Maximum Timeout for Window Watchdog(WWDG)

From the reference manual, section on Reset and Clock-control

APB1 frequency is determined by a pre-scaler/divider from AHB frequency; the divider can be 1, 2, 4, 8 or 16.

AHB frequency is determined by a pre-scaler/divider from SYSCLK; the divider can be 1, 2, 4, 8, 16, 64, 128, 256 or 512.

So how slow APB1 can be depends how slow you can run your processor and AHB bus.

Personally, I wouldn't normally run the processor slower than 8 MHz because that's what it starts at.

And from 8 MHz / 512 / 16 = 976 Hz approx.

Hope this helps,

Danish