Sending multiple byte data from Arduino as Slave on SPI

Question

I have been working around the teardown of an induction cooktop and thought of having a tweak in the system. So far, I have deduced following circuitry out of the cooktop- There are two devices communicating with each other in 3-wire SPI mode. The master, which controls power management and Induction circuitry, continuously sends commands and data to the slave. The slave, which is TM1628, receives the master commands and data, then accordingly displays the data on the seven segment or responds with the switch pressed, if any.

The above picture explains it better. Now, I thought of replacing TM1628 with an Arduino. So far so good. I gathered all the data that was being sent by the master and the reply sent by the slave.

But, when I started to code, I got stuck in one place. So the data as sent by the master and its response looks like this -

(Master to Slave)**Command1 - Command2 - Data1 - Data2 - ... - Data 14 - Command1 - Command2 - **(Slave to master)**Data1 - Data2 - ... - Data5 - **(And now the repetition)

When the master is sending the data, I have no issues as the data will continuously be read by Arduino and I can take the action accordingly. But when the Arduino (Working as a slave) needs to send continuously 5 streams of data, depending only on the clock input timing from the master, how could I know that one stream has been written and now the next data has to be written to the SPDR register.

Answer 1

With SPI there is no such thing as "sending" and "receiving". It is a full duplex operation every time. Whenever the master runs the clock the slave both receives bits from the master and simultaneously sends bits to the master.

These operations are inseparable.

Whatever mechanism you are using to recognise "A byte has arrived from the master" is the same mechanism that you use for filling the SPDR register. The bytes where you "send" data to the master are actually bytes where the master is sending you data, but you don't care about what that data is. Just like when the master is sending you "commands" you are sending the master data (which you haven't specified) and the master "doesn't care" what that data is.

It's up to you to keep track of how many bytes the master has clocked in and out, and to say "For bytes 19-23 the master is expecting valid data, so I need to make sure that I put data into the SPDR for those bytes".