USB low-speed mode with a PIC18F2550 with a 4MHz crystal

gangi · Joined: 14 Feb 2011 Posts: 2

Hello!!

I'm trying to use a pic18f2550 as a low-speed usb device. In the example codes I only found high-speed examples so it didn't helped me. Looking in the datasheet 2.2.1 section seems I have to put FSEN to 0 and FOSC3:FOSC0 to 1 but i didn't find any fuses with a similar name in the 18f2550.h file. Anyone knows what fuses I have to use to set low-speed mode?

Thanks in advance.

bkamen · Posted: Mon Feb 14, 2011 1:41 pm

Read the Datasheet's chapter titled "Oscillator" -- that should have what you need.

In cases of special oscillator settings, you'll also want to review the .H file for that PIC as well as the setup_oscillator() command.

-Ben
_________________
Dazed and confused? I don't think so. Just "plain lost" will do. :D

Ttelmah · Joined: 11 Mar 2010 Posts: 19515

XTPLL, PLL1, CPUDIV3

Two things confusing here, you have to select CPUDIV3, to get /4 (this is because the same bit pattern gives /3, when not using the PLL), and no fuse for FSEN.

FSEN, is not a fuse, but a software configured bit. Bit 2 of the UCFG register.
In USB.H, look for the line 'USB_USE_FULL_SPEED TRUE', and change this to 'FALSE'.

Obviously NOXINST, VREGEN, if using the internal regulator etc., are also needed.

Best Wishes

gangi · Joined: 14 Feb 2011 Posts: 2

Thanks Ttelmah!!

I didn't find the line 'USB_USE_FULL_SPEED TRUE' in the usb.h file but i found it in the pic18_usb.h file. Probably because my compiler version(4.088) is different than yours.

Thank you!

Ttelmah · Joined: 11 Mar 2010 Posts: 19515

Yes, they changed the USB includes around V4.100. The newer ones have the define I gave.

Best Wishes