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FC-handset-spec section 1.7.2: make contrast with MIDI ringtone chips
author Mychaela Falconia <falcon@freecalypso.org>
date Tue, 15 Jun 2021 19:57:05 +0000
parents 1cdd0f0a6e70
children
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1 Our dear Calypso has a dedicated digital output for driving old-fashioned
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2 cellphone buzzers - I (Mother Mychaela) previously assumed those buzzers to be
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3 piezoelectric, but now it appears that they are actually magnetic buzzers, not
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4 piezo. But irrespective of the physics of the actual transducer that is
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5 ultimately controlled by Calypso BU/PWT output, the Calypso output itself is
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6 purely digital, producing digital waveforms, and we (FreeCalypso) need to
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7 understand exactly what the chip puts out. The findings presented here have
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8 been obtained by observing Calypso BU/PWT output with an oscilloscope; the hw
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9 platform used by the Mother for these experiments was a BenQ M32 module - this
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10 module has the familiar Calypso+Iota chipset inside, it has BU/PWT brought out,
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11 and the breakout board supplied with these modules allowed for very quick
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12 experimentation.
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13
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14 BU mode
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15 =======
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16
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17 BU functionality of the BU/PWT output is implemented in the ARMIO block,
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18 together with GPIO and keypad functions. Here are the new findings which were
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19 previously unknowable without oscilloscope observation:
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20
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21 * In "normal" buzzer operation when ARMIO_LOAD_TIM is in the [1,255] range
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22 (i.e., not 0), the output frequency is 13 MHz / 512 / (ARMIO_LOAD_TIM + 1).
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23 This part matches our previous understanding. The highest frequency that can
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24 be produced is 12.6953125 kHz, and the lowest is 99.182129 Hz.
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25
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26 * When ARMIO_LOAD_TIM is set to 0 and the buzzer is enabled, BU output is NOT a
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27 25.390625 kHz tone (per the formula above) as we previously thought - instead
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28 BU output will be constant high if BUZZER_LEVEL_REG is set to maximum 63, or
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29 pure 203.125 kHz PWM exactly like LT otherwise.
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30
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31 * PWM power level control for BU works exactly like the one for LT - see the
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32 companion Calypso-PWM-light article.
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33
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34 * The interesting question is exactly how PWM power level control and tone
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35 generation combine, and the answer turned out to match the terse description
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36 in CAL207: BU is the output of an AND gate; one input to this AND gate is the
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37 output of the timer that produces tones between 99 Hz and 12.7 kHz (or
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38 constant high if ARMIO_LOAD_TIM is set to 0), and the other input to the AND
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39 gate is a PWM block strictly identical to LT.
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40
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41 * For a practical example, consider what happens when a 12.7 kHz (max frequency)
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42 tone is emitted in combination with PWM. The "on" part of the waveform at
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43 this tone frequency equals 512 periods of CLK13M, whereas the full cycle of
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44 LT-like PWM is 64 periods of CLK13M. Thus the waveform seen on an o'scope
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45 consists of 8 repeated PWM cycles, then a "pause" of 512 CLK13M periods (the
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46 "off" part of the tone waveform), then the full cycle repeats. Furthermore,
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47 the tone waveform and the PWM waveform fed to the internal AND gate are not
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48 synchronized, and the resulting slight misalignment is easily visible on an
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49 o'scope.
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50
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51 Pirelli DP-L10
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52 ==============
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53
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54 In the Pirelli DP-L10 phone Calypso BU output is repurposed to control the
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55 vibrator. Pirelli's official fw sets ARMIO_LOAD_TIM to 0 and BUZZER_LEVEL_REG
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56 to 63; when operated in this manner, BU becomes a mere on/off output under
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57 software control just like a GPIO. If someone wishes to run the vibrating motor
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58 at a lower speed (i.e., exercise a form of "analog" control), the correct way to
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59 perform such feat would be by dialing down BUZZER_LEVEL_REG (PWM control between
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60 1/64 and 64/64) - setting ARMIO_LOAD_TIM to a nonzero value does not seem to
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61 make any sense when the buzzer has been replaced with a vibrator.
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62
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63 PWT mode
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64 ========
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65
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66 The principal difference between BU and PWT modes is that BU mode generates
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67 "arbitrary" tone frequencies by dividing from CLK13M/512, whereas PWT mode
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68 generates predefined musical note frequencies, specifically 48 notes of the
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69 chromatic scale from F4 through E8 in the scientific pitch notation. The
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70 frequency range of PWT mode is narrower than BU mode (349 Hz to 5274 Hz, as
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71 opposed to 99 Hz to 12.7 kHz), and PWT does not support constant high output
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72 for Pirelli-style vibrator driving - however, BU mode suffers from a major
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73 problem in that its repertoire of possible tone frequencies does not correspond
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74 to musical notes. Given the desire to have ringtone melodies that are composed
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75 of standard musical notes, TI added the new PWT hardware block that is
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76 specifically designed to produce musical note frequencies.
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77
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78 Here are some observations about PWT made with o'scope experiments:
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79
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80 * The description of FRC_REG in CAL207 is correct: bits [1:0] select the octave,
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81 bits [5:2] select the semitone within the octave. The definitions in the
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82 non-functional (not actually used) mmiBuzzer.h header file in TI's BMI source
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83 are wrong - or more precisely, the definitions under #if 0 are the correct
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84 ones, whereas the enabled-for-compilation definitions are wrong.
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85
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86 * PWM power control works on the same principle as in BU mode: the output of
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87 the musical tone frequency generator and the output of the PWM block are
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88 simply ANDed together. The 64-level PWM block itself is very similar to the
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89 ones in LT and BU, but it runs 8 times slower, i.e., one PWM period is 512
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90 cycles of CLK13M, as opposed to 64 cycles of CLK13M in BU and LT, and each
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91 "quantum" of this slower PWM equals 8 cycles of CLK13M.