[ECOS] Re: MPC555 serial receive drops bytes
Wed May 7 16:32:00 GMT 2008
Steven Clugston wrote:
> I've been developing an application which makes use of the mpc555 serial
> driver (xxx555_serial_with_ints.c).
> With the exception fixed, I was still getting overruns so I moved the
> code out of the DSR code into the ISR to improve latency. Since its only
> moving a byte out of a register, I thought it doesn't really need a DSR.
> This improved matters, but the overrun was still occuring.
This sounds like your ISR code can take more than one character time to
execute. Assuming 8-N-1 data format, a 57600 baudrate works out to 173.6
microseconds per character. You don't say what speed your CPU is
running, but I suspect 173.6 uS works out to a *lot* of CPU
instructions. You should not need to sit in the ISR waiting for the line
to go idle.
I had the same problem with the Coldfire serial driver. The original
driver simply masked out the interrupt and called the DSR. That worked
great until you got some other DSR taking more than one character time
to execute. The serial DSRs got posted, but they didn't run in time. The
solution for my driver was to move the data in/out of the UART to a
circular buffer in the ISR.
I suspect this issue may occur on other platforms as well.
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