SPI Receive Clocking on E128

[EagleBeak]

From looking at legacy code, it appears that to generate the SPI clock for the master to receive data from the slave, you must place a byte in the data register, which makes sense because the data coming from the slave comes in on a different pin.

The SPI guide under CPHA = 0 Transfer Format says:

"Data that was previously in the master SPI Data Register should now be in the slave data register and the data that was in the slave data register should be in the master."

So for an SPI slave device that takes a two byte command and returns two bytes,  one would:

1. Transfer command byte 1

2. Transfer command byte 2

3. Write a dummy value into the data register to clock in the response from the slave

4. Retrieve the value in the data register, which is the byte from the slave.

5. Repeat steps 3. & 4.

Is this correct?

Thanks,

Eagle Beak

[Alban]

Eagle Beak,

I agree fully as long as you cable all the wires.

In single wire mode (connecting MISO/MOSI together with pull-up) the "dummy" data must be 0xFF not to interfere with the incoming data on the wired-or bus.

Alban.

[EagleBeak]

Alban,

Makes sense about the dummy data being 0xFF. The legacy code I was using just placed whatever happened to be in the A register into the SPIDR but we do have both wires hooked up. I think it would be good to send 0xFF anyway to prevent any possible problems from the slave device and it makes measuring it on the scope less confusing.

Thanks!

Eagle Beak

[bigmac]

Hello Eagle Beak,

Yes, that would seem basically correct.  What is the SPI slave device you are using?

Regards,

Mac

[EagleBeak]

Mac,

The slave device is an X9111, a programmable resistor used as a gain pot on an op-amp circuit.

Eagle Beak

[bigmac]

Hello Eagle Beak,

The correct setup for the X9111 would appear to be CPHA = 0 and CPOL = 0.  You could also use the single data line (MIMO)mode, as Alban suggested, if to your advantage.

Regards,

Mac