ATmega328P datasheet(145/448 Pages) ATMEL

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145

8161D–AVR–10/09

ATmega48PA/88PA/168PA/328P

17.2.1

Registers

The Timer/Counter (TCNT2) and Output Compare Register (OCR2A and OCR2B) are 8-bit reg-

isters. Interrupt request (shorten as Int.Req.) signals are all visible in the Timer Interrupt Flag

(TIMSK2). TIFR2 and TIMSK2 are not shown in the figure.

The Timer/Counter can be clocked internally, via the prescaler, or asynchronously clocked from

the TOSC1/2 pins, as detailed later in this section. The asynchronous operation is controlled by

the Asynchronous Status Register (ASSR). The Clock Select logic block controls which clock

source he Timer/Counter uses to increment (or decrement) its value. The Timer/Counter is inac-

tive 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 (OCR2A and OCR2B) are compared with the

Timer/Counter value at all times. The result of the compare can be used by the Waveform Gen-

erator to generate a PWM or variable frequency output on the Output Compare pins (OC2A and

OC2B). See Section “17.5” on page 146. for details. The compare match event will also set the

Compare Flag (OCF2A or OCF2B) 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 the section.

17.3

Timer/Counter Clock Sources

The Timer/Counter can be clocked by an internal synchronous or an external asynchronous

clock source. The clock source clk

T2 is by default equal to the MCU clock, clkI/O. When the AS2

bit in the ASSR Register is written to logic one, the clock source is taken from the Timer/Counter

Oscillator connected to TOSC1 and TOSC2. For details on asynchronous operation, see ”ASSR

– Asynchronous Status Register” on page 164. For details on clock sources and prescaler, see

”Timer/Counter Prescaler” on page 156.

17.4

Counter Unit

The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit. Figure

17-2 on page 146 shows a block diagram of the counter and its surrounding environment.

Table 17-1.

Definitions

BOTTOM

The counter reaches the BOTTOM when it becomes zero (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 OCR2A Register. The assignment is depen-

dent on the mode of operation.

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