General Clock Question, how is it calculated?

Jakezilla3 · Joined: 23 Feb 2009 Posts: 5

Hi all, I am using a 18F4455, and have a question about the clock. I am using a USB bootloader and attached are a few of my includes.

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

I have a question. If you want the PIC's CPU clock to run at 24 MHz,
why are setting the #use delay() statement to 48 MHz ? The #use delay
statement is what tells the compiler how to build the library code for
delay_us() and delay_ms(), as well as setting up the registers for
various peripherals. The #use delay() should always match the
actual CPU clock speed.

Jakezilla3 · Joined: 23 Feb 2009 Posts: 5

You need to set it to 48MHz for USB communication, or so I thought. And I also thought that it takes that number and divides it by 4. So if you have a 20MHz external clock, and all the fuses set correctly, you put 20,000,000 in the delay statement, one clock tick is 1/(20MHz/4) = .2us

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

I took your fuse settings and made a test program. I don't have your PIC
but I have a related one. The LED blinks at 1 Hz. So the PIC is running
at 48 MHz with your settings.

Jakezilla3 · Joined: 23 Feb 2009 Posts: 5

I assume you have a 20MHz external clock?

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Jakezilla3 · Joined: 23 Feb 2009 Posts: 5

I am sorry if I am not explaining my problem/question very well.

From the help file

Ttelmah · Guest

First thing.
The _#USE DELAY_ statement, is for the CPU, not the USB. The USB clock is separate from this.

Start with your external clock. 20MHz. Selecting HSPLL, says 'feed the CPU, from the clock coming from the USB PLL'. PLL5, says 'divide incoming clock by 5, to feed the USB PLL'. So you have 4MHz feeding this. The PLL performs a multiplication by 24, to give the USB master clock at 96MHz. Now, all the Microchip data sheet diagrams in a sense, slightly 'go wrong' at this point. This is divided by 2, to feed the USB system (as shown), but the feed to the CPU divider, also really comes off this 48MHz /2 output. CPUDIV1, then gives a CPU clock at 48MHz. Now this is 'Fosc' for the CPU. If you want to run the CPU at 24MHz, simply select CPUDIV2, and change the delay statement to match this. If you want to run at 20MHz, select 'HS', instead of 'HSPLL', and CPUDIV1, with a 20MHz DELAY statement. You can happily run the CPU, at almost any frequency you want (actually 'handling' the USB timings can become borderline, down at about 4MHz), even if you want using the RC clock. All that matters for USB, is that the external crystal, divided by the PLLx statement, gives 4Mhz. Smile

The 'external clock', on the oscillator pins, is not the internal clock, with HSPLL selected. If you want to use the crystal frequency to run the CPU, use HS. The USB will still run OK.

Then if you now look at the timer diagram (Fig 12.1), note that it's clock, is either a separate external input, or "Fosc/4 internal clock". This is the 'extra' division by 4. In fact almost all the internal peripherals use this clock (commonly called the 'instruction' clock, since it corresponds to one instruction time).

Best Wishes

Jakezilla3 · Joined: 23 Feb 2009 Posts: 5

Thank you so much for that excellent description! I knew that everything works but I just have a hard time accepting things happening behind the scenes unless I know how they are working.

So just a quick recap
external clock (OSC1) = 20MHz
internal clock (FOSC) = 48MHz
instruction clock (FOSC/4) = 12MHz