SPI slave response delayed by 1 transfer

[Joseph-DiGiovanni]

First off thank you very much for your work getting the teensy to act as an SPI slave. I'm having a strange issue though and was hoping you would recognize it and have a simple solution.

I have a master Teensy LC sending a few transfers and a Teensy 3.2 slave just sending back a matching response. I believe I understand SPI thoroughly and know responses are expected on the next transfer, but I'm actually getting the responses after 2 transfers. My slave code just responds with the received data and my master code is using the standard SPI library that works with everything else so I'm at a loss.

All I'm doing on the master is:

Serial.Println(SPI.transfer(0xFF), HEX);
Serial.Println(SPI.transfer(0x01), HEX);
Serial.Println(SPI.transfer(0x02), HEX);
Serial.Println(SPI.transfer(0x03), HEX);
Serial.Println(SPI.transfer(0x04), HEX);

but the returned result is:

0x03  // I know the first result will be junk left over from the last sequence, so this is ignored
0x04  // This is where I would expect the 0xFF back from the slave
0xFF  // response arrives 1 byte late as well as subsequent bytes
0x01
0x02

My slave code doesn't differ much from the example, the onReceive function looks like this:

void myFunc()
{
  while (mySPI.active())
  {
    if (mySPI.available())
    {
      uint16_t rx = mySPI.popr();
      mySPI.pushr(rx);
    }
  }
}

From my understanding this code should just reflect any received data back as a response on the next transfer.

The SPImode argument is set to 8 on my slave, setting it to 16 actually makes the response delayed by another 2 bytes so that's not the issue. Thanks in advance for any help you can offer

[tonton81]

it is the way it is designed, I too had this issue and was unable to find a solution for it, probably because the initial FIFO frame includes a command for the clock setup, not sure. You'll have to just push it out with another dummy transfer

[Joseph-DiGiovanni]

That's unfortunate. Might want to put that info in the readme or something