8 years, 11 months ago.

wait_ms() and Ticker slow to a crawl, but only after about an hour?!

Ok, so I have this weird as hell problem that is impossible to debug. I have code like this:

void flash()
{
	led1 = 1;
	wait_ms(300);
	led1 = 0;
}

int main()
{
	Ticker t;
	t.attach(flash, 1.0f);
	for (;;)
	{
		led2 = 1;
		wait_ms(300);
		led2 = 0;
		wait_ms(300);
	}
}

If I compile it and upload it to my board (a custom one without a 32 kHz crystal, so I'm using the HRM1017 target), then it works fine. For about an hour. I've narrowed it down to between 80 and 110 minutes, assuming it isn't stochastic. Edit: see below.

After that, the for() loop flashing still works, but when the ISR runs it stays on for ages. I'm not sure exactly how long, but 60 seconds wouldn't be implausible. It does eventually exit the ISR (turning off led1) and then led2 flashes again.

On an earlier test I found that the ticker was running every 512 seconds instead of every 1 second. I thought it might be something to do with the RTC overflow ISR but that is every 8.5 minutes (2^24/32768 seconds) and it definitely lasts longer than that.

(Yes I know it's not a good idea to block for so long in ISRs but the code *should* work and it does for an hour. Why should it then stop?)

So the questions are: 1. Why does the ISR get called so infrequently? 2. Why does wait_ms() take longer than it should in the ISR, but not in the main loop? 3. Why does this only happen after about an hour?!

This is really weird.

Edit 1

Ok I timed it more precisely a couple of times. Both times it was about 63 minutes and 20 seconds, which is exactly 3800 seconds. Stranger and stranger... That doesn't correspond with any interesting binary numbers I would expect. E.g. 3800*32768 in hex is 0x76C0000. I'm going to try changing the ticker timeout and see if that affects things.

Answer

This was fixed in revision 97 of the mBed code.

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Last updated: 27 Aug 2020

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