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annotate MCSI-apparent-bug @ 19:f68ca40fa5c1

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author Mychaela Falconia <falcon@freecalypso.org>
date Fri, 13 Sep 2019 07:28:00 +0000
parents 8a3fd530a647
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1 NOTE: OUTDATED ARTICLE!!!
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2
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3 The following article was written in 2018-10; it is now 2019-03 and we have got
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4 MCSI working - see the new Calypso-digital-voice article. The original article
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5 below is only for history preservation.
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6
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7 Original article with outdated info follows
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8 ===========================================
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9
7
43829da82312 MCSI-apparent-bug article written
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10 The Calypso chip provides a little-used auxiliary synchronous serial interface
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11 called MCSI. It is an auxiliary interface to the DSP part of the Calypso
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12 (controlled solely by the DSP and not directly accessible to the ARM part); it
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13 is not clear what other uses TI may have had in mind for this interface, but
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14 one advertised feature of TI's TCS211 chipset+firmware solution is Bluetooth
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15 support in the form of using MCSI as a digital voice channel to be connected to
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16 TI's Bluetooth Island chip. In FreeCalypso we are not interested in Bluetooth
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17 per se, but we are interested in getting a digital voice channel out of our
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18 modem for GSM voice calls in order to compete with the mainstream proprietary
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19 GSM modem modules which do provide such digital voice channels.
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20
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21 Our current development board (FCDEV3B) has MCSI brought out on a header for
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22 experimentation, and we had high hopes that we could use TI's so-called
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23 "Bluetooth headset" mode (the mode in which the voice path is connected between
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24 GSM and "Bluetooth" on MCSI) to get a usable digital voice channel out of our
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25 modem. However, experiments have revealed the following:
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26
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27 1) The good part: when our TCS211-based fw is given a command to switch into
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28 the Bluetooth headset mode (auw 0 2 through fc-tmsh or our own added AT@VPATH=2
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29 command), MCSI comes alive: a 520 kHz bit clock appears on MCSI_CLK and an 8 kHz
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30 frame sync signal appears on MCSI_FSYNCH.
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31
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32 2) The bad part: as soon as I dial a voice call after having enabled the
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Mychaela Falconia <falcon@freecalypso.org>
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33 Bluetooth headset mode as above, the MCSI clock and frame sync signals
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34 *disappear* at some point in the voice call setup process! I can make them
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Mychaela Falconia <falcon@freecalypso.org>
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35 reappear by switching to AT@VPATH=0 (the regular GSM-to-ABB voice routing mode)
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36 and then back to AT@VPATH=2 (merely repeating AT@VPATH=2 or auw 0 2 is not
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Mychaela Falconia <falcon@freecalypso.org>
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37 sufficient, one has to switch to another mode and then back to Bluetooth
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Mychaela Falconia <falcon@freecalypso.org>
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38 headset), and then the MCSI clock & frame sync disappearance no longer happens
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Mychaela Falconia <falcon@freecalypso.org>
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39 on subsequent calls - they stay on. I can similarly make these MCSI clock &
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40 frame sync signals come on and stay on if I issue the initial AT@VPATH=2 command
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Mychaela Falconia <falcon@freecalypso.org>
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41 *after* the first voice call has been connected - but it is not clear at all
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42 exactly which step in the call setup process one would need to wait for.
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43
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44 To me (Mother Mychaela) it would make sense for the DSP to keep MCSI clocks off
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45 during idle and only turn them on during active voice calls (doing so would
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46 avoid erratic clock output as the modem goes into and out of deep sleep during
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47 idle), but the behaviour we are seeing seems like TI tried to do what I just
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48 said, but got the logic reversed.
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49
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50 This erratic behaviour with MCSI clocks getting turned off when they should be
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51 getting turned on makes this interface unusable beyond lab experimentation in
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52 my maternal opinion - dunno about others, but I would not be able to market a
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53 commercial FreeCalypso modem product with a digital voice channel via MCSI if
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54 the behaviour of the interface is clearly buggy and requires hacky workarounds
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55 on the part of the end user. For this reason no further work has been done to
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56 see if the MCSI_TXD line actually puts out sensible voice downlink bits and if
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57 the MCSI_RXD line accepts voice uplink bits in a sensible format when an active
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58 voice call is in progress and the clocks are on.
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59
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60 As I see it, there are 3 ways we can solve this digital voice channel problem:
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61
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62 1) The most ideal option would be to convince TI to dig up and release the
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Mychaela Falconia <falcon@freecalypso.org>
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63 source for the Calypso DSP ROM: getting that source and being able to study it
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64 would solve all of our problems at the root. We would then understand exactly
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65 what the DSP does with regard to MCSI, and either change its behaviour to a
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66 more desirable one by way of a DSP patch, or adapt our ARM-side fw or even the
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67 external control interface definition to what will then be fully known DSP
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68 behaviour.
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69
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70 2) If TI no longer have the source for the Calypso DSP ROM or are unable to
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71 find it because of the length of time that has passed (about 15 y), or if they
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72 have it but we are not able to convince them to release it, we could reverse-
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Mychaela Falconia <falcon@freecalypso.org>
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73 engineer this ROM by disassembling the ROM image that has been read out.
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74 However, reversing this ROM thoroughly enough to serve as a replacement for the
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75 lost original source would be an extremely costly undertaking: if I were to do
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76 it, I would need to be given a full year to work on it on a full-time basis, at
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77 the cost of about 150 kUSD, and I find it doubtful that any other professional
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78 reverser of sufficient qualification level would do it for less. Needless to
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79 say, it is highly unlikely that anyone in our ultra-marginalized FreeCalypso
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80 community can spare 150 kUSD, and if someone does have that kind of money to
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81 spend toward the liberation of the Calypso DSP, the sensible course of action
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82 would be to first offer that money to TI in exchange for the DSP ROM source
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Mychaela Falconia <falcon@freecalypso.org>
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83 before giving it to a reverser.
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84
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85 3) If someone has an actual (as opposed to hypothetical) money-backed business
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86 need for a Calypso-based modem with a digital voice interface, the fastest and
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Mychaela Falconia <falcon@freecalypso.org>
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87 cheapest solution would be to forego MCSI and tap into VSP instead, as described
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88 in the FCDEV3B-repackaging article in the section titled "Tapping VSP for the
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Mychaela Falconia <falcon@freecalypso.org>
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89 digital voice interface". This option would require designing and building a
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90 new PCB, which would probably cost some 10 to 15 kUSD, but it is far less than
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91 the 150 kUSD that would be needed for DSP ROM RE, and the timeline would be
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Mychaela Falconia <falcon@freecalypso.org>
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92 much shorter too. Unlike MCSI, VSP is used in the regular voice path going
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Mychaela Falconia <falcon@freecalypso.org>
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93 through the ABB, hence it is *known* to be good with no possibility of nasty
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94 surprises.