Datasheet

175

ATmega640/1280/1281/2560/2561

2549K–AVR–01/07

W hen the modulator is enabled the type of modulation (logical AND or OR) can be

selected by the PORTB7 Register. Note that the DDRB7 controls the direction of the

port independent of the COMnx1:0 bit setting.

Timing Example Figure 65 illustrates the modulator in action. In this example the Timer/Counter1 is set to

operate in fast PWM mode (non-inverted) and Timer/Counter0 uses CTC waveform

mode with toggle Compare Output mode (COMnx1:0 = 1).

Figure 65. Output Compare Modulator, Timing Diagram

In this example, Timer/Counter2 provides the carrier, while the modulating signal is gen-

erated by the Output Compare unit C of the Timer/Counter1.

The resolution of the PWM signal (OC1C) is reduced by the modulation. The reduction

factor is equal to the number of system clock cycles of one period of the carrier (OC0A).

In this example the resolution is reduced by a factor of two. The reason for the reduction

is illustrated in Figure 65 at the second and third period of the PB7 output when

PORTB7 equals zero. The period 2 high time is one cycle longer than the period 3 high

time, but the result on the PB7 output is equal in both periods.

1 2

OC0A

(CTC Mode)

OC1C

(FPWM Mode)

PB7

(PORTB7 = 0)

PB7

(PORTB7 = 1)

(Period)

clk

I/O