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Serial Communications Interface Module

Question asked by Joao Brito on Dec 28, 2009
Latest reply on Dec 29, 2009 by bigmac

I'm trying to do a simple RS-232 link that must work at 9600bps.

The problem is that the xtal conected to my cpu (MC68HC908AP64A) has a frequency of 4MHz. And according to the cpu datasheet the baud rate is given by the following formula:

baud rate = CGMXCLK / (64 x PD x BD)

Since PD allowed values are 1; 3; 4 or 13, and BD allowed values are 1; 2; 4; 8; 16; 32; 64; or 128, there is no combination that results at approximately 9600. The maximum speed that I can successfully transmit/receive is 4800bps. Strangely I've already seen the same hardware configuration working at 9600bps!

How can I manage to do that?
Is there any way to double the internal clock speed using the same xtal?

King Regards.