ATmega16 datasheet(94/315 Pages) ATMEL

94 / 315 page

ATmega16(L)

2466E–AVR–10/02

Figure 44. Compare Match Output Unit, Schematic

The general I/O port function is overridden by the Output Compare (OC1x) from the

Waveform Generator if either of the COM1x1:0 bits are set. However, the OC1x pin

direction (input or output) is still controlled by the Data Direction Register (DDR) for the

port pin. The Data Direction Register bit for the OC1x pin (DDR_OC1x) must be set as

output before the OC1x value is visible on the pin. The port override function is generally

independent of the Waveform Generation mode, but there are some exceptions. Refer

to Table 44, Table 45 and Table 46 for details.

The design of the output compare pin logic allows initialization of the OC1x state before

the output is enabled. Note that some COM1x1:0 bit settings are reserved for certain

modes of operation. See “16-bit Timer/Counter Register Description” on page 104.

The COM1x1:0 bits have no effect on the input capture unit.

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 44 on page 104. For fast

PWM mode refer to Table 45 on page 105, and for phase correct and phase and fre-

quency correct PWM refer to Table 46 on page 105.

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 93.)

For detailed timing information refer to “Timer/Counter Timing Diagrams” on page 102.

PORT

DDR

OCnx

Waveform

Generator

COMnx1

COMnx0

FOCnx

clk

I/O

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