FUSES for PIC18F4550 to run at 32kHz , 1 MHz or 4 MHz ?

[tcruise7771]

Hello i am running PWM with PIC18F4550 with 20MHz crystal at 48MHz speed .

[Ttelmah]

The minimum clock you can run USB at is typically 8MHz.
So do you want USB?.
If so, you cannot clock the CPU this slow. If not, why use the 4550?.....

You can use the CPUDIV divider to drop the speed a lot.
So without changing anything else:
CPUDIV1 - clock 48MHz
CPUDIV2 - clock 32Mhz
CPUDIV3 - clock 24MHz
CPUDIV4 - clock 16MHz

Then if you use the HS fuse, instead of HSPLL
CPUDIV1 - clock 20MHz
CPUDIV2 - clock 10MHz
CPUDIV3 - clock 6.66Mhz
CPUDIV4 - clock 5Mhz

Then if you select the INTRC fuse, instead of HS/HSPLL, the _CPU_ will switch to using the INTRC oscillator block. You still need the crystal attached to run USB, and the PLL5 fuse to set this up.
You can then use CLOCK=8MHz, or CLOCK=4MHz to select these frequencies from the INTRC source _but_, I know USB runs OK at 8MHz, however at 4MHz, there were timing issued. Going below here, and there is no point in using the 4550.

Now, seriously, if the PWM needs to be this slow, just look at doing a software PWM using a timer interrupt. The whole 'point' of the hardware unit, is to give _high speeds_ beyond what simple software approaches can manage....

With care, 6.66MHz would possibly work, and this would allow the PWM, to go down to about 407Hz.

Best Wishes

[tcruise7771]

Thank you Ttelmah i saw other post of you explaining the OSC options for 4550 , and using thanks to you i got the pic running at 5MHz and slowed down the PWC cycle to ~305Hz. I am using those fuses

[Ttelmah]

Yes, just select INTEC, and set the clock to 31000. The compiler will program the internal oscillator to give this.
Remember there are two 32kish rates. 31250, and 31000. The former comes from the HFintosc, the second from the LF. The former is about +/-10% accurate, the latter about +/-16%.

Best Wishes

[tcruise7771]

Thanks Ttelmah , if i select INTEC what other fuses i would need to select to tell to use the first +/-10% , HF ? Does that 10% is constant during the PIC running time, or it changes like every second ?

[Ttelmah]

Just the clock rate.
Select 31000, or 31250.

You can always verify what the compiler does, by simply loading the file into MPLAB, running it's simulator, and letting the code run to the first instruction in the main. Then look at what the oscillator bits are set to.

You can use 'setup_oscillator' to physically ensure the right mode is selected after boot. You also have an option with this, to 'tweak' the timing, but obviously this then needs to be tuned for each chip.
So:

setup_oscillator(OSC_INTRC|OSC_31250,23);

sets the 'osctune' to 23.

Best Wishes