Extension board LEDs wiring fix

fix.cpp

#include "mbed.h"

// mosi, miso (unused really), sclk
SPI sw(p5, p6, p7); 

// lat (latch) pin of TLC59281 as shown in extension board schematic
DigitalOut lat(p8);

/* The extension board wiring from TLC59281 to 3 LEDs pairs are not consistent with
   remaining 5 LEDs pairs (green is wired as red, red is wired as green). 
   This function fixes that problem. This way, all LEDs will turn red
   when we send the following to TLC59281:
   0b0101010101010101  
   (where each "01" is a 2-bit value controlling the state of green/red LED pair)
   
   It switches position of bits in a number as shown in: https://stackoverflow.com/a/47990/4620679   */
unsigned short fix_extension_board_leds(unsigned short leds_state_bits) {
    for (int i = 10; i < 16; i += 2) { 
        char bit1 = (leds_state_bits >> i)   & 1;
        char bit2 = (leds_state_bits >> i+1) & 1;
        leds_state_bits ^= (-bit2 ^ leds_state_bits) & (1 << i);
        leds_state_bits ^= (-bit1 ^ leds_state_bits) & (1 << i+1);
    }
    return leds_state_bits;
}

int main() {
    lat = 0;
    sw.format(16,0);
    sw.frequency(1000000);
    while(1) {        
        sw.write( fix_extension_board_leds(0b0101010101010101) ); // 0x5555
        lat = 1;
        lat = 0;
        wait(1);
        sw.write( fix_extension_board_leds(0b0000000000000000) ); // 0x0000
        lat = 1;
        lat = 0;
        wait(1);
    }
}