ATmega8(L)
Compare Match Output
Unit
The Compare Output mode (COM1x1:0) bits have two functions. The waveform genera-
tor uses the COM1x1:0 bits for defining the Output Compare (OC1x) state at the next
Compare Match. Secondly the COM1x1:0 bits control the OC1x pin output source. Fig-
ure 36 shows a simplified schematic of the logic affected by the COM1x1:0 bit setting.
The I/O Registers, I/O bits, and I/O pins in the figure are shown in bold. Only the parts of
the general I/O Port Control Registers (DDR and PORT) that are affected by the
COM1x1:0 bits are shown. When referring to the OC1x state, the reference is for the
internal OC1x Register, not the OC1x pin. If a System Reset occur, the OC1x Register is
reset to “0”.
Figure 36. Compare Match Output Unit, Schematic
COMnx1
Waveform
Generator
COMnx0
FOCnx
D
Q
1
0
OCnx
Pin
OCnx
D
Q
PORT
D
Q
DDR
clkI/O
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 36, Table 37 and Table 38 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
95.
The COM1x1:0 bits have no effect on the Input Capture unit.
85
2486M–AVR–12/03
ATMEL [ ATMEL ]
