Is it possible to transfer 32 bits in a single write sequence in SPI mode using Spark Core

Firmware
#1

I am trying to transfer a hexadecimal value (32bit) to the slave using SPI.transfer(0x43BD50A3), does this work ?

#2

The SPI.transfer() function is expecting a byte - I imagine the compiler produced a warning about truncating the 32-bit value?

What will happen is that only the least significant byte (0xA3 here) is sent.

To send 4 bytes, have 4 calls to SPI.transfer():

SPI.transfer(0x43);
SPI.transfer(0xBD);
SPI.transfer(0x50);
SPI.transfer(0xA3);

Here’s some background on why sending more than one byte could be confusing:

Since the STM32 is little-endian, it stores the value in memory like this

0xA3, 0x50, 0xBD, 0x43

And writing this out as a single value would write the digits in the reverse order to the way they were original written, which may not be what you were expecting!

#3

@mdma, Thanks for your explanation, should there be a delay between each SPI transfer (to prevent data collision) ? i.e to ensure that the second byte is transferred only after the first byte has been fully transmitted to the slave?

#4

The transfer waits for the data to be received, so you don’t need a delay. in fact it does more than waiting, it fetches the byte sent from the slave also (since SPI is a full-duplex protocol.)

So, no delays needed!

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