Frequency problem with the PIC18FXX2

[Electronics4you]

Hi All,

Searching in the datasheet (PIC18FXX2) for workable crystals I found this peace of text: "A Phase Locked Loop circuit is provided as a programmable option for users that want to multiply the frequency
of the incoming crystal oscillator signal by 4. For an input clock frequency of 10 MHz, the internal clock frequency will be multiplied to 40 MHz. This is
useful for customers who are concerned with EMI due to high frequency crystals." Does it mean, that I can connect a 10 MHz crystal to the PIC and then get a 4 times higher work frequency? Seem a little strange to me, because normally it's 1/4 the frecuency. Any comments to that?

What's tha maximum fecuency it can work with?

 

[Nigel Goodwin]

Hi All,

Searching in the datasheet (PIC18FXX2) for workable crystals I found this peace of text: "A Phase Locked Loop circuit is provided as a programmable option for users that want to multiply the frequency
of the incoming crystal oscillator signal by 4. For an input clock frequency of 10 MHz, the internal clock frequency will be multiplied to 40 MHz. This is
useful for customers who are concerned with EMI due to high frequency crystals." Does it mean, that I can connect a 10 MHz crystal to the PIC and then get a 4 times higher work frequency? Seem a little strange to me, because normally it's 1/4 the frecuency. Any comments to that?

The quote above quite clearly explains it, the 18Fxx2 contains an optional PLL multiplier, this allows you to use an external crystal 1/4 of the required clock frequency. So a 10MHz external crystal can give 40MHx internally - and, as suggested above, because the external crystal is only 10MHz the EMI levels will be lower.

An extra advantage is in the availablity of crystals, 40MHz ones are most commonly overtone ones, and wouldn't run at 40MHz - a 40MHz fundamental crystal would be more expensive and fragile.

What's tha maximum fecuency it can work with?

MicroChip specify 40MHz internally, so a 10MHz external crystal, assuming you're using the multiplier?.