User guide

141
ATmega162/V
2513E–AVR–09/03
resulting in incorrect Waveform Generation. Similarly, do not write the TCNT2 value
equal to BOTTOM when the counter is down-counting.
The Setup of the OC2 should be performed before setting the Data Direction Register
for the port pin to output. The easiest way of setting the OC2 value is to use the Force
Output Compare (FOC2) strobe bit in Normal mode. The OC2 Register keeps its value
even when changing between Waveform Generation modes.
Be aware that the COM21:0 bits are not double buffered together with the compare
value. Changing the COM21:0 bits will take effect immediately.
Compare Match Output
Unit
The Compare Output mode (COM21:0) bits have two functions. The waveform genera-
tor uses the COM21:0 bits for defining the Output Compare (OC2) state at the next
Compare Match. Also, the COM21:0 bits control the OC2 pin output source. Figure 62
shows a simplified schematic of the logic affected by the COM21:0 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 COM21:0
bits are shown. When referring to the OC2 state, the reference is for the internal OC2
Register, not the OC2 pin.
Figure 62. Compare Match Output Unit, Schematic
The general I/O port function is overridden by the Output Compare (OC2) from the
waveform generator if either of the COM21:0 bits are set. However, the OC2 pin direc-
tion (input or output) is still controlled by the Data Direction Register (DDR) for the port
pin. The Data Direction Register bit for the OC2 pin (DDR_OC2) must be set as output
before the OC2 value is visible on the pin. The port override function is independent of
the Waveform Generation mode.
PORT
DDR
DQ
DQ
OCn
Pin
OCn
DQ
Waveform
Generator
COMn1
COMn0
0
1
DATA BUS
FOCn
clk
I/O