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). See “Output Compare
Unit” on page 147. 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 Table 63 are also used extensively throughout the document.
Table 63. Definitions
BOTTOM The counter reaches the BOTTOM when it becomes 0x00.
MAX
TOP
The counter reaches its MAXimum when it becomes 0xFF (decimal 255).
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.
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 143.
Counter Unit
The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit. Figure
62 shows a block diagram of the counter and its surroundings.
Figure 62. Counter Unit Block Diagram
TOVn
(Int.Req.)
DATA BUS
Clock Select
count
Edge
Detector
Tn
clkTn
clear
TCNTn
Control Logic
direction
( From Prescaler )
bottom
top
Signal description (internal signals):
count Increment or decrement TCNT2 by 1.
direction Select between increment and decrement.
clear
clkTn
top
Clear TCNT2 (set all bits to zero).
Timer/Counter clock, referred to as clkT0 in the following.
Signalize that TCNT2 has reached maximum value.
146
ATmega128(L)
2467P–AVR–08/07
ATMEL [ ATMEL ]
