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2467S–AVR–07/09
ATmega128
The double buffered Output Compare Register (OCR2) is compared with the Timer/Counter
value at all times. The result of the compare can be used by the waveform generator to generate
a PWM or variable frequency output on the Output Compare Pin (OC2).
for details. The compare match event will also set the compare flag (OCF2)
which can be used to generate an output compare interrupt request.
Definitions
Many register and bit references in this document are written in general form. A lower case “n”
replaces the Timer/Counter number, in this case 2. However, when using the register or bit
defines in a program, the precise form must be used (i.e., TCNT2 for accessing Timer/Counter2
counter value and so on).
The definitions in
are also used extensively throughout the document.
Timer/Counter
Clock Sources
The Timer/Counter can be clocked by an internal or an external clock source. The clock source
is selected by the clock select logic which is controlled by the clock select (CS22:0) bits located
in the Timer/Counter Control Register (TCCR2). For details on clock sources and prescaler, see
“Timer/Counter3, Timer/Counter2, and Timer/Counter1 Prescalers” on page 144
.
Counter Unit
The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit.
shows a block diagram of the counter and its surroundings.
Figure 62.
Counter Unit Block Diagram
Signal description (internal signals):
count
Increment or decrement TCNT2 by 1.
direction
Select between increment and decrement.
clear
Clear TCNT2 (set all bits to zero).
clk
T
n
Timer/Counter clock, referred to as clk
T0
in the following.
top
Signalize that TCNT2 has reached maximum value.
Table 63.
Definitions
BOTTOM
The counter reaches the BOTTOM when it becomes 0x00.
MAX
The counter reaches its MAXimum when it becomes 0xFF (decimal 255).
TOP
The counter reaches the TOP when it becomes equal to the highest
value in the count sequence. The TOP value can be assigned to be the
fixed value 0xFF (MAX) or the value stored in the OCR2 Register. The
assignment is dependent on the mode of operation.
DATA BUS
TCNTn
Control Logic
count
TOVn
(Int.Req.)
Clock Select
top
Tn
Edge
Detector
( From Prescaler )
clk
Tn
bottom
direction
clear
