Datasheet

Figure 20-2. Counter Unit Block Diagram
DATA BUS
TCNTn Control Logic
count
TOVn
(Int. Req.)
TOPBOTTOM
direction
clear
TOS C1
T/C
Oscillator
TOS C2
Pres ca le r
clk
I/O
clk
Tn
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
). clk
T2
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, refer to Modes of Operation .
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.
20.5. 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 extreme values in some modes of operation
(refer to Modes of Operation).
The following figure shows a block diagram of the Output Compare unit.
Atmel ATmega32A [DATASHEET]
Atmel-8155I-ATmega32A_Datasheet_Complete-08/2016
142