ATtiny43U datasheet(83/182 Pages) ATMEL

83 / 182 page

8048B–AVR–03/09

generation. Similarly, do not write the TCNTn value equal to BOTTOM when the counter is

down-counting.

The setup of the OCnx should be performed before setting the Data Direction Register for the

port pin to output. The easiest way of setting the OCnx value is to use the Force Output Com-

pare (nx) strobe bits in Normal mode. The OCnx Registers keep their values even when

changing between Waveform Generation modes.

Be aware that the COMnx1:0 bits are not double buffered together with the compare value.

Changing the COMnx1:0 bits will take effect immediately.

12.6

Compare Match Output Unit

The Compare Output mode (COMnx1:0) bits have two functions. The Waveform Generator uses

the COMnx1:0 bits for defining the Output Compare (OCnx) state at the next Compare Match.

Also, the COMnx1:0 bits control the OCnx pin output source. Figure 12-4 on page 83 shows a

simplified schematic of the logic affected by the COMnx1:0 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 COMnx1:0 bits are shown. When referring

to the OCnx state, the reference is for the internal OCnx Register, not the OCnx pin. If a system

reset occur, the OCnx Register is reset to “0”.

Figure 12-4. Compare Match Output Unit, Schematic (non-PWM Mode)

The general I/O port function is overridden by the Output Compare (OCnx) from the Waveform

Generator if either of the COMnx1:0 bits are set. However, the OCnx pin direction (input or out-

put) is still controlled by the Data Direction Register (DDR) for the port pin. The Data Direction

ble on the pin. The port override function is independent of the Waveform Generation mode.

PORT

DDR

OCn

OCnx

Waveform

Generator

COMnx1

COMnx0

FOCn

clk

I/O

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