Datasheet

73
2586N–AVR–04/11
ATtiny25/45/85
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 out-
put) 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 visi-
ble on the pin. The port override function is independent of the Waveform Generation mode.
The design of the Output Compare pin logic allows initialization of the OC0x state before the out-
put is enabled. Note that some COM0x[1:0] bit settings are reserved for certain modes of
operation. See “Register Description” on page 80.
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 on the next Compare Match. For compare output
actions in the non-PWM modes refer to Table 11-2 on page 80. For fast PWM mode, refer to
Table 11-3 on page 81, and for phase correct PWM refer to Table 11-4 on page 81.
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 Out-
put mode (COM0x[1:0]) bits. The Compare Output mode bits do not affect the counting
PORT
DDR
DQ
DQ
OCn
Pin
OCnx
DQ
Waveform
Generator
COMnx1
COMnx0
0
1
DATA BUS
FOCn
clk
I/O