ATmega328P datasheet(149/448 Pages) ATMEL

149 / 448 page

149

8161D–AVR–10/09

ATmega48PA/88PA/168PA/328P

17.6.1

Compare Output Mode and Waveform Generation

The Waveform Generator uses the COM2x1:0 bits differently in normal, CTC, and PWM modes.

For all modes, setting the COM2x1:0 = 0 tells the Waveform Generator that no action on the

OC2x Register is to be performed on the next compare match. For compare output actions in the

non-PWM modes refer to Table 17-5 on page 159. For fast PWM mode, refer to Table 17-6 on

page 159, and for phase correct PWM refer to Table 17-7 on page 160.

A change of the COM2x1:0 bits state will have effect at the first compare match after the bits are

written. For non-PWM modes, the action can be forced to have immediate effect by using the

FOC2x strobe bits.

17.7

Modes of Operation

The mode of operation, i.e., the behavior of the Timer/Counter and the Output Compare pins, is

defined by the combination of the Waveform Generation mode (WGM22:0) and Compare Output

mode (COM2x1:0) bits. The Compare Output mode bits do not affect the counting sequence,

while the Waveform Generation mode bits do. The COM2x1:0 bits control whether the PWM out-

put generated should be inverted or not (inverted or non-inverted PWM). For non-PWM modes

the COM2x1:0 bits control whether the output should be set, cleared, or toggled at a compare

match (See Section “17.6” on page 148.).

For detailed timing information refer to ”Timer/Counter Timing Diagrams” on page 153.

17.7.1

Normal Mode

The simplest mode of operation is the Normal mode (WGM22:0 = 0). In this mode the counting

direction is always up (incrementing), and no counter clear is performed. The counter simply

overruns when it passes its maximum 8-bit value (TOP = 0xFF) and then restarts from the bot-

tom (0x00). In normal operation the Timer/Counter Overflow Flag (TOV2) will be set in the same

timer clock cycle as the TCNT2 becomes zero. The TOV2 Flag in this case behaves like a ninth

bit, except that it is only set, not cleared. However, combined with the timer overflow interrupt

that automatically clears the TOV2 Flag, the timer resolution can be increased by software.

There are no special cases to consider in the Normal mode, a new counter value can be written

anytime.

The Output Compare unit can be used to generate interrupts at some given time. Using the Out-

put Compare to generate waveforms in Normal mode is not recommended, since this will

occupy too much of the CPU time.

17.7.2

Clear Timer on Compare Match (CTC) Mode

In Clear Timer on Compare or CTC mode (WGM22:0 = 2), the OCR2A Register is used to

manipulate the counter resolution. In CTC mode the counter is cleared to zero when the counter

value (TCNT2) matches the OCR2A. The OCR2A defines the top value for the counter, hence

also its resolution. This mode allows greater control of the compare match output frequency. It

also simplifies the operation of counting external events.

The timing diagram for the CTC mode is shown in Figure 17-5. The counter value (TCNT2)

increases until a compare match occurs between TCNT2 and OCR2A, and then counter

(TCNT2) is cleared.

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