User guide

139
ATmega162/V
2513E–AVR–09/03
Counter Unit The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit.
Figure 60 shows a block diagram of the counter and its surrounding environment.
Figure 60. 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
T
2 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 dec-
remented at each timer clock (clk
T
2). clk
T
2 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
T
2 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 142.
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.
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 gen-
erator 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 sig-
nals 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 142).
Figure 61 shows a block diagram of the output compare unit.
DATA BUS
TCNTn Control Logic
count
TOVn
(Int.Req.)
topbottom
direction
clear
TOSC1
T/C
Oscillator
TOSC2
Prescaler
clk
I/O
clk
Tn