Refined low-power code #78
Description
Hi @PaulZC,
The other Paul, Robin, and I have recently been revisiting the Artemis low-power code. It turns out that we've been incorrectly using the am_hal_pwrctrl_memory_deepsleep_powerdown() function (see: https://forum.sparkfun.com/viewtopic.php?f=169&t=55163#p224296).
Because the Artemis isn't necessarily writing to the SRAM starting at the first memory location, retaining the lowest 64K is not advisable (see: https://forum.sparkfun.com/viewtopic.php?f=169&t=50904&p=224374#p224374). We've basically been powering down the first 64K of SRAM and have been lucky that no code was actually written to those banks.
Based on these findings, the recommended method of powering down the memory looks to be:
//Power down cache, flash, SRAM
am_hal_pwrctrl_memory_deepsleep_powerdown(AM_HAL_PWRCTRL_MEM_ALL); // Power down all flash and cache
am_hal_pwrctrl_memory_deepsleep_retain(AM_HAL_PWRCTRL_MEM_SRAM_384K); // Retain all SRAM
This will power down all the cache (? uA) and flash (~110 uA) but retain all of the SRAM. The power consumption difference between 32K and 384K is <1 uA, which is negligible, even for me.
Additionally, upon wake, it is not necessary to power up the memory and the following line of code can be safely removed as it doesn't actually do anything:
//Power up SRAM, turn on entire Flash
am_hal_pwrctrl_memory_deepsleep_powerdown(AM_HAL_PWRCTRL_MEM_MAX);
Cheers,
Adam