Electronics > Microcontrollers

PIC18F 48MHz clock or better

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PIC18F with USB peripheral like PIC18F2550 accept up to 48 MHz external clock, they divide up to /12 and then obtain a 48MHz clock from the internal PLL.

I now see on the datasheet that PIC18F26K20 works with external clock (EC mode) at 64MHz (see page 374).

However, why would you need such a high clock? Doesn't the PLL maintain the clock precision of a lower frequency clock?

Jon Chandler:
The 18F25K20 can use an external 16 MHz and the 4x PLL multiplier to operate at 64 MHz.

Running the PIC18F26K20 at 64MHz you can use its Capture module to measure time with a resolution of 62.5ns. The 1% precision of the internal clock is completely despicable if you are going to measure a very low period.


--- Quote from: AcHmed99 on April 27, 2011, 09:10:57 pm ---I've never used an 18F before just 10F's through to 16F's. My understanding from the data sheet is that the PLL can only be used with the internal RC oscillator.

--- End quote ---

It sounds like you have barely looked at the datasheet. A block diagram of the oscillator section clearly shows the x4 PLL can be driven from the external oscillator. The ten different oscillator modes are documented and The AC timing characteristics section defines external clock and PLL timing requirements.

In EC mode you can supply a clock up to 64MHz (48MHz for extended temperature range parts). HS mode supports crystals up to 25MHz or 16MHz if the PLL is used. A 16MHz crystal is the simplest way to get a 64MHz clock and 62.5ns resolution for the timers.


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