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### 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)):
}
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