RP2040 SleepMemory

[dhalbert]

Implements alarm.sleep_memory on RP2040. Implementation taken from nRF. It's just a 256-byte block of RAM, since RP2040 does not power down RAM when sleeping.

Fixes #5081. That issue says other things must be done, including removing a watchdog that zeros out RAM. But I didn't do that and it still works?!

https://forums.adafruit.com/viewtopic.php?p=973288 asked about RP2040 sleep memory, and it seemed easy to implement.

Test program, tested with fake deep sleep and with power just via a 5V adapter:

import alarm
import board
import os
import time

time.sleep(1)

u = board.UART()
u.baudrate = 115200

LEN = 16

print(alarm.wake_alarm)
print("previous bytes:", alarm.sleep_memory[0:LEN])
u.write(b"previous bytes: " + str(alarm.sleep_memory[0:LEN]) + b"\r\n")

time_alarm = alarm.time.TimeAlarm(monotonic_time=time.monotonic() + 5)
b = os.urandom(LEN)
print("storing", b)
alarm.sleep_memory[0:LEN] = b

u.write(b"storing " + str(b) + b"\r\n")
time.sleep(1)

alarm.exit_and_deep_sleep_until_alarms(time_alarm)

[anecdata]

alarm.sleep_memory survives supervisor.reload(), and microcontroller.reset() into NORMAL and SAFE_MODE (more extensive persistence than espressif port where alarm.sleep_memory does not survive microcontroller.reset()). Nice, thanks @dhalbert

[tannewt]

Thanks!