Re: Launchpad clock jitter: What should I expect?

Reginald Beardsley [email protected] [msp430] wrote:
> I'm seeing more jitter than I would like in the length of a pulse (e.g. 120 us) generated by timer A in PWM mode  using ACLK from the 32 KHz xtal on a G2553 Launchpad.  An eyeball estimate is a standard deviation of 15 ns.  I've pulled the header pins that connect to the xtal and removed the zero ohm jumpers.  That improved things a lot, but I'd like better if possible. Is there anything else I should consider?
>
> This is plugged into a solderless breadboard with a 10x scope probe connected to a short  wire plugged into the row for P1.6.  LED2 is disconnected.
>
> Is it reasonable to think that I can improve on this with the Launchpad?  I found a Freescale application note showing jitter well under 1 ns using a 32 KHz xtal feeding the PLL to generate 48 MHz USB timing.  This leads me to think it's a noise issue with the Launchpad design or my bench area (several computers running) in close proximity), but I could  be all wet.

Hi Reg,
I had recently worked with the same device using the FET. Same, I wanted 
the chip on a breadboard, not the launch pad. I had a pretty rough time 
with the 32k xtal while debugging. I found that the noise from the TEST 
pin was all over the XOUT, and sometimes even keeping the clock from 
running. I'd bet that without the debugger and using the ACLK out to 
check the clock will be stable. And for anyone that runs into the clock 
not running while debugging, I found that a 56pF on XOUT to ground keeps 
the TEST noise down so it works.

Best, Dan.



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Posted by: Dan Bloomquist <[email protected]>
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