USB 1.x

General discussion relating to the library modules supplied with the compiler

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NigelMills
Posts: 22
Joined: Mon Jan 13, 2014 2:33 pm

USB 1.x

Post by NigelMills » Tue Apr 15, 2014 3:42 pm

Hi guys,

Courtesy of my colleague Danny Scott, I have a functioning USB 2 build running on my hardware, but I've had to up the clock frequency from 8MHz to 48MHz. I'm only using it as a replacement for a UART for serial comms, and I'd be happy using USB 1.1 if I could lower the processor clock back down to the original 8MHz again.

Can anyone point me at any documentation that describes using Jerry Messina's USB library at lower clock speeds for USB 1.1 ?

Best regards,

Nigel

Jerry Messina
Swordfish Developer
Posts: 1469
Joined: Fri Jan 30, 2009 6:27 pm
Location: US

Re: USB 1.x

Post by Jerry Messina » Tue Apr 15, 2014 4:49 pm

Nigel,

On most of the chips you need to get 48MHz (or some multiple) into the USB engine. You can usually run the cpu at a different frequency if you like, but how you do that depends on the device. There's usually a different clock tree for the uC. One thing... you need to respond to enumeration requests at a pretty quick pace (~100us), otherwise you'll have problems.

You need to be a USB full-speed device to be spec compliant with the CDC class, so while you can sometimes get low-speed to work it's not a good idea.

NigelMills
Posts: 22
Joined: Mon Jan 13, 2014 2:33 pm

Re: USB 1.x

Post by NigelMills » Tue Apr 15, 2014 6:49 pm

Jerry,

Many thanks, I will investigate.

Nigel

Jerry Messina
Swordfish Developer
Posts: 1469
Joined: Fri Jan 30, 2009 6:27 pm
Location: US

Re: USB 1.x

Post by Jerry Messina » Tue Apr 15, 2014 7:06 pm

I meant to add... I think Danny was using the 67J94, if so look at the CPDIV[1:0] bits in OSCCON4.

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