
140
ATmega64A [DATASHEET]
8160D–AVR–02/2013
The Timer/Counter can be clocked internally, via the prescaler, or by an external clock source on the T2 pin. The
Clock Select logic block controls which clock source and edge the Timer/Counter uses to increment (or decrement)
its value. The Timer/Counter is inactive when no clock source is selected. The output from the Clock Select logic is
referred to as the timer clock (clk
T2).
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
will also set the Compare Flag (OCF2) which can be used to generate an Output Compare interrupt request.
18.2.2
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 (that is, TCNT2 for accessing Timer/Counter2 counter value and so on).
The definitions in
Table 18-1 are also used extensively throughout this section.
18.3
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 Reg-
18.4
Counter Unit
The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit.
Figure 18-2 shows a
block diagram of the counter and its surroundings.
Figure 18-2. Counter Unit Block Diagram
Signal description (internal signals):
count
Increment or decrement TCNT2 by 1.
Table 18-1.
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