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2552K–AVR–04/11
ATmega329/3290/649/6490
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
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 (TCCR2A). There are close connections between how the
counter behaves (counts) and how waveforms are generated on the Output Compare output
OC2A. For more details about advanced counting sequences and waveform generation, see
The Timer/Counter Overflow Flag (TOV2) 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
(OCR2A). Whenever TCNT2 equals OCR2A, the comparator signals a match. A match will set
the Output Compare Flag (OCF2A) at the next timer clock cycle. If enabled (OCIE2A = 1), the
Output Compare Flag generates an Output Compare interrupt. The OCF2A Flag is automatically
cleared when the interrupt is executed. Alternatively, the OCF2A Flag can be cleared by soft-
ware 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 Com-
pare Output mode (COM2A1:0) 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
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
Tn