18F26K80 Serial setting sanity check and 4xPLL.

[CuriousOne]

I had issues with 20mhz external oscillator as well - needed to fine tune capacitors for it to run.
Don't remember exactly, but had to replace recommended 22pf with 10pf or something like that.

[Ioannis]

may be you have too much stray capacitance on your pcb or breadboard.

Ioannis

[HenrikOlsson]

Crystals, crystal oscillators and ceramic resonators are three different things.

• Crystals needs load capacitors (whose value CAN sometimes be picky). The crystal uses the oscillator inside the PIC.
• Crystal oscillators have the crystal, load capacitors and oscillator in one package and drives the CLKIn pin of the PIC at roughly TTL levels. The oscillator IN the PIC is not used.
• Ceramic resonator, like crystal oscillators, does not need external capacitors but unlike crystal oscillator, they do use the oscillator inside the PIC.

The PLL is a block that is tacked on after the oscillator or CLKIn signal.

Apparently Peter is using a resonator and if that resonator works when the PIC is configured to NOT use the PLL, ie it's configured to run and does run at 20MHz then I'd say the external circuitry is OK. As we know, the PIC is specified to run at up to 64MHz so expecting it to run at 80MHz might be a bit of a stretch. The fact that it works on one device doesn't mean it works on all devices. Then again, I would expect it to either not run at all or start up and randomly crash - not run at the base frequency.

A) Get it to work at whatever frequency resonator you put at it's input.
B) Get the PLL to work while using an inout frequency that does nor overclock the PIC.
C) Now that you know the PLL is indeed working, try the overclocking thing.

/Henrik.

[tumbleweed]

the PIC is specified to run at up to 64MHz so expecting it to run at 80MHz might be a bit of a stretch.

Pretty much what I raised back in post 2. My part ran at 80 using an external clock+PLL, but that doesn't mean much.

Ceramic resonator, like crystal oscillators

You mention that crystal oscillators have logic-level outputs, so I think in your comparison a resonator is closer to a plain crystal (but as you mention they usually include the caps).