wait_us() jitter..
Topic last updated
31 Jul 2010, by
Igor Martinovski.
1 reply
digitalOut,
wait
Hi,
Has anyone noticed that wait_us() has about 500ns of jitter. It's not much, but at short wait times it could cause some problems. I took some waveform traces. The wait time shown below is either 3.5us or 4us. I can understand why the pulse is not exactly 3us wide, but what gets me is that it changes between two values.. Also, for wait_us(2), the pulse is very stable. And at one point during testing, the pulse was stable for only even numbers of wait..
I thought it was a problem with the DigitalIO implementation, but when I use a simple NOP loop delay, it can produce a (very) stable pulse down to 150ns, shown below. The issue is not much of a problem beyond 10us, so I don't think its much of a bother to most people, I was just curious..
The figure below is from the following code snippet:
DigitalOut out(p14);
...
while (1){
out=1;
wait_us(3);
out=0;
wait_us(1000);
}
Replies
Hi Igor,
This is a great analysis! Yes, the jitter you are seeing is due to the fact all the mbed time functions work from a running 1us ticker, hence you will get only as acurate as within 1us of the requested time.
Simon
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Hi,
Has anyone noticed that wait_us() has about 500ns of jitter. It's not much, but at short wait times it could cause some problems. I took some waveform traces. The wait time shown below is either 3.5us or 4us. I can understand why the pulse is not exactly 3us wide, but what gets me is that it changes between two values.. Also, for wait_us(2), the pulse is very stable. And at one point during testing, the pulse was stable for only even numbers of wait..
I thought it was a problem with the DigitalIO implementation, but when I use a simple NOP loop delay, it can produce a (very) stable pulse down to 150ns, shown below. The issue is not much of a problem beyond 10us, so I don't think its much of a bother to most people, I was just curious..
The figure below is from the following code snippet:
DigitalOut out(p14); ... while (1){ out=1; wait_us(3); out=0; wait_us(1000); }