FreeCalypso > hg > freecalypso-tools
annotate doc/Deep-sleep-support @ 872:b026f1156852
CHANGES: document fc-pwt-comp
author | Mychaela Falconia <falcon@freecalypso.org> |
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date | Mon, 28 Mar 2022 06:30:09 +0000 |
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doc/Deep-sleep-support article written
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1 All standard phones and modems based on the Calypso chipset from TI implement |
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2 several different power saving modes, called sleep modes, and one of these sleep |
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3 modes has a profound impact on the operation of the externally visible UART |
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4 interfaces provided by the device. The power saving mode in question is called |
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5 deep sleep, and the phone or modem can only enter this deep sleep mode when it |
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6 is in the so-called idle state, meaning that it is camped on a cell and is ready |
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7 to receive incoming calls, messages or GPRS packets - deep sleep cannot be |
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8 entered while in an active call or in the middle of packet data transfer. When |
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9 a Calypso GSM device is idle with deep sleep enabled, it will only wake up at |
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10 preprogrammed intervals to listen on the paging channel, and will stay in deep |
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11 sleep in between these paging windows. Calypso GSM devices also enter deep |
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12 sleep when they are completely idle with no radio network connection. |
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13 |
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14 When a Calypso GSM device enters deep sleep, the main VCXO or VCTCXO that runs |
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15 at 13 or 26 MHz and provides all other clocks in normal operation is completely |
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16 stopped (powered off), and the only clock that remains running is the 32.768 kHz |
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17 watch crystal oscillator. The preprogrammed wakeup timing (waking up to listen |
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18 on the paging channel at the right time) is driven by this 32.768 kHz clock, but |
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19 the Calypso can also be woken up ahead of the programmed time by an external |
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20 interrupt such as a button press on the phone keypad. |
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21 |
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22 This deep sleep mode provides a very important power saving measure (the |
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23 extremely low current draw that is achieved during deep sleep is not possible |
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24 without stopping the fast clock), but it presents a real challenge for the |
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25 external UART interfaces. Consider what happens when an external host sends |
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26 some characters to one of Calypso's UARTs (either the AT command interface or |
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27 RVTMUX) while the GSM device is in deep sleep. In normal operation a UART |
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28 requires a clock of 16x the baud rate (some vendors' UARTs can make do with |
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29 only 8x the baud rate) in order to receive asynchronous incoming characters, |
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30 and in the Calypso these UART clocks come from the 13 MHz master clock - but |
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31 that master clock is stopped during deep sleep! |
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32 |
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33 Calypso UARTs have some special asynchronous (non-clock-dependent) logic that |
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34 causes a wakeup signal to be generated if some incoming traffic is detected at |
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35 a UART while in deep sleep, but the first character that triggers this wakeup |
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36 will be lost: the asynchronous logic can detect that there is "something |
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37 happening" on the UART RxD line, but it cannot catch the actual byte content |
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38 without a clock: the *only* clock available during deep sleep is 32.768 kHz, |
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39 and even at 9600 baud one would need a clock several times faster than this |
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40 rate in order to receive and register an incoming byte. Furthermore, wakeup |
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41 from deep sleep takes a non-trivial length of time, thus if someone tries to |
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42 send lots of data to a Calypso UART while in deep sleep, quite a bit more than |
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43 just the first character will be lost: I did some experiments to characterize |
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44 the delay which needs to be inserted between the first "sacrificial" wakeup |
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45 character and the subsequent character which is expected to be received |
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46 correctly, and 40 ms wasn't enough, whereas 60 ms did the trick. |
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47 |
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48 So how can one have reliable communication with a Calypso GSM device over a |
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49 UART if the GSM device goes into and out of deep sleep at times which are |
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50 unpredictable to the external host and if sending characters to the Calypso |
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51 during deep sleep causes those characters to be lost? The solution involves a |
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52 special protocol: |
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53 |
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54 1) On the Calypso side, TI's reference firmware implements a UART activity |
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55 timer: every time some characters are received at a UART, the timer is reset to |
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56 10 s, and until that timer expires, the GSM device is not allowed to go into |
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57 deep sleep. |
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58 |
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59 2) Host systems sending command traffic to Calypso modems need to keep track of |
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60 how much time has elapsed since the last time they sent something to the modem, |
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61 and if enough time has elapsed that the modem is now allowed to enter deep |
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62 sleep, the host needs to perform a precautionary wakeup transmission before the |
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63 actual desired one. |
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64 |
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65 What is a precautionary wakeup transmission? The idea is to send something to |
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66 the modem can be either accepted or lost by the latter: if the modem happens to |
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67 be awake at the time, the transmission will be received normally, and if the |
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68 modem is in deep sleep, the transmission will be lost but will cause the modem |
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69 to wake up and start the 10 s UART activity timer. Our FC host tools currently |
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70 use the following wakeup transmissions: |
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71 |
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72 * On the AT command channel we send A-delay-T-delay-CR, i.e., AT and a carriage |
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73 return (3 characters total) with delays inserted in between; each of the two |
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74 delays is currently set to 30 ms based on empirical testing. We expect the |
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75 response to be either AT<newline>OK<newline> (echo of command followed by OK |
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76 response) if the modem was awake or just <newline>OK<newline> if we woke it up: |
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77 if we are waking the modem from deep sleep, our initial characters will trigger |
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78 the wakeup sequence but will themselves be lost, and the modem is expected to |
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79 be awake with UARTs working by the time the CR comes in; we make use of a quirk |
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80 of TI's AT command interpreter implementation in that sending a CR by itself |
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81 produces a <newline>OK<newline> response. |
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82 |
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83 * On the RVTMUX interface we send a string of 64 zero bytes followed by 100 ms |
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84 of delay; it is certainly overkill, but this approach was implemented back in |
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85 2013 (near the very beginning of FreeCalypso) and has worked without any |
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86 problems ever since, hence we are not changing it. |
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87 |
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88 In the case of RVTMUX, our serial communication engine through which everything |
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89 funnels is rvinterf. Rvinterf will do the "wakeup shot" the first time it sends |
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90 anything to the target, and for all subsequent transmissions it will consider |
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91 the time since the last transmission: if it is greater than a set threshold |
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92 (7 s by default), the wakeup shot is sent again. Thus there will be no |
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93 extraneous wakeup shots and associated delays during reasonably continuous |
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94 back to back communication, but the wakeup shot delay will be incurred if |
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95 rvinterf is killed and restarted or if a non-trivial pause occurs in the |
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96 communication flow. |
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97 |
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98 In the case of AT commands, our fcup-* tools described in the User-phone-tools |
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99 article go through a back-end program called fcup-atinterf which does the serial |
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100 talking, and the latter helper program is responsible for the wakeup logic. |
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101 However, fcup-atinterf is not a daemon like rvinterf, it is run anew for every |
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102 fcup-* user command, hence every fcup-* command currently involves the wakeup |
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103 delay step. It is certainly inefficient, but the underlying philosophy values |
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104 reliability over efficiency. |
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105 |
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106 The one remaining use case which has not been addressed at all yet is the GSM |
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107 07.10 MUX; the current plan is to investigate it after the fc-host-tools-r9 |
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108 release and after we get FCDEV3B V2 boards which will hopefully be free from |
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109 the sleep mode bug that afflicts FCDEV3B V1. |