Create byte to send to MCP23017 from boolean tests


I need to send a byte to an MCP23017 to activate a series of relays based upon the results of some boolean tests that compare input conditions from other inputs. How does one efficiently create a byte from these boolean tests

e.g. True, False, False, False, False, False, True, False >> 10000010

Thanks, Jeff

This is a tad clunky, but it’s the first thing that came to mind:

#define BIT8  7
#define BIT7  6
#define BIT6  5
#define BIT5  4
#define BIT4  3
#define BIT3  2  
#define BIT2  1
#define BIT1  0

byte byteToSend;

void setup() {
  int testVal;

  byteToSend = 0;
  PackByte(BIT4, true);       // Send: 00001010 (= 10 DEC)
  PackByte(BIT2, true);


void PackByte(int whichBit, int whichState)
  byteToSend |= whichState << whichBit;

void loop() {

Making use of bitfields is an options

struct BITS
  byte b1: 1;
  byte b2: 1;
  byte b3: 1;
  byte b4: 1;
  byte b5: 1;
  byte b6: 1;
  byte b7: 1;
  byte b8: 1;

void setup() {


void loop() {

    BITS logic;
    byte value;
  } bits;

  bits.logic.b1 = false;
  bits.logic.b2 = true;
  bits.logic.b3 = false;
  bits.logic.b4 = false;
  bits.logic.b5 = false;
  bits.logic.b6 = false;
  bits.logic.b7 = false;
  bits.logic.b8 = true;

  Serial.println(bits.value, HEX);

You can directly set the bits in the struct from your logic. The union allows access to either the bit form or the byte form.


void PackByte(int whichBit, int whichState)

 byteToSend |= whichState << whichBit;

This only works for true. It won’t strike a bit with false.

Because true OR false = true.

To make it work for both true and false, do this:

void PackByte(int whichBit, int whichState)
  if (whichState)
    byteToSend |= 1 << whichBit;
    byteToSend &= ~(1 << whichBit);

Because true AND NOT true is false.

@Jobi-Wan: You are correct.

Thank you all, I will poke around with these and see what happens.