Question - USB with Internal RC Oscillator

Discussion created by lpcware Employee on Jun 15, 2016
Latest reply on Jun 15, 2016 by lpcware
Content originally posted in LPCWare by Clovis on Tue Jul 24 08:26:40 MST 2012
Hi everyone. I'm introducing myself in the world of CortexM3 and I would really appreciate your help in this matter.

I've been succesfully testing some example codes for USB HID with an LPC1769 on a LPCXpresso Embedded Artist's Base board that I've found on the Internet.

I would like to modify this examples to operate the USB HID using the internal RC oscillator in order to simplify a future PCB Design.

I've been reading LPC17xx's User Manual, Clocking and Power Control Section.
The internal RC Oscillator runs at a 12MHz frequency, and the system PLL is capable of taking it as input and multipliyng this frequency four times, so I could generate a 48MHz main clock. This clock would allow USB HID to work fine.

HOWEVER, I've seen SEVERAL times in the user manual that it's strongly suggested (almost as if it were an obbligation) to use the system oscillator (external) and not the internal RC oscillator when using the PLL0 output for USB clocking.

Is it impossible to use the IRC?
Has anyone faced this issue before?

Thanks in advance!