ROM-Based USB HID changing sys clock to 48MHz?

Discussion created by lpcware Employee on Jun 15, 2016
Latest reply on Jun 15, 2016 by lpcware
Content originally posted in LPCWare by ktownsend on Mon Feb 15 14:39:11 MST 2010
I have been trying to debug a number of issues related to timing on the LPC1343 (specifically with SPI and UART), and the problem seems to occur after USB HID is initialised (following the examples from NXP). I'm running the system clock at 72MHz, but as soon as I init the rom-based USB I have the impression that the system clock is being dropped down to 48MHz, throwing off all the other peripherals that were configured for 72MHz. (I suspected this when my UART, set at 57600, worked fine before USB init but I had to drop it to 38400 after the init.  If I set the clock to 48MHz, though, instead of 72 and everything works fine).

Since I can't see any of the rom-based code, and the only oscillascope I have on hand is limited to 25MHz, I can't confirm if the system clock is indeed being set to 48MHz after USB is initialised ... perhaps someone from NXP can offer some insight into whether my suspicion is true or not? I definately wouldn't have expected that behaviour, though.  Are you oblidged to run the main clock at 48MHz if you want to use USB?

Original Attachment has been moved to: 1100119_LPC1100-FFT-Example.zip