ATMEga64L ATMEL Corporation, ATMEga64L Datasheet - Page 95

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

Compare Match Output
Unit
Compare Output Mode and
Waveform Generation
2490G–AVR–03/04
The Compare Output mode (COM01:0) bits have two functions. The Waveform Genera-
tor uses the COM01:0 bits for defining the Output Compare (OC0) state at the next
Compare Match. Also, the COM01:0 bits control the OC0 pin output source. Figure 37
shows a simplified schematic of the logic affected by the COM01:0 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 COM01:0
bits are shown. When referring to the OC0 state, the reference is for the internal OC0
Register, not the OC0 pin.
Figure 37. Compare Match Output Unit, Schematic
The general I/O port function is overridden by the Output Compare (OC0) from the
Waveform Generator if either of the COM01:0 bits are set. However, the OC0 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 OC0 pin (DDR_OC0) must be set as output
before the OC0 value is visible 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 OC0 state before
the output is enabled. Note that some COM01:0 bit settings are reserved for certain
modes of operation. See “8-bit Timer/Counter Register Description” on page 102.
The Waveform Generator uses the COM01:0 bits differently in Normal, CTC, and PWM
modes. For all modes, setting the COM01:0 = 0 tells the Waveform Generator that no
action on the OC0 Register is to be performed on the next Compare Match. For com-
pare output actions in the non-PWM modes refer to Table 53 on page 103. For fast
PWM mode, refer to Table 54 on page 103, and for phase correct PWM refer to Table
55 on page 103.
COMn1
COMn0
FOCn
clk
I/O
Waveform
Generator
D
D
D
PORT
DDR
OCn
Q
Q
Q
ATmega64(L)
1
0
OCn
Pin
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