ATMEga64L ATMEL Corporation, ATMEga64L Datasheet - Page 148

• Manufacturer Part Number: ATMEga64L
• Description: 8-bit AVR Microcontroller with 64K Bytes In-System Programmable Flash
• Manufacturer: ATMEL Corporation

Using the Output Compare
Unit
Compare Match Output
Unit
148
ATmega64(L)
Since writing TCNT2 in any mode of operation will block all Compare Matches for one
timer clock cycle, there are risks involved when changing TCNT2 when using the Output
Compare channel, independently of whether the Timer/Counter is running or not. If the
value written to TCNT2 equals the OCR2 value, the Compare Match will be missed,
resulting in incorrect waveform generation. Similarly, do not write the TCNT2 value
equal to BOTTOM when the counter is downcounting.
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 Out-
put Compare (FOC2) strobe bits 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.
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 64
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. If a System Reset occur, the OC2 Register is reset to “0”.
Figure 64. 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
COMn1
COMn0
FOCn
clk
I/O
Waveform
Generator
D
D
D
PORT
DDR
OCn
Q
Q
Q
1
0
2490G–AVR–03/04
OCn
Pin