Hello,
I have a device that sends one byte over the serial line every 10ms.
Using c, I wrote an application that opens the serial port and reads
bytes in an infinite loop. I disabled all blocking (O_NONBLOCK, VMIN=0,
VTIME=0, B115200). My CPU spends ~100% of its time calling read()
[which almost always returns 0].
I compute the time each byte shows up using gettimeofday(). By
differencing the time of successive samples, I can compute the time it
took each byte to arrive. Since the bytes are transmitted at 100Hz, I
expect to find that delta_time is 10ms.
For several seconds I get good results with delta_time = 10ms with a
noise of ~50us
Then performance deteriorates and I get 10ms + with a noise of ~50us and
a bias that cycles through 0ms, 5ms, 0ms -5ms.
Then results go back to good.
See a graph of this here (y axis is delta_timeval, x axis is time in sec):
http://img218.imageshack.us/img218/4944/plot1t.gif
http://img12.imageshack.us/img12/9693/plot2.gif
http://img10.imageshack.us/img10/5995/plot3.gif
Q: What is the source of the alternating +/- 5ms bias that comes and
goes every few seconds?
Possible answers:
1. My external device is sending the bytes strangely (I don't believe
this, but I can use an oscilliscope to confirm).
2. read() doesn't return within 1ms of the data coming in to the serial
port.
3. gettimeofday() does not return a time good to 1ms
4. none of the above
Thank you for your help!
Chris
PS: I am using 7.2-RELEASE
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