Datasheet

122
7728G–AVR–06/10
ATtiny87/ATtiny167
Figure 12-6. Compare Match Output Logic
12.8.1 Compare Output Function
The general I/O port function is overridden by the Output Compare (OC1A/B) from the Wave-
form Generator if either of the COM1A/B1:0 bits are set and if OCnxi respective bit is set in
TCCR1D register. However, the OC1A/Bi 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 OC1A/Bi
pin (DDR_OC1A/Bi) must be set as output before the OC1A/B 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 12-1, Table 12-2 and Table 12-3 for details.
The design of the Output Compare pin logic allows initialization of the OC1A/B state before the
output is enabled. Note that some COM1A/B1:0 bit settings are reserved for certain modes of
operation. See “16-bit Timer/Counter Register Description” on page 132.
The COM1A/B1:0 bits have no effect on the Input Capture unit.
12.8.2 Compare Output Mode and Waveform Generation
The Waveform Generator uses the COM1A/B1:0 bits differently in normal, CTC, and PWM
modes. For all modes, setting the COM1A/B1:0 = 0 tells the Waveform Generator that no
action on the OC1A/B Register is to be performed on the next compare match. For compare
output actions in the non-PWM modes refer to Table 12-1 on page 132. For fast PWM mode
refer to Table 12-2 on page 133, and for phase correct and phase and frequency correct PWM
refer to Table 12-3 on page 133.
A change of the COM1A/B1:0 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 FOC1A/B strobe bits.
12.9 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 (COM1A/B1:0) bits. The Compare Output mode bits do not affect the counting
sequence, while the Waveform Generation mode bits do. The COM1A/B1:0 bits control
whether the PWM output generated should be inverted or not (inverted or non-inverted PWM).
For non-PWM modes the COM1A/B1:0 bits control whether the output should be set, cleared
PORT
DQ
DQ
OCnxi
Pin
OCnx
DQ
Waveform
Generator
COMnx1
OCnxi
COMnx0
0
1
DATA BUS
FOCnx
clk
I/O
DDR