u-blox / low-power-sleep

This class provides an API to assist with low power behaviour on an STM32F437 micro, as used on the u-blox C030 board. If you need to operate from battery for any significant period, or are mains powered and don't want to take the planet down with you, you should design your code with this in mind. This library uses the https://developer.mbed.org/users/Sissors/code/WakeUp/ library and so could be extended to support all of the MCUs that library supports.

Dependencies:   WakeUp

Dependents:   example-low-power-sleep aconnoCellularGnss

readme.txt@1:4f2c412dc013, 2017-04-10 (annotated)

Committer:
rob.meades@u-blox.com
Date:
Mon Apr 10 11:40:13 2017 +0100
Revision:
1:4f2c412dc013
Child:
3:442c9afc0229
Add files to repo, removing temp.txt placeholder.

Who changed what in which revision?

UserRevisionLine numberNew contents of line
rob.meades@u-blox.com 1:4f2c412dc013 1 This class provides an API to assist with low power behaviour on an STM32F437 micro. If you need to operate from battery for any significant period, or are mains powered and don't want to take the planet down with you, you should design your code with this in mind. This library uses the https://developer.mbed.org/users/Sissors/code/WakeUp/ library and so could be extended to support all of the MCUs that library supports.
rob.meades@u-blox.com 1:4f2c412dc013 2
rob.meades@u-blox.com 1:4f2c412dc013 3 The principle is that the STM32F437 is put into Stop mode (typical current consumption 230 uA @ 1.8 V), where the main clocks are stopped and it is running from the internal 32 kHz oscillator. Wake-up from this state is achieved using an alarm from the RTC. Since lack of a clock involves suspending the RTOS and the RTOS uses microsecond timing in a uint32_t variable, the maximum sleep time is 4294 seconds, or just over 1 hour. However, intermediate wake-ups are handled internally by this library in order to permit sleep times of up to one month.
rob.meades@u-blox.com 1:4f2c412dc013 4
rob.meades@u-blox.com 1:4f2c412dc013 5 In addition to this, it is possible to save significantly more power by putting the STM32F437 into Standby mode (typical current consumption 2.8 uA @ 1.8 V). In this mode, as well as the main clocks being stopped, all of RAM is also powered down; only the 4 kbyte Backup SRAM is retained. If you are able to design your code such that none of the following need be maintained across a low-power sleep cycle:
rob.meades@u-blox.com 1:4f2c412dc013 6
rob.meades@u-blox.com 1:4f2c412dc013 7 * RTOS timers,
rob.meades@u-blox.com 1:4f2c412dc013 8 * dynamically allocated variables (i.e. those on the stack or the heap, for instance allocated with 'new'),
rob.meades@u-blox.com 1:4f2c412dc013 9 * more than 4 kbytes of statically allocated variables,
rob.meades@u-blox.com 1:4f2c412dc013 10
rob.meades@u-blox.com 1:4f2c412dc013 11 ...then it may be possible to use this Standby mode. Of course, designing your code to work in this way is quite a specific task. The code will begin running at main() again on expiry of the low-power sleep period and will need to determine how to behave based on the information it saved in the 4 kbyes of Backup SRAM. But, given the huge saving in power, such a design may be worthwhile.
rob.meades@u-blox.com 1:4f2c412dc013 12
rob.meades@u-blox.com 1:4f2c412dc013 13 Finally, if your code has another means of retaining state across a low-power sleep cycle then even the Backup SRAM can be powered down, reducing the typical current consumption to 2.3 uA @ 1.8 V.
rob.meades@u-blox.com 1:4f2c412dc013 14
rob.meades@u-blox.com 1:4f2c412dc013 15 Note: these functions take possession of Alarm A on the RTC hardware block of the microcontroller.
rob.meades@u-blox.com 1:4f2c412dc013 16 Note: it is not possible to make these functions threadsafe versus set_time(), so please ensure that set_time() can never be active at the same moment as one of these calls.