Fusebit of PIC18F4550 with USB

defoxe · Joined: 30 Jan 2015 Posts: 8

Hello,
I am not sure why working configuration of fusebit for PIC18F4550 with USB and quartz 20MHz is:

temtronic · Posted: Tue May 09, 2017 4:02 pm

You'll need to look at the 4550 'header' file that's the first line of your program to see how 'CPUDIV1' is defined.
It's been years since I used the 4550 but CPUDIV1 probably means 1st divider config for CPU, which is /2 , giving 48MHz clock.
I know I'd print out the page that had the 'clock flow' diagram and highlight the signal path I wanted.....as some pathways were NOT allowed if you wanted USB operation and had a 'wrong' xtal value.

Jay

Ttelmah · Joined: 11 Mar 2010 Posts: 19215

This is one of those oddities caused by the CCS naming, and the chip itself.

If you connect the clock directly (not via the PLL), the four clock divisions available are /1 /2 /3 & /4. CCS called these CPUDIV1, CPUDIV2, CPUDIV3 & CPUDIV4.
However the same dividers, when used from the PLL, give /2, /3, /4 & /6.
So the divider called CPUDIV3, gives /4 from the PLL!... Similarly all the other dividers give different ratios.

I pointed this out to CCS, and said that it really would be better to call these by their 'binary' titles, or by names that made this clear. So (something like) CPUDIV00 CPUDIV01 (so the binary values), or CPUDIV1_2 CPUDIV2_3, giving the two divisions. Unfortunately since the existing vales had been published for some time, they felt it better to keep them.

All of the listed values are supported for USB. The ones highlighted are those supported for low speed USB. This requires a 24MHz clock.

defoxe · Joined: 30 Jan 2015 Posts: 8

Thank you for answers, but I must have spend a little bit more time on this.

Ttelmah · Joined: 11 Mar 2010 Posts: 19215

The key point is to read the text under the table you show: