question about serial communication

Bas Wijnen wijnen at debian.org
Fri May 28 02:39:13 EDT 2010


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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