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


Posted by: Reginald Beardsley




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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 p...@yahoo.com [msp430] wrote:

Subject: [msp430] Timer A jitter on G2 LaunchPad
To: m...
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




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