PIC18F 48MHz clock or better

[AcHmed99]

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[scrat]

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.

[johnmx]

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.

[Rufus]

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.

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.

[johnmx]

The PLL can be used in any of the following conditions:
•  When the primary clock is HSPLL
•  When the primary clock is HFINTOSC and the selected frequency is 16 MHz
•  When the primary clock is HFINTOSC and the selected frequency is 8 MHz

So just use a 16MHz crystal (HSPLL mode) if you don’t want to use the internal RC.

Alternatively, you can use an external oscillator running at 64MHz (EC mode), but this option is more expensive.

[scrat]

As from the datasheet http://ww1.microchip.com/downloads/en/DeviceDoc/41303G.pdf at page 27 (figure 2-1) and 385 (table 26-6), and as stated above by rufus, running at 64MHz can be achieved by:

External clock at 16MHz, PLL 4x (EC/ECIO + PLL, not specified in the table, but schematic says it's possible)
External clock at 64MHz, no PLL (EC/ECIO)
Quartz at 16MHz, PLL 4x (HSPLL)
Internal oscillator at 16MHz, PLL 4x (HFINTOSC + PLL)

[ziq8tsi]

External clock at 16MHz, PLL 4x (EC/ECIO + PLL, not specified in the table, but schematic says it's possible)

There is definitely no ECPLL mode.  All sixteen bit patterns for FOSC<3:0> in CONFIG1H are defined, and none of them enable EC input and PLL simultaneously.

You can use an external 16MHz crystal with PLL, or an external 64MHz clock without.

[scrat]

External clock at 16MHz, PLL 4x (EC/ECIO + PLL, not specified in the table, but schematic says it's possible)

There is definitely no ECPLL mode.  All sixteen bit patterns for FOSC<3:0> in CONFIG1H are defined, and none of them enable EC input and PLL simultaneously.

You can use an external 16MHz crystal with PLL, or an external 64MHz clock without.

Yes, you're right! Edited above to correct this. Now it's like johnmx stated.

An alternative is PIC18F2550/4550, which let you use the PLL (which is much more flexible) with external clock generators at many multiples of 4MHz (but result in a fixed 48MHz clock)

[Leo Bodnar]

The internal clock has a precision of +/-1 %. I dont recall the drift over time and temperature.

If you want to have extremely flexible PLL settings have a look at dsPIC33 series or some PIC24.  They have fully adjustable division and multiplication PLL coefficients.

[metalphreak]

An alternative is PIC18F2550/4550, which let you use the PLL (which is much more flexible) with external clock generators at many multiples of 4MHz (but result in a fixed 48MHz clock)

The fixed 48Mhz clock is entirely due to the USB module.

[scrat]

An alternative is PIC18F2550/4550, which let you use the PLL (which is much more flexible) with external clock generators at many multiples of 4MHz (but result in a fixed 48MHz clock)

The fixed 48Mhz clock is entirely due to the USB module.

Yes, but that clock is also usable as the CPU and CCPWM peripheral one.