question about serial communication

Tao Jin jintao.pku at gmail.com
Fri May 28 07:34:34 EDT 2010


Hi Bas

Thanks for the response! Yes this is an open source school project.  
And sorry for the typo about the link speed.

Actually i am pretty sure the latency is not on SoC.  I used  
oscilloscope to monitor the signal on uart link and the latency  
between incoming byte and echo byte is very short.

Also I used a USB to serial adapter that connects to pc and test a  
selfloop which connects the uart rx and tx pins using jump wires. I  
still see the latency around 10 ms. Btw I used a user space c program  
to read the ttyusb device. Any chance that may be the reason?

Thanks again

Jintao

On May 28, 2010, at 2:39, Bas Wijnen <wijnen at debian.org> wrote:

> Hi,
>
> On Thu, May 27, 2010 at 08:15:11PM -0400, Tao Jin wrote:
>> I ran across this mailing list on the web.  I have a question about
>> serial comm. in embedded system.  Hopefully this is the right place  
>> to
>> ask such question.
>
> If you're working on an open hardware project, I think this is the  
> right
> place.  I'll just assume you are. :-)
>
>> I am working a program that has zigbee cc2530 SoC communicate with
>> host device, mobile phone, through serial interface.  The baud rate  
>> is
>> set as 115200 Kbps at both zigbee chip and host device.
>
> This is insanely high.  I think you mean 115200 bps, which is a normal
> value.
>
>> When I ping 1 byte from host dev to zigbee, and zigbee echo back the
>> response,  the RTT was around 10ms.  I think this has to do with some
>> uncertain delay on the serial link.  Anyone could show some guidance
>> how to get around the high overhead on serial link?  Or this is
>> something unavoidable?
>
> The speed of the link itself is not uncertain at all.  One byte using
> 8n1 is 10 bits, so with 115200 bps, that costs about 87 μs.   
> It's a
> round trip, so that must be doubled; the total transfer time is then
> 0.2 ms.
>
> The other 9.8 ms must be lost in the logic: The SoC sending an
> interrupt and finding out that it is from the serial port, making up a
> response, and the phone doing similar things.
>
> One thing which may introduce a significant delay, is if you put the  
> SoC
> to sleep.  When waking up, it will normally wait until all the clocks
> have stabalized.
>
> Hope this helps,
> Bas
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