Watchdog — `pymcu.hal.watchdog`#

from pymcu.hal.watchdog import Watchdog

Hardware watchdog timer. Resets the MCU if firmware stops calling feed() within the configured timeout — useful for recovering from unexpected hang states in production firmware.

class `Watchdog`#

`Watchdog(timeout_ms: const[uint16] = 500)`#

The timeout must be a compile-time constant — it is resolved to a WDP prescaler at build time.

Timeout table (ATmega328P, ~3 V, 25 °C)#

`timeout_ms`	WDP	Actual timeout
16	`0b000`	~16 ms
32	`0b001`	~32 ms
64	`0b010`	~64 ms
125	`0b011`	~125 ms
250	`0b100`	~250 ms
500	`0b101`	~500 ms
1000	`0b110`	~1 s
2000	`0b111`	~2 s
4000	`0b1000`	~4 s
8000	`0b1001`	~8 s

Methods#

Method	Description
`enable()`	Enable the watchdog with the configured timeout
`disable()`	Disable the watchdog (timed-disable sequence: WDCE+WDE)
`feed()`	Reset the watchdog counter (`WDR` instruction)

Example#

from pymcu.hal.watchdog import Watchdog
from pymcu.hal.uart import UART

def main():
    uart = UART(9600)
    wdt = Watchdog(timeout_ms=500)
    wdt.enable()

    while True:
        uart.write_str("alive\n")
        wdt.feed()      # must be called within 500 ms

Safety notes#

Call wdt.feed() on every code path that may take longer than the timeout (including ISRs that block the main loop).
The watchdog persists across soft resets on ATmega328P. If disable() is not called at startup and the WDT flag is set in MCUSR, the device will reset in a loop. Clear MCUSR and call disable() at the top of main() to recover from this state.

Watchdog-based ultra-low-power wake#

Combine with the power module for periodic wake from deep sleep:

from pymcu.hal.power import sleep_power_down
from pymcu.hal.watchdog import Watchdog

wdt = Watchdog(timeout_ms=1000)

@interrupt(0x0018)    # WDT vector (ATmega328P)
def on_wdt():
    pass              # just wake; do nothing in ISR

def main():
    wdt.enable()
    asm("sei")
    while True:
        sleep_power_down()   # ~1 s @ ~0.1 µA, then run loop body
        do_periodic_task()

ATmega328P register map#

Register	Address	Description
`WDTCSR`	`0x60`	WD Control/Status (WDP, WDEN, WDIE, WDIF, WDCE, WDE)
`MCUSR`	`0x54`	MCU Status Register (WDRF, BORF, EXTRF, PORF)