ATmega162 datasheet(143/324 Pages) ATMEL

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143

2513K–AVR–07/09

ATmega162/V

Compare Output Mode

and Waveform

Generation

The Waveform Generator uses the COM21:0 bits differently in Normal, CTC, and PWM modes.

For all modes, setting the COM21:0 = 0 tells the Waveform Generator that no action on the OC2

PWM modes refer to Table 61 on page 150. For fast PWM mode, refer to Table 62 on page 150,

and for phase correct PWM refer to Table 63 on page 150.

A change of the COM21:0 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

FOC2 strobe bits.

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 (WGM21:0) and Compare Output

mode (COM21:0) bits. The Compare Output mode bits do not affect the counting sequence,

while the Waveform Generation mode bits do. The COM21:0 bits control whether the PWM out-

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

the COM21:0 bits control whether the output should be set, cleared, or toggled at a Compare

Match (See “Compare Match Output Unit” on page 142.).

For detailed timing information refer to “Timer/Counter Timing Diagrams” on page 147.

Normal Mode

The simplest mode of operation is the Normal mode (WGM21:0 = 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.

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