### Circuit

http://www.uchobby.com/index.php/2008/01/08/arduino-audio-dac-options/
    
    47kohm resistor off pin, then 0.01uF cap to ground and speaker in parallel,
    optionally with a pot or pair of resistors (sum=100khom?) acting as a
    voltage divider for the headphones.

### MCU configuration

Set interrupt timer for 8KHz.

Set PWM period to 255, prescaler to 1; this gives a period of 3.54
microseconds, or about 282KHz. Further filtering may not even be needed?

### Example Code

#define PWM_OUT_PIN     BOARD_LED_PIN

HardwareTimer pwm(1);
HardwareTimer gen(1);

uint8_t[64] sine_lookup;
int counter = 0;

/*
from math import sin
count = 64
print [int(127+127*sin(3.14159268*2*i/count)) for i in range(count)]
*/
unsigned char sine_lookup[] __FLASH__ = {127, 139, 151, 163, 175, 186, 197, 207, 216,
    225, 232, 239, 244, 248, 251, 253, 254, 253, 251, 248, 244, 239, 232, 225,
    216, 207, 197, 186, 175, 163, 151, 139, 126, 114, 102, 90, 78, 67, 56, 46,
    37, 28, 21, 14, 9, 5, 2, 0, 0, 0, 2, 5, 9, 14, 21, 28, 37, 46, 56, 67, 78,
    90, 102, 114};

void setup() {
    // configure PWM output
    pinMode(PWM_OUT_PIN, OUTPUT);
    pwm.setChannel1Mode(TIMER_PWM);
    pwm.setPrescaleFactor(1);
    pwm.setOverflow(255);       // 8-bit resolution
    pwm.setCompare(1, 128);        // initialize to "zero"

    // configure 8KHz ticker and interrupt handler
    gen.pause();
    gen.setPeriod(125);         // 8Khz
    gen.setChannel1Mode(TIMER_OUTPUT_COMPARE);
    gen.refesh()


    // get things started!
    gen.resume()
}

void loop() {
    // USB serial interaction code goes here...
}

void handler_sample(void) {
    pwm.setCompare(1, sin_8bit(counter, 800));  // 100Hz sine wave
    // pwm.setCompare(1, execute(counter);
    counter++;
}

uint8_t sin_8bit(int counter, int period) {
    int high, low;
    float t = (t % period) / (float)period;
    float weight = t - (t % 1);
    low = sine_lookup[63*t];
    if (63*t >= )
        high = sine_lookup[0];
    else
        high = sine_lookup[1+63*t];

    return (int)(low * weight + high * (1.0 - weight)):
}