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CHANGES: document fc-add-ramps
author Mychaela Falconia <falcon@freecalypso.org>
date Sat, 18 Dec 2021 21:36:39 +0000
parents 19cabe7c8e08
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19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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1 All standard phones and modems based on the Calypso chipset from TI implement
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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2 several different power saving modes, called sleep modes, and one of these sleep
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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3 modes has a profound impact on the operation of the externally visible UART
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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4 interfaces provided by the device. The power saving mode in question is called
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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5 deep sleep, and the phone or modem can only enter this deep sleep mode when it
19cabe7c8e08 doc/Deep-sleep-support article written
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6 is in the so-called idle state, meaning that it is camped on a cell and is ready
19cabe7c8e08 doc/Deep-sleep-support article written
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7 to receive incoming calls, messages or GPRS packets - deep sleep cannot be
19cabe7c8e08 doc/Deep-sleep-support article written
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8 entered while in an active call or in the middle of packet data transfer. When
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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9 a Calypso GSM device is idle with deep sleep enabled, it will only wake up at
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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10 preprogrammed intervals to listen on the paging channel, and will stay in deep
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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11 sleep in between these paging windows. Calypso GSM devices also enter deep
19cabe7c8e08 doc/Deep-sleep-support article written
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12 sleep when they are completely idle with no radio network connection.
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13
19cabe7c8e08 doc/Deep-sleep-support article written
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14 When a Calypso GSM device enters deep sleep, the main VCXO or VCTCXO that runs
19cabe7c8e08 doc/Deep-sleep-support article written
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15 at 13 or 26 MHz and provides all other clocks in normal operation is completely
19cabe7c8e08 doc/Deep-sleep-support article written
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16 stopped (powered off), and the only clock that remains running is the 32.768 kHz
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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17 watch crystal oscillator. The preprogrammed wakeup timing (waking up to listen
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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18 on the paging channel at the right time) is driven by this 32.768 kHz clock, but
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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19 the Calypso can also be woken up ahead of the programmed time by an external
19cabe7c8e08 doc/Deep-sleep-support article written
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20 interrupt such as a button press on the phone keypad.
19cabe7c8e08 doc/Deep-sleep-support article written
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21
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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22 This deep sleep mode provides a very important power saving measure (the
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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23 extremely low current draw that is achieved during deep sleep is not possible
19cabe7c8e08 doc/Deep-sleep-support article written
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24 without stopping the fast clock), but it presents a real challenge for the
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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25 external UART interfaces. Consider what happens when an external host sends
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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26 some characters to one of Calypso's UARTs (either the AT command interface or
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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27 RVTMUX) while the GSM device is in deep sleep. In normal operation a UART
19cabe7c8e08 doc/Deep-sleep-support article written
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28 requires a clock of 16x the baud rate (some vendors' UARTs can make do with
19cabe7c8e08 doc/Deep-sleep-support article written
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29 only 8x the baud rate) in order to receive asynchronous incoming characters,
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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30 and in the Calypso these UART clocks come from the 13 MHz master clock - but
19cabe7c8e08 doc/Deep-sleep-support article written
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31 that master clock is stopped during deep sleep!
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32
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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33 Calypso UARTs have some special asynchronous (non-clock-dependent) logic that
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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34 causes a wakeup signal to be generated if some incoming traffic is detected at
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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35 a UART while in deep sleep, but the first character that triggers this wakeup
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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36 will be lost: the asynchronous logic can detect that there is "something
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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37 happening" on the UART RxD line, but it cannot catch the actual byte content
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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38 without a clock: the *only* clock available during deep sleep is 32.768 kHz,
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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39 and even at 9600 baud one would need a clock several times faster than this
19cabe7c8e08 doc/Deep-sleep-support article written
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40 rate in order to receive and register an incoming byte. Furthermore, wakeup
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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41 from deep sleep takes a non-trivial length of time, thus if someone tries to
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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42 send lots of data to a Calypso UART while in deep sleep, quite a bit more than
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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43 just the first character will be lost: I did some experiments to characterize
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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44 the delay which needs to be inserted between the first "sacrificial" wakeup
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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45 character and the subsequent character which is expected to be received
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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46 correctly, and 40 ms wasn't enough, whereas 60 ms did the trick.
19cabe7c8e08 doc/Deep-sleep-support article written
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47
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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48 So how can one have reliable communication with a Calypso GSM device over a
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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49 UART if the GSM device goes into and out of deep sleep at times which are
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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50 unpredictable to the external host and if sending characters to the Calypso
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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51 during deep sleep causes those characters to be lost? The solution involves a
19cabe7c8e08 doc/Deep-sleep-support article written
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52 special protocol:
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53
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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54 1) On the Calypso side, TI's reference firmware implements a UART activity
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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55 timer: every time some characters are received at a UART, the timer is reset to
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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56 10 s, and until that timer expires, the GSM device is not allowed to go into
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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57 deep sleep.
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58
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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59 2) Host systems sending command traffic to Calypso modems need to keep track of
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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60 how much time has elapsed since the last time they sent something to the modem,
19cabe7c8e08 doc/Deep-sleep-support article written
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61 and if enough time has elapsed that the modem is now allowed to enter deep
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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62 sleep, the host needs to perform a precautionary wakeup transmission before the
19cabe7c8e08 doc/Deep-sleep-support article written
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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
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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66 the modem can be either accepted or lost by the latter: if the modem happens to
19cabe7c8e08 doc/Deep-sleep-support article written
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67 be awake at the time, the transmission will be received normally, and if the
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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68 modem is in deep sleep, the transmission will be lost but will cause the modem
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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69 to wake up and start the 10 s UART activity timer. Our FC host tools currently
19cabe7c8e08 doc/Deep-sleep-support article written
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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
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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73 return (3 characters total) with delays inserted in between; each of the two
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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74 delays is currently set to 30 ms based on empirical testing. We expect the
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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75 response to be either AT<newline>OK<newline> (echo of command followed by OK
19cabe7c8e08 doc/Deep-sleep-support article written
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76 response) if the modem was awake or just <newline>OK<newline> if we woke it up:
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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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
19cabe7c8e08 doc/Deep-sleep-support article written
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79 be awake with UARTs working by the time the CR comes in; we make use of a quirk
19cabe7c8e08 doc/Deep-sleep-support article written
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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
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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84 of delay; it is certainly overkill, but this approach was implemented back in
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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85 2013 (near the very beginning of FreeCalypso) and has worked without any
19cabe7c8e08 doc/Deep-sleep-support article written
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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
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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89 funnels is rvinterf. Rvinterf will do the "wakeup shot" the first time it sends
19cabe7c8e08 doc/Deep-sleep-support article written
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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
19cabe7c8e08 doc/Deep-sleep-support article written
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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
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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99 article go through a back-end program called fcup-atinterf which does the serial
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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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
19cabe7c8e08 doc/Deep-sleep-support article written
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102 fcup-* user command, hence every fcup-* command currently involves the wakeup
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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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
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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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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Mychaela Falconia <falcon@freecalypso.org>
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108 release and after we get FCDEV3B V2 boards which will hopefully be free from
19cabe7c8e08 doc/Deep-sleep-support article written
Mychaela Falconia <falcon@freecalypso.org>
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109 the sleep mode bug that afflicts FCDEV3B V1.