ATmega16A datasheet(119/352 Pages) ATMEL

119 / 352 page

119

8154B–AVR–07/09

ATmega16A

Figure 17-2. Counter Unit Block Diagram

Signal description (internal signals):

count

Increment or decrement TCNT2 by 1.

direction

Selects between increment and decrement.

clear

Clear TCNT2 (set all bits to zero).

clk

Timer/Counter clock.

top

Signalizes that TCNT2 has reached maximum value.

bottom

Signalizes that TCNT2 has reached minimum value (zero).

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 (TCCR2). There are close connections between how the

counter behaves (counts) and how waveforms are generated on the Output Compare output

OC2. For more details about advanced counting sequences and waveform generation, see

“Modes of Operation” on page 122.

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

the WGM21:0 bits. TOV2 can be used for generating a CPU interrupt.

17.5

Output Compare Unit

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

(OCR2). Whenever TCNT2 equals OCR2, the comparator signals a match. A match will set the

Output Compare Flag (OCF2) at the next timer clock cycle. If enabled (OCIE2 = 1), the Output

Compare Flag generates an output compare interrupt. The OCF2 Flag is automatically cleared

when the interrupt is executed. Alternatively, the OCF2 Flag can be cleared by software by writ-

ing 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 WGM21:0 bits and Compare Output

mode (COM21:0) bits. The max and bottom signals are used by the waveform generator for han-

dling the special cases of the extreme values in some modes of operation (“Modes of Operation”

on page 122). Figure 17-3 shows a block diagram of the output compare unit.

DATA BUS

TCNTn

Control Logic

count

TOVn

(Int.Req.)

top

bottom

direction

clear

TOSC1

T/C

Oscillator

TOSC2

Prescaler

clk

I/O

clk

ATmega16A Datasheet(PDF) 119 Page - ATMEL Corporation

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