For a couple of days I have been learning about SPI and decided to give it a go with 2 small devices.

  • DEVICE 1: Atmega328p (MASTER)

As far as I understand:

  • master sets SS pin low
  • master moves data to SPDR
  • as soon as data gets in to SPDR clock starts transmitting and data shift out
  • at the same time SLAVE shift data out too
  • so by the end of 8 clock pulses SLAVE received 1 byte and master received 1 byte too
  • SS pin goes high again

Scenario #1

I am sending the address byte to MFRC522 from Atmega328p, and expecting a reply. According to the documentation, the first response from SLAVE is "X" I take it as 0x00.

On the next byte sent to the SLAVE, I receive the response from the first byte I sent, and so on until the end.

Since it has some kind of offset would I need to send 0x00 to the slave to read that last byte of information?


1 Answer 1


Table 6 on p.11 of the datasheet is pretty explicit:

Yes, to receive the Nth data byte, you need to transmit 00 in the (N+1)th transfer.

  • \$\begingroup\$ 00 is 0b00000000 ? that what confused me:) thanks man \$\endgroup\$ Commented Oct 25, 2018 at 14:34
  • 3
    \$\begingroup\$ Most often it doesn't actually matter what the master sends while the slave is replying with data words; all that is actually needed are the clock cycles. \$\endgroup\$ Commented Oct 25, 2018 at 16:20

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