question about different speeds on a pic18f452

[kam]

Hi all,

I'm working on a 18f452, and it comes with 8 different oscillator modes (lp, xt, hs, hs+pll, rc, rcio, ec and ecio). These range from 455kHz ~ 25mHz. I am currently using XT (4Mhz) because, well, thats what I am using!

Can someone explain to me why I would not always want to use the fastest speed? I'm assuming thats it got to do with heat and power, faster=more power used/needed, low=less power etc...

Also, when using the "#use delay(clock=4000000)", does that have to 100% match with the osc used? I am using a 4mHz osc, so I have the clock set to 4,000,000...

Just trying to get a better understanding of this

Thanks!

~Kam (^8*

[Ttelmah]

More speed, means more power consumed, and more RF noise. Both are significant factors in designs. For a battery operated system, you may well want to run at very slow frequencies (like 32768Hz), which are nice and easy for timing calculations, and keep consumption to a minimum.
The #use delay statement, tells the compiler how fast the chip is running, and nothing else. This is used for calculations of the baud rates in serial communications, and the number of loops to be used for timings (delay_us, delay_ms etc.).
In your case, you could for instance select the hs+pll mode in the fuses, and change the #use delay to 16MHz. The chip would run 4* as fast (the PLL generates an internal clock at four times the external oscillator frequency), external RF from the oscillator would remain the same, since it only runs as 4Mhz, while there would be a slight increase in power consumption, and RF noise from the chip itself.

Best Wishes

[kam]

Thanks for the info! makes sense...

Now...I looked at setting my speed to HS+PLL, and I could not find a fuse to do this, I can find the HS, but not HS+PLL

Here are all the fuses as listed in the h file

[rwyoung]

H4
_________________
Rob Young
The Screw-Up Fairy may just visit you but he has crashed on my couch for the last month!

[kam]

Thanks for the info!

~Kam (^8*