PIC18F2550 & PIC18F2550 crystal settings

[ratgod]

Hi there,
I had a good search through the net and through datasheets and I have not found one simple explaination on how to set the crystal settings on the 2/4550.

Here is my attempt to try and puts some information together and hopefully with help we can create a table that everyone can understand.

[Ttelmah]

In a sense, you are starting at the wrong 'end' with this. Start with a key question. Do you want to use USB?.
If the answer here is 'no', then you can progress to the normal fuse settings, using XT, HS, EC, or ECIO.
If however the answer is'yes', then a second question arises. Fast, or standard mode?. If standard mode, then you are basically stuck with a 24MHz clock.

For Fast mode, you now get the big requirement, that the USB 'section', needs to receive a clock at exactly 4MHz. There is an internal prescaler (selected by PLLx fuses, with 'x' as 1,2,3,4,5,6,10 or 12), so your external clock is limited to 4,8,12,16,20,24 or 48MHz.
Now you must use the external clock for USB (internal oscillator is not accurate enough), so you have the choices for oscillator types at this point, of:

[ratgod]

Thanks for your reply, it does make things a little cleared.

Am I understanding correctly; Fast USB needs 96Mhz internally so you need to provide 4Mhz by PLL and/or Division, while Standard USB requires 24Mhz internally? this is why you can have a 24Mhz master clock directly using the 'NOUSBDIV' fuse?

When considering oscillator types; I got these from the CCS wizard.

[Ttelmah]

You can get oscillators with just three legs...
Generally though four pins is more normal.
Key I'd say, is whether the module needs power. This implies it has an internal amplifier, and is then almost certainly an oscillator.
The PIC itself provides the required amplification stage, with all the modes containing 'HS', or 'XT', and can then be used with a resonator/crystal that does not oscillate "on it's own".
Generally, avoid resonators for USB. Unfortunately, they are used in a hell of a lot of cheaper USB kit (mainly because they of the price), which then gives problems when the temperature range goes outside normal 'office' tolerances. Typically resonators have tolerances about 500* 'worse' than standard crystals, and will rarely meet the USB specification, over more than about a 20 degree range, 'at best'...

Best Wishes

[ratgod]

I finally got the Serial-USB demo going last night on my breadboard, I had to fiddle with the crystal but its finally working, many thanks for the help with crystal settings.

Do you know if the 470nF cap from the pic's Vusb pin to ground is critical? my demo seems to work with it in or out.

[Ttelmah]

Depends on the chip, and how the system is powered (USB or local supply). On some chips a larger capacitor seems essential toevenget them to work...

Best Wishes

[aaaaamartin]

look here

http://www.sprut.de/electronic/pic/8bit/18f/programm/usbbootloader/usbboot.htm

for a graphic and explanation (in german) on how to setup the xtal.

Best wishes Martin