Hello
I am going to implement a USB Device using LPC1768 and I need the isochronous transfer type for my application. To this purpose I studied the keil's USBAudio example. It is written in the datasheet of the MCU that the frame interrupt is generated every 1 ms, but this interrupt occurs very much more than one time in 1 ms in USBAudio. Where am I wrong?
The OSC of LPC1768 is unstable - Place the crystal near to the XTAL1/2 pins. OR The frequency of USB clock (PLL) is wrong.
The USB SIE on LPC1768 generates FRAME interrupt timing using a dedicated timer and USB clock. And then, you’ll see it without USB connection. When SOF comes from host, the SIE makes the timer synchronized to the SOF timing by DPLL (Digital PLL). This circuitry is known as SOF recovery, which provides steady SOF interval even when missing SOF occurs by communication error.
In this reason, FRAME timing is affected by the quality and accuracy of USB clock so much
Tsuneo
Thank you Tsuneo.
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