ATmega162 datasheet(139/324 Pages) ATMEL

139 / 324 page

139

2513K–AVR–07/09

ATmega162/V

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

source the 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 (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 140. 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 59 are also used extensively throughout the section.

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 “Asyn-

chronous Status Register – ASSR” on page 152. For details on clock sources and prescaler, see

“Timer/Counter Prescaler” on page 156.

Table 59. 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 OCR2 Register. The

assignment is dependent on the mode of operation.

ATmega162 датащи(PDF) 139 Page - ATMEL Corporation