Watchdog triggers before timeout on RP2 (specifically WS5100S-EVB-PICO)

[AndyThirtover]

Example

from machine import WDT wdt=WDT(timeout=600000) expect a 10 minute timeout

Wait a few seconds and the system will reboot.

MicroPython v1.19.1 on 2022-06-18; W5100S-EVB-Pico with RP2040

Discovered when testing the following:

from machine import WDT, reset
import uasyncio as asyncio
import network

"""
   WATCHDOG -- Reboot system if Network not seen for 600 seconds
   or any other system crash
"""

mon_period = const(600000)
dog_period = const(600000+1000)

wdt = WDT(timeout=dog_period)

async def monitor():
    while True:
        wdt.feed()
        await asyncio.sleep_ms(mon_period) #  i.e. shorter than dog period
        print ('Watchdog period COMPLETE')

Regards

Andy

[robert-hh]

From the data sheet:

The watchdog (Section 4.7) has a 24-bit counter, that decrements every tick, starting from a user defined value set in LOAD register. There is a logic error which means the counter is decremented twice per tick, instead of once per tick. In a recommended setup where the tick occurs at 1μs intervals, this halves the maximum time between resetting the watchdog counter from ~16.7 seconds to ~8.3 seconds.

So the documentation should be clarified.

[AndyThirtover]

Thanks Robert.

Are you suggesting that I should do something like this:

from machine import WDT

wdt = WDT(timeout=16777216)

async def monitor():
    while True:
        wdt.feed()
        await asyncio.sleep_ms(8000) #  i.e. 8 seconds, which is shorter than 8.3
        print ('Watchdog period COMPLETE')

Regards

Andy

[robert-hh]

yes, only that the largest timeout value you can supply to WDT() is 8388 or (2**23 - 1)//1000. The time unit is still ms, but the internal counter runs a 1MHz and counts twice per cycle. So you have to feed the dog latest after 8 seconds.

[dpgeorge]

So the documentation should be clarified.

Yes. And also probably raise an exception in the WDT constructor if the timeout argument is out of range.

[robert-hh]

Sure, will do.

[dpgeorge]

Addressed by 47c45d0e7fda10f4a62b7017d48ae452203b22f0