SPI slave send and receive 1..n bytes - what if less bytes are sent?

[TecDroiD]

Hi,

i want to utilize an unknown number of SPI slaves which will send and receive e.g. a byte of data each. patching them together has to be as easy as plugging an ethernet cable BUT some data is used by multiple slaves.

Let's say I have a maximum of 32 possible slaves but currently, I only use 6 of them, so the master will only need to send 6 bytes. It pulls SS low, sends its bytes and pushes SS high again. Slaves have a send and a receive buffer with 32 bytes each.

So I'd like to know if (and how) it is possible to send/receive into the SPI buffers as long as SS is low. When SS goes high or the buffers are full, it should run a callback function to do my software magic.. Is this possible?

Simple workaround would be to send/receive data byte by byte but this would take much of my calculating power.

[waclawek.jan]

"Buffer" is already a software construct imposed upon you by Cube/HAL, so just simply don't use it.

SPI as such works on frames (single byte, halfword, in newer STM32 any number of bits between 4-16, according to SPI's setting). Read the SPI chapter in Reference Manual.

JW

[TecDroiD]

Well, thank you but that doesn't answer my question.

I want to enable my slaves which can work parallel to each other to receive all data sent BUT they don't know how many bytes there are.. only a maximum is sent. And my buffers are just arrays.. why shouldn't I use them? What else is HAL_SPI_TransmitReceive_DMA made for?

[S.Ma]

Which SPI topology multi-drop or daisy chain ?

[S.Ma]

If you have 32 slaves and 1 NSS, beware of the bus loading (max bitrate) for both mutidrop and daisy chain mode (which is ADC/DAC type).

There is a tricky way to do daisy chain with fixed multiple bytes shift register, if this works for you. Otherwise better use DMA in cyclic mode with TX/RX buffer big enough to cover the biggest message. Use NSS to prepare/process the transited data into buffers.

Remember that in slave mode with DMA, there are some quirks to make it work ok.

[waclawek.jan]

> What else is HAL_SPI_TransmitReceive_DMA made for?

It's made for transmitting and receiving *fixed-length* data.

JW

[AScha.3]

just small update:

[waclawek.jan]

There's no reason to shout.

Yes, indeed, there are STM32 with SPI going up to 32 bits, but most of them don't. AFAIK only 'H7 and 'U5 do.

JW

[Tesla DeLorean]

What you send, ie how much, determines the clocking of the bus, so typically it is a symmetrical transaction, where you either stuff padding data, or ignore some coming back.

[waclawek.jan]

I believe the question was about SPI in slave mode.

JW