topic Re: External clock, SCI and PWM in CodeWarrior Development Tools

External clock, SCI and PWM

mahkamehvarza — Tue, 15 Sep 2015 16:40:52 GMT

I am using a MC9S08QE32. I need to have a 125KHz square wave on one

of the PWM channels and a 115200 baudrate on one of the SCIs. I am

using a 16MHz crystal oscillator, and so far I have figured out the

Processor expert settings to get the 125KHz on PWM but cannot figure out

how to get the 115200 on the SCI at the same time.

How can I achieve this? If it is not possible with this clock source, what

changes do I need to make to achieve this? using an external clock source

is highly preferred.

Thanks

Re: External clock, SCI and PWM

TICS_Fiona — Wed, 16 Sep 2015 02:45:51 GMT

Hi,

Since you are using a 16MHz external crystal oscillator, the bus clock is 8M.

The SCI band Rate is 8M /(16*[ SBR12:SBR0]) . The nearest baud rate to 115200 is 131072bps, in this case SBR should be set to 4. The error is more than 10%. So 115200 is not a recommended SCI speed for your application based on the current hardware design.

Fiona Kuang

Technical Information & Commercial Support

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Re: External clock, SCI and PWM

mahkamehvarza — Wed, 16 Sep 2015 17:05:54 GMT

Fiona,

Thanks for your reply. 115200 baud rate is a requirement and so is the 125KHz square wave. Can you think of a way that I can have both at the same time?

I was looking at using an external clock source for one timer and the changing the 16MHz xtal to a 7.3728MHz to achieve the 115200 baud rate. But I just cannot figure out how to set up a timer module to work off of an external clock source in PE.

Any suggestions?