ATtiny44 datasheet(73/236 Pages) ATMEL

73 / 236 page

8006H–AVR–10/09

ATtiny24/44/84

equals the OCR0x value, the Compare Match will be missed, resulting in incorrect waveform

generation. Similarly, do not write the TCNT0 value equal to BOTTOM when the counter is

down-counting.

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 Com-

pare (0x) 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.

11.6

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 11-4 on page 73 shows a

simplified schematic of the logic affected by the COM0x1:0 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 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 11-4. 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 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

ATtiny44 Datasheet(PDF) 73 Page - ATMEL Corporation