TMS320F28377D: UNSTABLE CLOCK MAYBE?

Peter Adamcik

Part Number: TMS320F28377D

Hi Guys, I'm not sure if my F28377D's CPU clock is running unstable or my scope is not reading the clock speed accurately.

I've setup the XCLKOUT PIN 72 and reading the CPU clock speed with my scope. All is working there.

I've set the CPU speed to 172.4Mhz and reading the clock with my scope it reads 172.4Mhz and after a few seconds my scope reads 166.5 then jumps back up to 172.4Mhz.

This is not good for time critical applications like the one I'm working on now. I need it to be 100% stable at the CPU speed I set it at.

Is the CPU not running at a stable speed or is it my scope? I've never had this problem before. Strange :)

Please let me know your expertise.

Thanks Guys

Peter.

over 5 years ago

0 Vivek Singh over 5 years ago

TI__Guru** 115191 points

Hi Peter,

CPU clock should be stable but IOs do not support such high speed clock hence you do not see it stable on scope. Please divide down the XCLKOUT by 4 and that should make it stable on scope.

Regards,

Vivek Singh

0 Peter Adamcik over 5 years ago in reply to Vivek Singh

Expert 1425 points

Hi Vivek, thank you for the reply. I'll check it out.

If I wanted to clock an external IC using the xclockout pin at high speed would that make the external IC run unstable?

Pete

0 Vivek Singh over 5 years ago in reply to Peter Adamcik

TI__Guru** 115191 points

Hi Peter,

As long as output frq is less than 50MHz it should be ok. Below is the XCLKOUT spec we have in device manual and user need to test the system to make sure it works as expected.

Regards,

Vivek Singh

0 Peter Adamcik over 5 years ago in reply to Vivek Singh

Expert 1425 points

Hi Vivek, So to read a stable xclockout I have to divide the xclockout clock by 4.

To measure the actual CPU's clock speed I'll set the PLL to 200Mhz and with xclockout divided by 4 the xclockout pin should read 50Mhz on my scope, Correct?

Thanks Vivek :)

Peter

0 Vivek Singh over 5 years ago in reply to Peter Adamcik

TI__Guru** 115191 points

Hi,

To measure the actual CPU's clock speed I'll set the PLL to 200Mhz and with xclockout divided by 4 the xclockout pin should read 50Mhz on my scope, Correct?

Yes, that is correct.

Regards,

Vivek Singh

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TMS320F28377D: UNSTABLE CLOCK MAYBE?