ATmega165 datasheet(103/336 Pages) ATMEL

103 / 336 page

103

ATmega165/V

2573G–AVR–07/09

Compare Output Mode and

Waveform Generation

The Waveform Generator uses the COM1x1:0 bits differently in normal, CTC, and PWM

modes. For all modes, setting the COM1x1:0 = 0 tells the Waveform Generator that no

action on the OC1x Register is to be performed on the next compare match. For com-

pare output actions in the non-PWM modes refer to Table 48 on page 113. For fast

PWM mode refer to Table 49 on page 113, and for phase correct and phase and fre-

quency correct PWM refer to Table 50 on page 114.

A change of the COM1x1:0 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 FOC1x 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 (WGM13:0) and

Compare Output mode (COM1x1:0) bits. The Compare Output mode bits do not affect

the counting sequence, while the Waveform Generation mode bits do. The COM1x1:0

bits control whether the PWM output generated should be inverted or not (inverted or

non-inverted PWM). For non-PWM modes the COM1x1:0 bits control whether the out-

put should be set, cleared or toggle at a compare match (See “Compare Match Output

Unit” on page 102.)

For detailed timing information refer to “Timer/Counter Timing Diagrams” on page 111.

Normal Mode

The simplest mode of operation is the Normal mode (WGM13:0 = 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 16-bit value (MAX = 0xFFFF) and

then restarts from the BOTTOM (0x0000). In normal operation the Timer/Counter Over-

flow Flag (TOV1) will be set in the same timer clock cycle as the TCNT1 becomes zero.

The TOV1 Flag in this case behaves like a 17th bit, except that it is only set, not cleared.

However, combined with the timer overflow interrupt that automatically clears the TOV1

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 Input Capture unit is easy to use in Normal mode. However, observe that the maxi-

mum interval between the external events must not exceed the resolution of the counter.

If the interval between events are too long, the timer overflow interrupt or the prescaler

must be used to extend the resolution for the capture unit.

The Output Compare units can be used to generate interrupts at some given time. Using

the Output Compare to generate waveforms in Normal mode is not recommended,

since this will occupy too much of the CPU time.

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