ATMEGA640 datasheet(119/436 Pages) MICROCHIP

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ATmega640/V-1280/V-1281/V-2560/V-2561/V [DATASHEET]

DS40002211A

The setup of the OC0x should be performed before setting the Data Direction Register for the port pin to output.

The easiest way of setting the OC0x value is to use the Force Output Compare (FOC0x) strobe bits in Normal

mode. The OC0x Registers keep their values even when changing between Waveform Generation modes.

Be aware that the COM0x1:0 bits are not double buffered together with the compare value. Changing the

COM0x1:0 bits will take effect immediately.

16.6 Compare Match Output Unit

The Compare Output mode (COM0x1:0) bits have two functions. The Waveform Generator uses the COM0x1:0

bits for defining the Output Compare (OC0x) state at the next Compare Match. Also, the COM0x1:0 bits control the

OC0x pin output source. Figure 16-4 shows a simplified schematic of the logic affected by the COM0x1:0 bit set-

ting. 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 COM0x1:0 bits are shown. When referring to the OC0x

state, the reference is for the internal OC0x Register, not the OC0x pin. If a system reset occur, the OC0x Register

is reset to “0”.

Figure 16-4. 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 COM0x1:0 bits are set. However, the OC0x pin direction (input or output) is still controlled by the Data Direc-

tion 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 visible on the pin. The port override function is independent of the Waveform Gen-

eration mode.

The design of the Output Compare pin logic allows initialization of the OC0x state before the output is enabled.

Note that some COM0x1:0 bit settings are reserved for certain modes of operation. See “Register Description” on

page 126.

16.6.1

Compare Output Mode and Waveform Generation

The Waveform Generator uses the COM0x1:0 bits differently in Normal, CTC, and PWM modes. For all modes,

setting the COM0x1:0 = 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 16-2 on page 126. For

fast PWM mode, refer to Table 16-3 on page 126, and for phase correct PWM refer to Table 16-4 on page 127.

A change of the COM0x1:0 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.

PORT

DDR

OCnx

Waveform

Generator

COMnx1

COMnx0

FOCn

clk

I/O

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