ATmega88 datasheet(137/340 Pages) ATMEL

137 / 340 page

137

7530H–AVR–02/09

ATmega48/88/168 Automotive

Depending on the mode of operation used, the counter is cleared, incremented, or decremented

at each timer clock (clk

T2). clkT2 can be generated from an external or internal clock source,

selected by the Clock Select bits (CS22:0). When no clock source is selected (CS22:0 = 0) the

timer is stopped. However, the TCNT2 value can be accessed by the CPU, regardless of

whether clk

T2 is present or not. A CPU write overrides (has priority over) all counter clear or

count operations.

The counting sequence is determined by the setting of the WGM21 and WGM20 bits located in

the Timer/Counter Control Register (TCCR2A) and the WGM22 located in the Timer/Counter

Control Register B (TCCR2B). There are close connections between how the counter behaves

(counts) and how waveforms are generated on the Output Compare outputs OC2A and OC2B.

For more details about advanced counting sequences and waveform generation, see “Modes of

Operation” on page 140.

The Timer/Counter Overflow Flag (TOV2) is set according to the mode of operation selected by

the WGM22:0 bits. TOV2 can be used for generating a CPU interrupt.

15.4

Output Compare Unit

The 8-bit comparator continuously compares TCNT2 with the Output Compare Register

(OCR2A and OCR2B). Whenever TCNT2 equals OCR2A or OCR2B, the comparator signals a

match. A match will set the Output Compare Flag (OCF2A or OCF2B) at the next timer clock

cycle. If the corresponding interrupt is enabled, the Output Compare Flag generates an Output

Compare interrupt. The Output Compare Flag is automatically cleared when the interrupt is exe-

cuted. Alternatively, the Output Compare Flag can be cleared by software by writing a logical

one to its I/O bit location. The Waveform Generator uses the match signal to generate an output

according to operating mode set by the WGM22:0 bits and Compare Output mode (COM2x1:0)

bits. The max and bottom signals are used by the Waveform Generator for handling the special

cases of the extreme values in some modes of operation (“Modes of Operation” on page 140).

Figure 15-3 shows a block diagram of the Output Compare unit.

Figure 15-3. Output Compare Unit, Block Diagram

OCFnx (Int.Req.)

= (8-bit Comparator )

OCRnx

OCnx

DATA BUS

TCNTn

WGMn1:0

Waveform Generator

top

FOCn

COMnX1:0

bottom

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