Sunday, December 28, 2014

Hex/Binary Pebble Watchface

My girlfriend got me the Pebble Steel smart watch for Christmas. The watch runs on an ARM Cortex M3 with a modified FreeRTOS kernel (which is described more as a threading library than a traditional operating system).

When I get a device like this I always have to poke around the API and see what it has to offer. So I looked at the getting started guide and wrote an app that simply displays the time and date in hexadecimal and binary forms.

The API is in C, and you only have access to a subset of the standard library. For instance, instead of doing itoa() with a base of two to display the numbers as binary, I had to create the following monstrosity:

const char *bytebin(int n, int m, char *b, int l)
    for (b[0] = '\0'; m > 0 && l > 0; m >>= 1, --l)
        strcat(b, ((n & m) == m) ? "1" : "0");
    return b;

And this is how I use it:

    snprintf(binDateBuffer, sizeof(binDateBuffer), "%s/%s/%s", 
           bytebin(tick_time->tm_mon + 1, 0x8, binMon, 4), 
           bytebin(tick_time->tm_mday, 0x10, binDay, 5), 
           bytebin(tick_time->tm_year + 1900, 0x400, binYr, 11));

The development IDE is hosted in the cloud. You literally just type in some C code and press play. The web application then compiles it and uploads it to your phone which sends it to the watch via Bluetooth. This lets you jump right into developing without any setup.

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