External clock for hdlc application on EM board

I implemented an Hdlc application on the EM board and I want to use an external clock for TX and Rx.
I set naHdlcIoctl(channel,NAHDLC_TX_CLCK_EXT,(void*)TRUE);
as it is told in the Api and I also connected my external clock physically to RTS/SPI_CLK pin.
No transmission is occuring .Do i have some additionnal settings to do ? answer if you have any idea.