18F4450 Fuses

dbrown · Joined: 24 Jul 2004 Posts: 20

I am trying the us_ex_serial example, however I am having problems with two things. I am using compiler version 3.242. I have tried several different crystals, 4 mhz, 20 mhz, and 48 mhz. I have set the use delay to reflect the applicable crystal frequency. When I connect the pic to the serial port I get garbled characters, an indication to me that the baud rate is not correct. The other problem is that the Win 2K computer never seems to find the pic. I have installed the usb.sys files in windows system folder, and I have also copied the .inf files to the system folder. Is there somewhere that I can find a definition for the fuses.

HS, HSPLL are they two separate purposes? I assume HS is for crystal speed greater than 4 mhz, and HSPLL is part of the High speed phase lock loop.

I do not undestand the purpose of the usbdiv fuse.

If I understand correctly the cpudiv1, pll5 all relate to the chosen crystal.

Does the vregen fuse enable the internal 3.3V regulator.

I have read the data sheet for the '4550, for 4 mhz, why does it list xt,hs,ec,ecio in one cell with clock divisions, and then hspll, ecpll, xtpll, ecpio in another cell with clock divisions again?

Any help would be appreciated.

Thank you

David Brown

dbrown · Joined: 24 Jul 2004 Posts: 20

The pic I am using is a 18F4550, not 18F4450, that was a typo.

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Start with a very simple program, just to get the RS-232 serial port
working. Example:

dbrown · Joined: 24 Jul 2004 Posts: 20

I tried the simple program as listed above, and the serial interface works fine. If I want to use the usb with a 4 mhz crystal then I need to set pll/1 and cpudiv0.

Thanks for your help PCM.

Dave

arys · Joined: 09 Mar 2007 Posts: 31

sorry for interrupt but where can I see the "hello world" sentence?

Ttelmah · Guest

Going back to the first question, a 48MHz crystal, is not legitimate. You can use a 48Mhz external _oscillator_, but not a crystal. The internal oscillator is not specified to run above 25MHz.
Now, the 4MHz crystal, may also give problems. Though 'legitimate', on these PICs, I have seen problems with them. For some reason, the internal USB PLL, can behave 'oddly', with quite a few crystals. The chip works, and is clocking at the right rate, but the USB becomes unreliable.
So, I'd say, use a multiple of 4MHz. The 20MHz crystal (or an 8MHz, 12MHz etc.), would be my choice.
For the 20MHz crystal, you have the choice of running the processor, at 48MHz, 24MHz, 20MHz, 16MHz, 12MHz, 10MHz, 6.66MHz, or 5MHz. The normal fuse settings are:

Ttelmah · Guest

Going back to the first question, a 48MHz crystal, is not legitimate. You can use a 48Mhz external _oscillator_, but not a crystal. The internal oscillator is not specified to run above 25MHz.
Now, the 4MHz crystal, may also give problems. Though 'legitimate', on these PICs, I have seen problems with them. For some reason, the internal USB PLL, can behave 'oddly', with quite a few crystals. The chip works, and is clocking at the right rate, but the USB becomes unreliable.
So, I'd say, use a multiple of 4MHz. The 20MHz crystal (or an 8MHz, 12MHz etc.), would be my choice.
For the 20MHz crystal, you have the choice of running the processor, at 48MHz, 24MHz, 20MHz, 16MHz, 12MHz, 10MHz, 6.66MHz, or 5MHz. The normal fuse settings are: