101
ATmega8A [DATASHEET]
8159E–AVR–02/2013
Figure 18-2. 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
T2
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
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.
18.5
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 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 WGM21:0 bits and Compare Output mode (COM21:0)
bits. The max and bottom signals are used by the waveform generator for handling the special cases of the
Figure 18-3 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
Tn