What is the speed of the PIC18F4520?

Hi to all,

I have been studying the datasheet for the PIC18F4520 and am a bit puzzled about its clockable speed.

The sheet seems to be implying that I could use say an 8mHz Xtal and use the PICs PLL to get 32mHz, well thats how it reads to me anyway.

I was always led to believe that the maximum clock speed of a device was that of the Xtal used, so am I reading the sheet wrong?

Al

bigal_scorpio said:

The sheet seems to be implying that I could use say an 8mHz Xtal and use the PICs PLL to get 32mHz, well thats how it reads to me anyway.

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There's a 4x PLL multiplier - so for an XTAL freq (or internal oscillator freq) of 8MHz, the PLL can bump it up to 32MHz. There are four of these cycles to an instruction cycles, so the actual instruction rate is back to 8 million instructions per second.

The sheet says the max freq is 40MHz (10MIPS).

By the way, can I just use a high frequency crystal (40MHz) directly, instead of using a 10Mhz crystal with the PLL? If I can, what is the correct capacitor value to use? The datasheet only shows the capacitor value for crystal up to 20 MHz.

mdanh2002 said:

By the way, can I just use a high frequency crystal (40MHz) directly, instead of using a 10Mhz crystal with the PLL? If I can, what is the correct capacitor value to use? The datasheet only shows the capacitor value for crystal up to 20 MHz.

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You're better off using a 10MHz crystal with 4 x PLL enabled.

You'll get the same clockspeed and more than likely use less power.