PWM.md
How PWM is used for TTS

We'll analyze the code for speech output on Arduino pin 9, on an ATMega328. This is
hardware pin 15 (known here as OC1A).

A good introduction to PWM can be found 
]]>here]]>. This
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is also useful.

At it's most basic however, a PWM waveform is a square-wave with a varying 
duty-cycle. The duty-cycle is the ratio of time spent high to the total
cycle time. The AVR's built-in timers are used to determine both the
frequency and the duty-cycle.

Sound is initialised in soundOn():

```c++
    TCCR1A = 0;
    ICR1 = PWM_TOP;
    TCCR1B = _BV(WGM13) | _BV(CS10);
    TCNT1 = 0;
    TCCR1A |= _BV(COM1A1);

In this mode, known as "PWM, Phase and Frequency corrected", Timer-1
counts up from 0 to PWM_TOP and then back down to 0 again.


The PWM frequency in this mode is given by the formula:

> freq = Clock / (2 x Prescaler x TOP)

Here Clock = 16e6, Prescaler = 1 (set by CS10 above) and TOP = 1200/2.
This gives a fixed frequency of about 13kHz.

The PWM duty cycle is changed in sound():

```c++
    if (duty != OCR1A) {
        TCNT1 = 0;
        OCR1A = duty;
    }


Here the OCR1A value is used to set the value at which Timer-1's counter
causes the output on OC1A to flip:
- while it's counting up from 0 (BOTTOM) to OCR1A, OC1A is high,
- when it reaches OCR1A it sets OC1A low and counts to TOP (ICR1),
- when it reaches TOP it starts counting down again,
- when it reaches OCR1A again, it sets OC1A high and counts to BOTTOM (0).