Sending command through SPI on Arduino UNO

Question

I am assuming the default pin configuration is pin 10(SS), 11(MOSI), 12(MISO), 13(CLK) from a default setup, SPI.begin(). The code compiles OK. Not sure about the int valuex and using the SPI.transfer() function.

#include <SPI.h>

int value1 = 0x8026;
int value2 = 0x0801;
int value3 = 0x8027;
int value4 = 0x0000;

void setup (void)
{
  Serial.begin (9600);
  Serial.println ("Starting");
  digitalWrite(SS, HIGH);  // ensure SS stays high for now

  // Put SCK, MOSI, SS pins into output mode
  // also put SCK, MOSI into LOW state, and SS into HIGH state.
  // Then put SPI hardware into Master mode and turn SPI on
  SPI.begin ();

  // Slow down the master a bit
  SPI.setClockDivider(SPI_CLOCK_DIV4);

}  // end of setup

void loop (void)
{
  // enable Slave Select
  digitalWrite(SS, LOW); 

  SPI.transfer (1);   // initiate transmission
  SPI.transfer (value1);
  SPI.transfer (value2);
  SPI.transfer (value3);
  SPI.transfer (value4);

  // disable Slave Select
  digitalWrite(SS, HIGH);

delay (1000); 
}

transfer sends a byte only. See SPI.transfer16(intval); if necessary — DataFiddler
– DataFiddler, Commented Mar 17, 2019 at 16:59

Majenko · Accepted Answer · 2019-03-17 17:20:42Z

Not sure about the int valuex and using the SPI.transfer() function.

A couple of things to know:

You are storing an unsigned 16-bit literal into a signed 16-bit variable. You should be using uint16_t value1 = 0x8026; etc. Also, unless you are planning to change the values, they should be const.
SPI.transfer() transfers only 8 bits of data. You are only sending the lowest byte of your valuex variables. SPI.transfer16() sends an unsigned 16-bit value.

To illustrate, this is how your program should look:

#include <SPI.h>

const uint16_t value1 = 0x8026;
const uint16_t value2 = 0x0801;
const uint16_t value3 = 0x8027;
const uint16_t value4 = 0x0000;

void setup(void) {
    Serial.begin(9600);
    Serial.println("Starting");
    digitalWrite(SS, HIGH);  // ensure SS stays high for now
    // Put SCK, MOSI, SS pins into output mode
    // also put SCK, MOSI into LOW state, and SS into HIGH state.
    // Then put SPI hardware into Master mode and turn SPI on
    SPI.begin();
    // Slow down the master a bit
    SPI.setClockDivider(SPI_CLOCK_DIV4);
}  // end of setup

void loop(void) {
    // enable Slave Select
    digitalWrite(SS, LOW);
    SPI.transfer(1);    // initiate transmission
    SPI.transfer16(value1);
    SPI.transfer16(value2);
    SPI.transfer16(value3);
    SPI.transfer16(value4);
    // disable Slave Select
    digitalWrite(SS, HIGH);
    Delay(1000);
}

That will transfer the stream of 8-bit values:

0x01 0x80 0x26 0x08 0x01 0x80 0x27 0x00 0x00

Hi, and thank you. I believe the code is right but I do not get any signaling out of MOSI pin 11; it does go from high to low. I can 'see' (with an oscilloscope) SS pin 10 go low, I can see only 3 clock pulses on pin 13. I am not concerned with any incoming data. — howdyrichard
– howdyrichard, Commented Mar 20, 2019 at 14:57
Also, when I added 16 to SPI.transfer, transfer changed from blue to black color. — howdyrichard
– howdyrichard, Commented Mar 20, 2019 at 16:38
The code works perfect! Thank you and I wish I had sent this sooner. Thanks again! — howdyrichard
– howdyrichard, Commented Apr 16, 2019 at 17:37

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