Print 2 numbers stored in 24-bits in decimal format

Question

An implementation function from the nice little Wiegand library YetAnotherArduinoWiegandLibrary prints a hexadecimal representation of a facility # & card # (8-bit facility #, 16-bit card #) — 24 bits of a 26-bit stream that comes across the wire.

Example encoded 24-bit number: 7111097 (decimal). Hex equivalent: 6C81B9. The 6C portion is the facility # (decimal: 108). 81B9 is card # (decimal 33209).

This function from the library's example usage serial prints the hexadecimal format of the number:

void receivedData(uint8_t* data, uint8_t bits, const char* message) {
    Serial.print(message);
    Serial.print(bits);
    Serial.print("bits / ");
    //Print value in HEX
    uint8_t bytes = (bits+7)/8;
    for (int i=0; i<bytes; i++) {
        Serial.print(data[i] >> 4, 16);  //1st, 3rd, 5th hex digit
        Serial.print(data[i] & 0xF, 16); //2nd, 4th, 6th hex digit
    }
    Serial.println();
}

What I've tried to do unsuccessfully so far is to loop over the bits to also serial print the two decimal formatted numbers (facility:card) in the final output form: 108:33209

Duncan C · Accepted Answer · 2020-03-04T01:44:36.403

Your code loops from the high order byte to the low order byte, printing each byte as 2 hex digits.

If you want instead to print the first byte as a decimal value, and then the remaining bytes as another decimal value, then do this:

Print byte 0 in base 10:

    Serial.print(data[0]);  //first byte
    Serial.print(":");

Then loop through the remaining bytes, building a result as a decimal value, and print that:

unsigned long total = 0;
for (int i=1; i<bytes; i++) //Skip the first byte
{
    total = total << 8;  //Shift the previous total by 8 bits. 
                         //(The same as multiplying by 256, but faster)
    total += data[i];    //Add in the new value
}
Serial.println(total);

The sample code you posted is written to handle a variable number of bytes. The code above does the same thing with decimal results. It's much simpler if you know you will always have 3 bytes or 4 bytes:

For 3 bytes:

    Serial.print(data[0]);  //first byte
    Serial.print(":");
    long total = long(data[1])*256 + data[2]);
    Serial.println(long(total);

Print 2 numbers stored in 24-bits in decimal format

1 Answers1