and 230x current multiplication circuit 251 LED (based on a set reference current and 230x current multiplication circuit
246 inside the chip) regardless of variations in both Vbat and Vf! Of course the 252 inside the chip) regardless of variations in both Vbat and Vf! Of course the
247 requested current can only be sustained as long as Vbat >= Vf + Vds, where Vds 253 requested current can only be sustained as long as Vbat >= Vf + Vds_min, where
248 is the lowest drop voltage of the FETs inside MAX1916, and once Vbat falls below 254 Vds_min is the lowest drop voltage of the FETs inside MAX1916, and once Vbat
249 this point, the LED current will begin to decline. However, the beauty of this 255 falls below this point, the LED current will begin to decline. However, the
250 design is that no arbitrary artificial turnover points (like the 3.5 V point in 256 beauty of this design is that no arbitrary artificial turnover points (like the
251 our hacky design from the spring of 2020) need to be set: the battery discharge 257 3.5 V point in our hacky design from the spring of 2020) need to be set: the
252 point at which the LED current begins to decline will be whatever it comes to be 258 battery discharge point at which the LED current begins to decline will be
253 naturally, based on Vf (perhaps depending on temperature) and MAX1916 Vds, and 259 whatever it comes to be naturally, based on Vf (perhaps depending on
254 the decline is expected to be gradual. 260 temperature) and MAX1916 Vds_min, and the decline is expected to be gradual.
255 261
256 1.4.4.1. Backlight current selection and dimming 262 1.4.4.1. Backlight current selection and dimming
257 263
258 In the simplest MAX1916-based design, a fixed LED current is set by connecting 264 In the simplest MAX1916-based design, a fixed LED current is set by connecting
259 a resistor of appropriately computed value between MAX1916 SET pin and whatever 265 a resistor of appropriately computed value between MAX1916 SET pin and whatever
265 LCD backlight LED current will be selected by way of two Calypso GPIO pins 271 LCD backlight LED current will be selected by way of two Calypso GPIO pins
266 configured as outputs, and a 74LVC2G125 dual tristate buffer. Each tristate 272 configured as outputs, and a 74LVC2G125 dual tristate buffer. Each tristate
267 buffer's A input will be tied high, and the two Calypso GPIO outputs will be 273 buffer's A input will be tied high, and the two Calypso GPIO outputs will be
268 connected to buffer output enable inputs. There will be two resistors with 274 connected to buffer output enable inputs. There will be two resistors with
269 different carefully computed values, each connected between one of the two 275 different carefully computed values, each connected between one of the two
270 tristate buffer outputs and MAX1916 SET pin. One resistor will provide a small 276 tristate buffer outputs and MAX1916 SET pin. There will also be a third
271 current, the other will provide a large current, and each of these two currents 277 resistor connected between MAX1916 SET pin and the V-IO rail.
272 will be switchable on/off by Calypso GPIO signals switching the buffer outputs 278
273 between driving high (2.7-2.8 V) and Hi-Z. Resistor values will be chosen such 279 The values of the 3 just-described resistors will be selected so that the path
274 that the sum of both currents will be the 15 mA limit (the current is reckoned 280 through each SET resistor will add a different contribution to the total LED
275 per LED), whereas the small current alone will be whatever we need for the 281 current. The always-on SET resistor will provide the smallest current
276 battery-saving long-call dimmed mode. 282 corresponding to the long-call dimmed mode, whereas the sum of all 3
283 contribution currents will equal the 15 mA limit - the current is reckoned per
284 LED. The difference between the 15 mA maximum and the small dimmed mode current
285 will be split unequally between the two switched SET resistors, allowing a total
286 of 4 different backlight intensity levels to be selected via the two Calypso
287 GPIO outputs going to the 74LVC2G125 buffer's output enable inputs. The actual
288 currents will be determined some time around Sep-Oct of 2021 when we get our
289 lunalcd2 test board built.
277 290
278 1.4.5. Slight regression relative to Pirelli DP-L10 291 1.4.5. Slight regression relative to Pirelli DP-L10
279 292
280 The actual LCD backlight LED driving circuit inside the Pirelli phone is not 293 The actual LCD backlight LED driving circuit inside the Pirelli phone is not
281 known, but reverse engineering of Pirelli's firmware followed by experimentation 294 known, but reverse engineering of Pirelli's firmware followed by experimentation
282 reveals that backlight intensity variation is achieved via a form of PWM, using 295 reveals that backlight intensity variation is achieved via a form of PWM, using
283 Calypso PWL output - although PWL is used in an inverted sense, such that the 296 Calypso PWL output - although PWL is used in an inverted sense, such that the
284 backlight intensity increases with more 0s being put out on PWL, as opposed to 297 backlight intensity increases with more 0s being put out on PWL, as opposed to
285 more 1s. Thus regardless of the unknown actual circuit implementation, the 298 more 1s. Thus regardless of the unknown actual circuit implementation, the
286 backlight intensity appears to be continuously variable from 1/255 to 255/255, 299 backlight intensity appears to be continuously variable from 1/255 to 255/255,
287 which is certainly a much richer control than our crude selection of just 3 300 which is certainly a much richer control than our crude selection of just 4
288 possible LED currents. 301 possible LED currents.
289 302
290 In terms of what Pirelli's fw offers to end users, the backlight intensity in 303 In terms of what Pirelli's fw offers to end users, the backlight intensity in
291 the dimmed in-call state is always set to 1/255, without any way to change it, 304 the dimmed in-call state is always set to 1/255, without any way to change it,
292 whereas the backlight intensity in the active interaction state is selectable 305 whereas the backlight intensity in the active interaction state is selectable
1356 input connector, and the control signals for the charging circuit will need to 1369 input connector, and the control signals for the charging circuit will need to
1357 be connected to the 2x4 header. 1370 be connected to the 2x4 header.
1358 1371
1359 3.5. LCD module and backlight 1372 3.5. LCD module and backlight
1360 1373
1361 It is the Mother's desire to finalize the selection of LCD module for the actual 1374 The Mother's original intent was to finalize the selection of LCD module for the
1362 handset before embarking on the detailed design (as in schematics, BOM and 1375 actual handset before embarking on the detailed design (as in schematics, BOM
1363 layout) of FC Venus, and to implement the final LCD and the final backlight 1376 and layout) of FC Venus, and to implement the final LCD and the final backlight
1364 circuit on the Venus board. 1377 circuit on the Venus board. However, because of the delay in KWH020ST23-F01 LCD
1378 testing caused by Digi-Key part backorder issues (see section 1.4.2), this plan
1379 is being modified slightly: the design of FC Venus board is being ungated on the
1380 bet that KWH020ST23-F01 LCD module will pass all necessary qualification tests,
1381 but this Venus board design won't be sent out to PCB fab until those LCD tests
1382 actually pass.
1383
1384 The backlight circuit design of section 1.4.4.1, which is hoped to be final, is
1385 being included in the Venus board design.
1365 1386
1366 3.6. Keypad buttons 1387 3.6. Keypad buttons
1367 1388
1368 The complex mechanical arrangement of keypad buttons that will be needed for the 1389 The complex mechanical arrangement of keypad buttons that will be needed for the
1369 real handset will NOT be done on the Venus board. Instead there will be a 1390 real handset will NOT be done on the Venus board. Instead there will be a