AT91: Correct way of using PDC & USART with interrupts? (newbie)

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Hi,

Im trying to use interrupts to send data out of the USART, employing
the PDC. The chip im using is the AT91RM9200. I have configured the
USART in loopback mode, sending and receiving data at the same USART.
Ive configured the USART to interrupt whenever ENDRX is asserted.

I have a doubt regarding the way the ENDTX interrupt is asserted.
According to a timing diagram i got from the AT91CDROM related to the
USART and PDC (AT91RM9200-BasicPDC) ,the ENDRX interrupt is asserted
when the transmit counter reaches zero, and stays high till TNCR (next
counter) is written (i do that in the interrupt handler).

When the control enters the interrupt handler, interrupts will have
been enabled, but ENDTX will still be high before a new value is
written into TNCR..whats stopping more such interrupts from being
asserted? is it the fact that the new interupt wil have the same
priority as this one?

Also, i had interrupts enabled for ENDRX assertion..but i was getting
multiple interrupts, as soon as i switched on the device..the problem
went away only when i followed the foll procedure for setting the usart
and the PDC:

first configure the USARt but dont enable RX\TX
configure the PDC, load the transfer\receive buffer pointers n counts
then enable RX and TX through the usart.
only at the end enable interrupt for ENDRX on the USART

but still, in the us_irq_handler if i do not disable ENDRX interrupts,
im getting multiple interrupts. must i disable the interrupt? or will
just putting in new values for the RX buffer in the PDC stop more
interrupts?


thanx in anticipation
Mayank


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