Getting per-second heart rate on a watch face

Hey, I'm trying to make a watch face that displays your heart rate each second, my onPartialUpdate is:

function onPartialUpdate(dc) {
    // The width changes based on the numbers, for now lets just use a constant.
    var w = 50;
    var h = heartRateLabel.height;
    var y = heartRateLabel.locY;
    // The text is aligned to the right, so subtract the width to get x.
    var x = heartRateLabel.locX - w;

    dc.setClip(x, y, w, h);

    dc.setColor(Graphics.COLOR_BLACK, Graphics.COLOR_GREEN);
    dc.fillRectangle(x, y, w, h);



function getHeartRate() {
    var hrIterator = ActivityMonitor.getHeartRateHistory(1, true);
    var previous =;

    if (previous == null || previous.heartRate == ActivityMonitor.INVALID_HR_SAMPLE) {
        return "--";
    return previous.heartRate.format("%02d");

The problem is that it only updates once a minute.

I've tried replacing with some dummy data and I know onPartialUpdate is called once per second. Also WatchFaceDelegate#onPowerBudgetExceeded is not called.

I've seen plenty of faces on the store that do it. I'm running this on a Forerunner 645 Music.

Also, is there any way to get the simulator to provide a changing heartrate instead of the constant 80?