18F452 Question: what frequency to run at?

[Robot builder 101]

I was wondering what crystal/frequency I could run a 452 at. I was going to run it at 20MHz, but Im not sure and want to check. Also, I think I have to use 10pf caps, right? THanks.

 

[Sceadwian]

According to microchips refrence, the max chip speed is 40mhz. (100ns instruction cycle)
What frequency to run it at is completly up to your syncing and instruction cycle rate requirements. Unless you have VERY specific output clock rates or power requirements it's best to run a chip at it's rated frequency.

 

[ericgibbs]

I was wondering what crystal/frequency I could run a 452 at. I was going to run it at 20MHz, but Im not sure and want to check. Also, I think I have to use 10pf caps, right? THanks.

hi,
Also look at the 'phase locked loop' [PLL] information in the 18F452 datasheet.
A 10MHz xtal can/will give a 40MHz clock.

Eric

 

[picbits]

Or use the 8mhz internal oscillator and free up a couple of pins and components

 

[eng1]

Also, I think I have to use 10pf caps, right?

Suggested values for HS crystals are 15-33 pF, see table 2-2 in the datasheet.

 

[justDIY]

Or use the 8mhz internal oscillator and free up a couple of pins and components

the '452 despite it's generous ram, flash and numerous peripherals lacks an internal OSC.

the 'replacement' for the '452, the new 18F4525 offers all the same features, plus more ram, more flash and can run at 32MHZ off the internal 8mhz OSC.

@Robot Builder 101:

one thing to consider, the faster you run your PIC, the more energy it takes. There is a considerable difference in current consumption between 1mhz, 4mhz and above. Not that you have worry a lot about an extra 100mA going to the processor when you're talking about motors that consume 1000's of mA, but it is something to keep in mind if you're going to build a tiny or solar powered bot.

 

[Mike - K8LH]

I bought a bunch of 20-MHz crystals when starting out a few years ago but I've come to realize that 4-MHz, 8-MHz, 16-MHz or 32-MHz are better frequencies when dealing with the Timer prescalers when using the CCP modules.

The 18F4520 (or others) may be a better choice than the '452. It features an INTOSC module and many other improvements over the '452.

Good luck. Mike

 

[Robot builder 101]

Thanks! I talked to a few friends of mine, and they say they run their 452s at 10mHz. I think I am going to run it at 20mhz with 20 pf caps. This should work, as there is no precise timing method required. This is for my mini-sumo robot, which can be expected to be complete soon.

 

[Robot builder 101]

I checked the spec sheet, and it says it works. Thanks.