You mean - when you look at the waveform on your scope? This is not how you measure clock jitter unless it is gigantic and needs no measurements :) I meant somethingl ike this.
- I don't have a scope with that capability
How counter behaves when you connect USB? It just steadily display sloweadily display slower or faster clock? If it's not steady - how erratic it is?
- The frequency measurement updates a couple of times per second. When USB is not connected, the total difference between readings over a period of a minute or so is on the order of 0.1ppm. It's easy to see temperature effects if I heat or cool the board. With USB connected, the difference from one reading to the next is as much as 200ppm. This happens instantly when I connect USB. Some readings are higher, some are lower. There is no detectable pattern.
- I tried counting clock pulses over a 1 second period. Without USB connected, I get repeatability within 0.1ppm. With USB connected, the number of pulses counted jumps up and down randomly each second just like it does in the frequency measurement mode.
- The phenomenon is not a gradual drift. It is an instantaneous increase by 3 orders of magnitude in the instability of the frequency measurement. Because I see the identical problem with a demo board, I can be confident it is not caused by switching power supplies on my board. Oscillator power is regulated within the MSP430.
- Again, I'm sure other people are using USB and depending on the stability of the 32KHz clock. Unfortunately none of them seem to be reading this thread. .