ATmega162 datasheet(93/324 Pages) ATMEL

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2513K–AVR–07/09

ATmega162/V

Using the Output

Compare Unit

Since writing TCNT0 in any mode of operation will block all compare matches for one timer clock

cycle, there are risks involved when changing TCNT0 when using the output compare channel,

independently of whether the Timer/Counter is running or not. If the value written to TCNT0

equals the OCR0 value, the Compare Match will be missed, resulting in incorrect waveform gen-

eration. Similarly, do not write the TCNT0 value equal to BOTTOM when the counter is down-

counting.

The setup of the OC0 should be performed before setting the Data Direction Register for the port

pin to output. The easiest way of setting the OC0 value is to use the Force Output Compare

(FOC0) strobe bits in Normal mode. The OC0 Register keeps its value even when changing

between Waveform Generation modes.

Be aware that the COM01:0 bits are not double buffered together with the compare value.

Changing the COM01:0 bits will take effect immediately.

Compare Match

Output Unit

The Compare Output mode (COM01:0) bits have two functions. The Waveform Generator 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 36 shows a simplified sche-

matic 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. If a System Reset occur, the OC0

Figure 36. Compare Match Output Unit, Schematics

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 direction (input or output)

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

ter 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.

PORT

DDR

OCn

Waveform

Generator

COMn1

COMn0

FOCn

clk

I/O

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