PIC18F takes forever to start

__baltazar__ · Guest

SOS guys!
I have just switched to the PIC18F series devices. For a while I was confused with the PLL & the OSC setting but managed to go through it ( or I'm still missing something ...) Here's my problem. I am using a 20MHz crystal with 15pf Cs and played with PLL & CPU div settings to get a stable MCU clock. But everything seems to be futile. Either it takes about 2 minutes to get the PIC started or when it is running already , it hangs somewhere and slows down.
I'm trying to clock the MCU core at 48Mhz although it is not a requirement for the project.
Q1. What is the recommended most stable MCU clock I should use? (I don't need the USB periheral)
Q2. What is the best clcok setting then - there are more than a dozen?
Q3. Do I have the right understanding that I have 48MHz when PLL div 2 & CPU div 5 is set - that is at 20MHz crystal?

I would appreciate your help. I have not experienced such problem with PIC16. By the way , I am using here the brothers PIC18F2550 & 4550.

Matro · Guest

Q1. There is no clock frequency more stable than another as long as you stay in the range of the specification
Q2. The best clock setting is the most appropriate to your needs...

Please check the fuses and that the 15pF match with the Xtal datasheet.
You can post your fuses configuration for further help.

Matro

baltazar_aquino · Joined: 16 Mar 2008 Posts: 27

Thanks for prompt reply Matro

Here's the test code

drh · Posted: Sat Mar 22, 2008 9:54 am

The oscillator configuration options for this PIC are quite extensive. After a quick look at the data sheet, DS39632D, I think it shows a 16Mhz xtal as the highest for the mode you are using.
_________________
David

Ttelmah · Guest

Add:

#FUSES PUT, NOXINST, USBDIV, NODEBUG, NOLVP, NOWDT

A couple of these are 'optional' at this point, but NOLVP, is essential, unless you have the low voltage programming pins tied to the inactive state.

As has been said, there is really no 'best' frequency, for the master oscillator. Any multiple of 4MHz, up to the maximum supported by the chip. However higher frequencies, mean slightly more power cnsumption in the oscillator, and slightly more RF noise. 4MHz, can be fractionally more 'fussy' to get going, so personally, I tend to recommend 8MHz, or 12MHz.

Best Wishes

baltazar_aquino · Joined: 16 Mar 2008 Posts: 27

I'm still stuck on this. I even tried a lower crystal freq (11.0592mc) under HS mode only (no PLL) but still it won't run. I tested 2 chips already and I have a dozen of this for a target project. My old friend 877a will run the code at 20Mc in a breeze but this one truly disappointing. Crying or Very sad

Ttelmah · Guest

How simple is your test code?.
I'd suggest you try something _really_ simple, like a single LED flasher. My suspicion is that something else is hampering your startup, in the code. The chips do run fine, and start just like any other. There are a lot of other things 'different' about these chips.
Put a scope on the oscillator output. Is it starting?.
Have you got brownout enabled?. What voltage?.
Add NOFCMEN, NOPBADEN, and NOIESO to your fuses.

Best Wishes

baltazar_aquino · Joined: 16 Mar 2008 Posts: 27