STM32 Jitter PLL

[Curby]

Hi there,

I'm using a STM32F413VGT6, 8MHz crystal and creating a 72/96MHz clock with it (PLL). It works, but I have a significant jitter on it. Using the MCO_2 to take a look at it with the oscilloscope I can see the attached picture.

What can I do to reduce the jitter? Creating a 400kHz frequency with a timer gives me 400kHz +- 5kHz - not useful! Where does it come from?

Thanks!

[fourtytwo42]

If the clock is wandering around like that the most likely cause is the PLL is not locked and is hunting. Carefully check the spec and ensure the PLL is operating within it's lock range and also most devices have a PLL lock flag somewhere you can check.

[Twoflower]

That's probably not Jitter but an aliasing effect of your scope. With digital scopes you might look out for some effects like that.

[Curby]

Ok, here another screenshot. Creating a 420kHz frequency with a hardware-timer (output compare) jitters like this. It drifts from about 418-422kHz.
I don't think this is good enough and practically i need a better signal.

I'm using 8MHz-> /4 -> *72 -> / 2 = 72MHz PLL.

[ataradov]

Again, this looks like a scope artifact. Find another way to check the frequency.

Expand time base so you have one period on the screen and set infinite persistence mode and see if you can see significant variations in the period.

Scope frequency counter is an indication tool, not an actual measurement tool.

[Yansi]

And please don't forget that STM32 chips support spread spectrum (frequency modulation) for EMI suppression