File content as of revision 1:5abd129839e7:

#include "mbed.h"

/* 
*  next baby step towards towards a library for MCP23S17
*  I'm proposing to allow 8-bit and 16-bit conditioning, reads, and writes
*  and interrupt conditioning
* 
*  Unless someone asks for them, I'm not going to implement
*  multi-reads or writes where a sequence of many bytes are read from or written
*  to the same register
* 
* This is currently experimental code. I'm still checking that I've understood the chip API.
*/


// all register addresses assume IOVCON.BANK = 0 (POR default)
 
#define IODIRA 0x00
#define IODIRB 0x01
#define IOCON  0x0A
#define GPIOA  0x12
#define GPIOB  0x13
#define OLATA  0x14
#define OLATB  0x15

// Control settings

#define IOCON_BANK  0x80 // Banked registers
#define IOCON_BYTE_MODE 0x20 // Disables sequential operation. If bank = 0, operations toggle between A and B registers
#define IOCON_HAEN  0x08 // Hardware address enable

SPI spi(p5, p6, p7);
DigitalOut ncs(p20);  // not chip select; bring this low to enable the chip
char writeOpcode = 0x40; // A0, A1, A2 are tied to ground on the breadboard.
char readOpcode = writeOpcode | 1; // low order bit = 1 for read

void _write(char address, char data) {
    ncs = 0;
    spi.write(writeOpcode);
    spi.write(address);
    spi.write(data);
    ncs = 1;
}

char _read(char address) {
    ncs = 0;
    spi.write(readOpcode); 
    spi.write(address);
    char result = spi.write(0);
    ncs = 1;
    return result;
}

void init() {
    _write(IOCON, (IOCON_BYTE_MODE | IOCON_HAEN )); // Hardware addressing on, operations toggle between A and B registers
}

void directionA(char direction) {
     _write(IODIRA, direction); 
}

void directionB(char direction) {
     _write(IODIRB, direction); 
}

void outputA(char byte) {
    _write(OLATA, byte); 
}

void outputB(char byte) {
    _write(OLATB, byte); 
}

char inputA() {
    return _read(GPIOA);
}

int main() {
    init();
    directionA(0xFF); // all 8 bits set to input
    directionB(0x00); // all 8 bits set to ouptut
    while(1) {
        wait(0.2);
        // copy inputs from A to outputs on B
        outputB(inputA());
        
    }
}