91
2513L–AVR–03/2013
ATmega162/V
Counter Unit
The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit.
Figure34 shows a block diagram of the counter and its surroundings.
Figure 34. Counter Unit Block Diagram
Signal description (internal signals):
count
Increment or decrement TCNT0 by 1.
direction
Select between increment and decrement.
clear
Clear TCNT0 (set all bits to zero).
clk
Tn
Timer/Counter clock, referred to as clk
T0 in the following.
top
Signalize that TCNT0 has reached maximum value.
bottom
Signalize that TCNT0 has reached minimum value (zero).
Depending of the mode of operation used, the counter is cleared, incremented, or decremented
at each timer clock (clk
T0). clkT0 can be generated from an external or internal clock source,
selected by the clock select bits (CS02:0). When no clock source is selected (CS02:0 = 0) the
timer is stopped. However, the TCNT0 value can be accessed by the CPU, regardless of
whether clk
T0 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 WGM01 and WGM00 bits located in
the Timer/Counter Control Register (TCCR0). There are close connections between how the
counter behaves (counts) and how waveforms are generated on the output Compare Output
OC0. For more details about advanced counting sequences and waveform generation, see
The Timer/Counter Overflow (TOV0) Flag is set according to the mode of operation selected by
the WGM01:0 bits. TOV0 can be used for generating a CPU interrupt.
Output Compare
Unit
The 8-bit comparator continuously compares TCNT0 with the Output Compare Register
(OCR0). Whenever TCNT0 equals OCR0, the comparator signals a match. A match will set the
Output Compare Flag (OCF0) at the next timer clock cycle. If enabled (OCIE0 = 1 and Global
Interrupt Flag in SREG is set), the Output Compare Flag generates an output compare interrupt.
The OCF0 Flag is automatically cleared when the interrupt is executed. Alternatively, the OCF0
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
WGM01:0 bits and Compare Output mode (COM01:0) bits. The max and bottom signals are
used by the waveform generator for handling the special cases of the extreme values in some
Figure 35 shows a block diagram of the output compare unit.
DATA BUS
TCNTn
Control Logic
count
TOVn
(Int.Req.)
Clock Select
top
Tn
Edge
Detector
( From Prescaler )
clk
Tn
bottom
direction
clear