McBSP in SPI mode, and ADS8320

Started by Raymond Ranwez May 29, 2001
Hi everybody,
I am trying to connect an ADS8320 16-bit A/D converter to the 5410 McBSP1 through the SPI protocol.
I followed the SLAA118 Application Report, dated January 2001, except for the clock speed (now at 2.4 MHz instead of 1.2 MHz).
My problem is that I cannot do more than 75,000 conversions a second, and I expected at least 85,000 samples per second.
The timing is the following:
- The AD8320 CS\ is low during 10.2 usec,
- The interrupt (due to receive Ready) is entered 500 nsec later. Here, I restart a conversion by writing 0 into the DXR1 register (from inside the interrupt routine).
- The interrupt is exit 500 nsec after it was fired.
- The next CS\ signal goes low (only) 1.8 usec later. And this seems to be the problem. Why this delay?
(I expected less than 500 nsec)
(It seems also that the delay is inversely proportional to the clock speed, so, it seems related to the McBSP itself).
Also, I don't have the timing diagram shown in that application report: I don't have any clock signal when the CS\ is high (but this seems normal to me, but why those extra clock pulse during CS\ high in the application report?)
Has somebody in the group experienced speed problems with the SPI on a McBSP channel?
Thanks for your help
Raymond Ranwez
Hardware Project Engineer
Atech - Respironics Belgium