With output compare can change state of a pin on Port T (set, clear or toggle) at a particular time (when TCNT equals a desired value).

1.

In the main program:

(a)

Turn on timer subsystem (1 -> Bit 7 of TSCR reg)

(b)

Set prescaler (TMSK2 reg)

(c)

Set up PTx as OC (1 -> bit x of TIOS reg)

(d)

Set action on compare (TCTL 1-2 regs, OMx OLx bits)

OMx	OLx
0	0	Disabled
0	1	Toggle
1	0	Clear
1	1	Set

(e)

Clear Flag (1 -> bit x of TFLG1 reg; 0 -> all other bits)

(f)

Enable OC interrupt (1 -> bit x of TMSK1 reg)

(g)

Enable maskable interrupts (cli or enable();)

2.

In interrupt service routine

(a)

Set time for next action to occur (write TCx reg)

• For periodic events add time to TCx register

          TC2 = TC2 + delta_T;
  

(b)

Clear Flag (1 -> bit x of TFLG1 reg; 0 -> all other bits)

To generate square wave at a particular frequency (e.g., 1 kHz square wave on OC2):

1.

Determine the period (period = 1/frequency) (for 1 kHz, period = 1 ms)

2.

Determine the half-period (period/2) (for 1 kHz, period = 0.5 ms)

3.

Determine number of T-clock cycles to make a half-period - you may have to adjust timer prescaler to make sure number of cycles less than 0xffff (for 1 kHz and prescaler of 0, number of cycles = 0.5ms/0.125 s per cycle = 4000 cycles)

4.

Set of OCx pin to toggle on successful compare.

5.

In ISR, add half period to TCx (for 1 kHz square wave on OC2 and prescaler of 0,

    TC2 = TC2 + 4000;

Can generate a pulse-width-modulated signal using a single OC channel. However it is much easier to generate a PWM signal using the PWM subsystem, or using an OC channel together with OC7.