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Altera_Forum
Honored Contributor I
1,010 Views

DE0_Nano: bad PLL jitter

I implemented Terasic's "My First FPGA" tutorial, and the 5Mhz PLL output is erratic. 

 

The project uses an ALTPLL to divide the onboard 50MHz oscillator down to 5MHz. 

I routed the clock to a GPIO and monitor with my logic analyzer. 

But the GPIO output wavers between 4 and 6 MHz (clock is high/low for: 83ns, 83ns, 1167ns, 83ns, 83ns, 1167ns, ...) 

 

I used the Megafunction Wizard to add the ALTPLL. My input is 50 MHz, output is 5 MHz, ratio 1/10, duty cycle 50%. All options are disabled (reset on loss of lock, etc). I'm routing to GPIO_10 / PIN_F13, if that matters. 

 

Am I missing something? I'm pretty confident in the Logic analyzer; 5MHz is well within its bandwidth. 

 

The jitter doesn't matter for LEDs, but now I'm trying to clock serial data into an IC on a breadboard, and it's become annoying. 

 

Thanks, 

Sean
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Altera_Forum
Honored Contributor I
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Shouldn't happen. I would monitor PLL locked signal to get additional information. 

 

Self-reset at least won't hurt. I'm not absolutely sure how the PLL works without a global reset.
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