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Serial Latency Issues, Baud Rate Affects Buffering



Hi All,

We have an ETRAX100LX development board.  For testing, serial port 2 on the 
ETRAX is connected to a PC via a null modem cable.  In the production system, 
a wireless modem (limited to 4800 baud) will sit between the PC and the 
ETRAX. At this time, we are operating the serial ports without hardware or 
software flow control.

The PC runs a simple program which sends 7 bytes of data to the ETRAX via the 
Serial port at a rate of 50Hz (7 * 50 = 350 which is within the bandwidth 
capabilties of a 4800 baud modem).  Instrumentation in the PC program ensures 
that it sends the data at the proper rate and meets its timing deadlines.

The ETRAX runs a simple application which loops at a rate of 50Hz.  Each 
iteration, the serial port is read.  Ideally, we would like to read 7 bytes 
of data each loop iteration.

Initally, CONFIG_ETRAX_SERIAL_RX_TIMEOUT_TICKS was set to 5 and performance 
was poor.  On most iterations (at 115200 and 4800 and others) we would read 0 
bytes, when the buffer was finally flushed, we would read multiple 7 byte 
packets.  A bit research indicated that changing 
CONFIG_ETRAX_SERIAL_RX_TIMEOUT_TICKS to 0 or 1 would help the problem.

With CONFIG_ETRAX_SERIAL_RX_TIMEOUT_TICKS set to 0, if we set the baud rate 
between the PC and the ETRAX to 115200, the ETRAX behaves well, and reads 7 
bytes each iteration.  If we set the baud rate between the PC and the ETRAX 
to 4800, the ETRAX behaves poorly and reads 0 bytes on some loop iterations 
while reading greater than 7 bytes on other loop iterations.  Why does the 
baud rate affect the behavior of the buffering?  Is there anything we can do 
to fix this issue? 

We also tried setting the CONFIG_ETRAX_SERIAL_FAST_TIMER and the 
CONFIG_ETRAX_SERIAL_FLUSH_DMA_FAST options.  This made the situation worse 
and NO data was transmitted between the PC and the ETRAX.

With CONFIG_ETRAX_SERIAL_FAST_TIMER enabled, we were able to successfully 
transmit and receive 7 byte packets at 50Hz at 115200 baud.  When we tried to 
operate at 4800 baud, no data was received (a logic probe confirms the PC was 
transmitting data).

With CONFIG_ETRAX_SERIAL_FLUSH_DMA_FAST, no data was recevied.

Port 0 on the ETRAX is connected at 115200 to a different device.  We have 
seen no issues communicating with this device with any combination of 
options.  However, we do always run this port at 115200.

We are looking for suggestions on what to try next as we are out of ideas.  
Any help would be greatly appreciated,

Eric

-- 
Eric Sorton
Senior Member Research Staff
Institute for Scientific Research (ISR), Inc.
119 Roush Circle
Industrial Park Road
Fairmont, WV  26554
Email: esorton@xxxxxxx.org
URL: http://www.isrparc.org
Voice: 304-368-9300 x219
FAX: 304-534-4106