Port expander (MCP23017) and setWatch

I went ahead and tried the interval route, and seemed to have made it work. Possible downside is the interval eating resource for other stuff, but I think I can get away with it by being smart in clearing the interval when input is safely not needed.

My solution:

var i2c = new I2C();
i2c.setup({
    scl: B2,
    sda: B3
});
var address = 0; //this is the address, set by the address pins.

// for MCP23017
var port = require("MCP23017").connect(i2c, null, address);

//matrix
var colPins = [port.A7, port.A6, port.A5, port.A4];
var rowPins = [port.A3, port.A2, port.A1, port.A0];


//set col to low
for (var i in colPins) {
    colPins[i].mode("output");
    colPins[i].write(0);
}

//set row to 1 via pull up
for (var j in rowPins) {
    rowPins[j].mode("input_pullup");
}

var col = -1;
var row = -1;
var lastButton = null;

function mapToButton(c, r) {
    if (c === 3 && r === 3) return ("B1");
    if (c === 2 && r === 3) return ("B2");
    if (c === 1 && r === 3) return ("B3");
    if (c === 0 && r === 3) return ("B4");
    if (c === 3 && r === 2) return ("B5");
    if (c === 2 && r === 2) return ("B6");
    if (c === 1 && r === 2) return ("B7");
    if (c === 0 && r === 2) return ("B8");
    if (c === 3 && r === 1) return ("B9");
    if (c === 2 && r === 1) return ("B10");
    if (c === 1 && r === 1) return ("B11");
    if (c === 0 && r === 1) return ("B12");
    if (c === 3 && r === 0) return ("B13");
    if (c === 2 && r === 0) return ("B14");
    if (c === 1 && r === 0) return ("B15");
    if (c === 0 && r === 0) return ("B16");
}

function scanCol() {
    //set col to low
    var i, j;
    for (i in colPins) {
        colPins[i].mode("output");
        colPins[i].write(0);
    }

    //set row to 1 via pull up
    for (j in rowPins) {
        rowPins[j].mode("input_pullup");
    }
    for (i in rowPins) {
        if (rowPins[i].read() === 0) {
            col = i;
            break;
        }
    }
}

function scanRow() {
    //set row to low
    var i, j;
    for (i in rowPins) {
        rowPins[i].mode("output");
        rowPins[i].write(0);
    }

    //set row to 1 via pull up
    for (j in colPins) {
        colPins[j].mode("input_pullup");
    }
    for (i in colPins) {
        if (colPins[i].read() === 0) {
            row = i;
            break;
        }
    }
    if (col >= 0 && row >= 0) {
        var btn = mapToButton(col, row);
        if (lastButton !== btn) {
            print(btn);
            lastButton = btn;
        }
    }
}

setInterval(function () {

    col = -1;
    row = -1;
    scanCol();
    if (col >= 0) {
        scanRow();
    } else {
        lastButton = null;
    }
}, 50);

Would still love to hear if it is possible to use setWatch btw!