SPI. Full Duplex. How??

[fopetesl]

Still a Noob
I understand the principle that a SPI device can supposedly read and transmit data concurrently.

Problem for me is how it CAN be concurrent when I am asking for,say, a one byte result from a status register.
I think I understand that I must send two bytes, the second being a 'dummy'? to receive the desired result?
Problem in my mind is what data do I place in 'dummy' byte without prompting more response from the SPI device.

Similar conundrum if I send a one byte string and expect a 10 byte response. I send 11 bytes 10 of which are 'dummies'.

I read, sorry re-read, the SPI spec(s) and also for the MMA7455 without it being made clear.

I missed something obvious so I should revert to being the Village Idiot?

[joan]

With SPI the master sends a bit then receives a bit. Repeat that 8 times and a byte is transferred in both directions (full duplex). Repeat that for each byte.

You need to read the device datasheets to see how the bits are interpreted on a device by device basis.

You are right to suggest that a lot of bits will be irrelevant as they may contain no useful information. For instance the master might send an 8 bit command to request 100 bytes. All the bytes sent after the 8 bit command from the master to the slave would be dummies. The real data would be in the 100 bytes simultaneously sent from the slave to the master.

[fopetesl]

Thanks, Joan.
I sort of got most of that already.

It's the part about

With SPI the master sends a bit then receives a bit.

Until the 'slave' has received the request byte, eight bits if you like, it can't respond, (unless it can see the future ), so the 'master' needs to send a dummy byte to see the answer.

I'll take your advice and look at the data sheet again.

I'll try and come back, it might help someone else.