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MPC8xx HDLC Postambule

Question asked by Sebastien BLAVIER on Jan 13, 2016
Latest reply on Jan 19, 2016 by Serguei Podiatchev



I try to dialog 2 boards  through HDLC FM0 =>  MPC885 on both side.

(configuration , preambule 8 bits 0xFF, Glitch=OFF, Transparent mode=OFF, CRC=16 bits, TX=16 bytes, RX=32 bytes, X21=OFF, TBRTSM=0)


when i set the field TEND of GSMR_L to 1,  ie. TXD is always encoded, even when idles are sent  =>   no problem.


But when i set the field TEND to 0,  ie. TXD is encoded only when data is sent , on Rx board side the MPC885 log the error AB and skip the check of CRC on each received frame.


The datasheet $23.3 says

Note that an SCC in HDLC mode, or any other synchronous mode, must receive a minimum of eight

clocks after the last bit arrives to account for Rx FIFO delay.


So i think when the TEND field is set to 0, 8 clock are automaticly sent  but not with TEND set to 0 which is my problem.


To  send a second closing flag or a postambule ( $23.3 adding 8 clock at the end) , i set the field  NOF of the PSMR to  2.

Note NOF : Number of flags. Minimum number of flags between or before frames.


But in this case (NOF ==2)  only the opening flagis impact  (at least 2 opening flag)  never for the closing flag (always 1).


Somebody can help me to add the 8 clock at the end of closing flag ?