Re: Timer A jitter on G2 LaunchPad

To answer my own question.  I removed the pins from the header which were connected to the xtal leads.  This cut the jitter to ~ 60 ns which is much more in line w/ expectation.  There are still little antennae connected to the xtal, so I'd expect that a better board layout would reduce this further.  Removing the 0 ohm resistors did not reduce the jitter any further, but turning off the  lamp on my magnifier cut the jitter to ~ 20 ns.

Reg

--------------------------------------------
On Sat, 7/12/14, Reginald Beardsley [email protected] [msp430] <[email protected]> wrote:

 Subject: [msp430] Timer A jitter on G2 LaunchPad
 To: [email protected]
 Date: Saturday, July 12, 2014, 6:48 PM
 
 
  
 
 
 
   
 
 
     
       
       
       What sort of jitter should I see using ACLK on
 timer A w/ the 32 KHz xtal in place?  I'm seeing ~1.5 us
 on the falling edge of a nominally 30.5 us pulse.
 
 
 
 I'm generating a pulse train using timer A in up mode
 driving LED2.  I expect to see jitter in the pulse length
 using SMCLK derived from the DCO at the 8 MHz calibrated
 frequency, but I also see it if I change to ACLK for timer A
 (still DCO for SMCLK). 
 
 
 
 I get the expected pulse length using ACLK, but more jitter
 than I would like.   I've got the header pins sticking
 out the underside and the board plugged in to a breadboard,
 so conceivably I'm loading the xtal and/or picking up
 stray noise.  It clearly stops running if I load it w/ my
 finger.  Probing the xtal lines w/ a scope stops the
 oscillator which is not surprising considering the power
 level.
 
 
 
 Selecting ACLK on a LaunchPad w/o the xtal produces a very
 erratic display though nominally at the right frequency.  I
 assume that's because it's getting ACLK from VLO.
 
 
 
 Comments?  Am I doing something obviously wrong or is it
 just that my expectations are excessive?
 
 
 
 Thanks,
 
 Reg
 
 
 
 
 
     
      
 
     
     
 
 
 
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Posted by: Reginald Beardsley <[email protected]>
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