ATmega64A
(TIMSK). TIFR and TIMSK are not shown in the figure since these registers are shared by other
timer units.
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 (clkT2).
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 gener-
ate a PWM or variable frequency output on the Output Compare pin (OC2). For details, see
“Output Compare Unit” on page 148. The Compare Match event will also set the Compare Flag
(OCF2) which can be used to generate an Output Compare interrupt request.
17.2.2
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 (i.e., TCNT2 for accessing Timer/Counter2
counter value and so on).
The definitions in Table 17-1 are also used extensively throughout this section.
Table 17-1. Definitions
BOTTOM
MAX
The counter reaches the BOTTOM when it becomes 0x00.
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.
17.3 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 Register (TCCR2). For details on clock sources and pres-
caler, see “Timer/Counter3, Timer/Counter2 and Timer/Counter1 Prescalers” on page 143.
17.4 Counter Unit
The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit. Figure
17-2 shows a block diagram of the counter and its surroundings.
147
8160C–AVR–07/09
ATMEL [ ATMEL ]
