Hi All, I have a problem using an NI USB-8451 to read data from a 2-byte shift register with an active high chip select input, using SPI. When I examine the waveforms with a scope I can see the Chip select go high exactly as expected, followed some 800 Us later by two burst of 8 clocks. The MISO lines show the expected data during these bursts. Perfect so far and the data returned is correct. But I'm puzzled by two extra burst of 8 clocks, which occur another 900us or so later, just before the chip select is returned low. (The MISO line remains high this time, presumably because 1's have been shifted in from the open-circuit serial-in pin at the front of the shift register.)
The only activity in the program during that 900us interval is a call to the NI 845x SPI Extract Script Read. This returns the data previously read from the shift register. Should this be producing SPI clocks. If so why? If not where might these extra two bytes worth of clocks be coming from? They don't deem to be corrupting my data in this instance but I would like to understand what's going on for the future, as we have lots of SPI-based boards to write test programs for.
The vi doing the SPI interaction is attached.
Thank you
George
