
71
ATtiny25/45/85 [DATASHEET]
2586Q–AVR–08/2013
Figure 11-6.
Compare Match Output Unit, Schematic
The general I/O port function is overridden by the Output Compare (OC0x) from the Waveform Generator if either
of the COM0x[1:0] bits are set. However, the OC0x 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 OC0x pin (DDR_OC0x) must be
set as output before the OC0x value is visible on the pin. The port override function is independent of the Wave-
form Generation mode.
The design of the Output Compare pin logic allows initialization of the OC0x state before the output is enabled.
11.6.1
Compare Output Mode and Waveform Generation
The Waveform Generator uses the COM0x[1:0] bits differently in Normal, CTC, and PWM modes. For all modes,
setting the COM0x[1:0] = 0 tells the Waveform Generator that no action on the OC0x Register is to be performed
A change of the COM0x[1:0] 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 FOC0x strobe bits.
11.7
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 (WGM0[2:0]) and Compare Output mode (COM0x[1:0]) bits. The
Compare Output mode bits do not affect the counting sequence, while the Waveform Generation mode bits do.
The COM0x[1:0] bits control whether the PWM output generated should be inverted or not (inverted or non-
inverted PWM). For non-PWM modes the COM0x[1:0] bits control whether the output should be set, cleared, or
PORT
DDR
DQ
OCn
Pin
OCnx
DQ
Waveform
Generator
COMnx1
COMnx0
0
1
D
ATA
B
U
S
FOCn
clk
I/O