SPI.transfer(buffer, size) does not send data from buffer

Question

I am having trouble getting SPI.transfer(buffer, size) to work as expected.

Here is the code:

// inslude the SPI library:
#include <SPI.h>
// set pin 10 as the slave select for the digital pot:
const int slaveSelectPin = 10;
uint16_t SPI_message = 0xABCD;
void setup() {
 // set the slaveSelectPin as an output:
 pinMode(slaveSelectPin, OUTPUT);
 // initialize SPI:
 SPI.begin();
}
void loop() {
 SPI.beginTransaction(SPISettings(14000000, MSBFIRST, SPI_MODE0));
 SPI.transfer(0xEF);
 SPI.transfer(&SPI_message, 2);
 SPI.endTransaction();
 delayMicroseconds(300);
}

Here is the output of logic analyzer:

enter image description here

As can be seen the call for SPI.Transfer(0xEF) works as expected. However SPI.transfer(&SPI_message,2) sends 0x0000 instead of 0xABCD on the bus.

What am I doing wrong?

• Mikael Patel: SPI.transfer(&SPI_message,2) is destructive. The buffer is updated with the received values.
• domen: I assume this is Arduino? Can you decrease SPI clock - currently you seem to be at the limit of your logic analyser. Does it work if you split the second transfer into two 1B transfers? --- What Mikael said is probably the reason for your issue.
• balboa: @MikaelPatel But still I should be able to see 0xABCD on my MOSI line on the SPI bus? I am not trying here to read data from a variable, but I am observing signals on the line of the SPI.
• domen: Only the first time loop is executed, then SPI_message is overwritten and zeroes are sent. Try reinitialising it to 0xabcd in loop.
• balboa: @domen Exactly! this is what I was overseeing! Now it works.

Answer 1

SPI.transfer(&SPI_message,2) is unfortunately destructive. The buffer is updated with the received values. There is a SPI member function[1] that can be used for 16-bit data; SPI.transfer16(SPI_message).

To correct your sketch either initialize the buffer before each call (as recommended by @domen) or use the 16-bit transfer function.

There are other SPI APIs that included non-destructive transfer function e.g. read/write. Please see [2].

Cheers!

Ref.

1. https://www.arduino.cc/en/Reference/SPITransfer
2. https://github.com/mikaelpatel/Arduino-SPI/blob/master/src/SPI.h#L137