PIC internal oscillators

[Peepsalot]

Just a general question about the built in oscillators in most Microchip PICs. All the ones I have seen I think are 4mhz, when the max clock of the chip is usually 20mhz. Why are they never set for maximum speed? Is it just too hard to build that into a single chip or what?

Also, just wanted to clarify, since one instruction cycle happens every 4 clock pulses, I'm pretty sure. So the internal oscillator would make instructions at 1mhz basically, or is the internal oscillator rating saying that the instruction cycle would be 4mhz? (I know, branching instructions take two cycles.)

 

[justDIY]

as I understand it, the interal osc is an RC based clock, so it's affected by everything, including ambient temp and supply voltage

it's probably too difficult to make one stable at 10-20 or more mhz?

several of the 'newer' PICs have an 8mhz internal osc, so perhaps they are working on getting the speed higher.

I also wonder about how much space it must take, that all PICs don't include at least the basic 4mhz clock - a lot of times a project doesn't require an ultra stable clock, and it reduces complexity slightly by not having to use a xtal and caps or a resonator

 

[Jay.slovak]

several of the 'newer' PICs have an 8mhz internal osc, so perhaps they are working on getting the speed higher.

I don't think they will design faster Internal Oscilators than that. Newer 18F PICs have 8Mhz oscilator that can feed onchip 4xPLL, that will give you internal 32Mhz clock. I think this is fast enaugh (8MIPS without crystal).

 

[Nigel Goodwin]

As suggested, the 4MHz internal oscillator is only RC, I wouldn't like to try and design a 20MHz RC oscillator - although more modern PIC's have 8MHz ones.

If you want 20MHz, add a crystal!.