IRC as clock source for USBCLKSEL?

[lpcware]

Content originally posted in LPCWare by 1234567890 on Wed Jan 07 11:36:13 MST 2015
IRC has a fixed value of 12 MHz, but is the default input for USBCLKSEL, which is followed by a clock devider (and no PLL).

All other inputs of USBCLKSEL (system oscillator, USB PLL and main clock) can deliver frequencies >= 48 MHz which can be devided to the required 48 MHz for the USB peripheral.

So when is it useful to deliver 12 MHz (or less) to the USB peripheral?

[lpcware]

Content originally posted in LPCWare by xianghuiwang on Thu Mar 26 15:43:32 MST 2015
In general, running from 12MHz IRC is not helping with full speed USB in active mode.
In suspend mode, we expect USB device to consume much less power hence the chip needs to be set to power down mode. Before entering power down mode, the part should switch the system clock to IRC.
The IRC accuracy and frequency range on LPC1500 does support low speed USB operation. NXP did not certify LPC1500 for low speed USB operation, but users can reference the LPCOpen Low speed USB example of LPC11U6x to implement xtal-less low speed USB application if needed.

[lpcware]

Content originally posted in LPCWare by 1234567890 on Tue Feb 24 13:22:48 MST 2015
But with 12 MHz the peripheral is useful for what?
And a disabled clock uses no power.
I'm not that familiar with USB, so I don't understand it. Please explain it to me.

[lpcware]

Content originally posted in LPCWare by embd02161991 on Thu Jan 08 20:06:17 MST 2015
Hi,

It will be useful to select IRC as clock source when you enter a USB suspend mode to reduce your power consumption even further.

Thanks,
NXP Technical Support