ATMEGA48 datasheet(153/374 Pages) ATMEL

153 / 374 page

153

2545J–AVR–12/06

ATmega48/88/168

The clock source for Timer/Counter2 is named clk

T2S. clkT2S is by default connected to the main

system I/O clock clk

IO. By setting the AS2 bit in ASSR, Timer/Counter2 is asynchronously

clocked from the TOSC1 pin. This enables use of Timer/Counter2 as a Real Time Counter

(RTC). When AS2 is set, pins TOSC1 and TOSC2 are disconnected from Port C. A crystal can

then be connected between the TOSC1 and TOSC2 pins to serve as an independent clock

source for Timer/Counter2. The Oscillator is optimized for use with a 32.768 kHz crystal.

For Timer/Counter2, the possible prescaled selections are: clk

T2S/8, clkT2S/32, clkT2S/64,

clk

T2S/128, clkT2S/256, and clkT2S/1024. Additionally, clkT2S as well as 0 (stop) may be selected.

Setting the PSRASY bit in GTCCR resets the prescaler. This allows the user to operate with a

predictable prescaler.

16.11 Register Description

16.11.1

TCCR2A – Timer/Counter Control Register A

• Bits 7:6 – COM2A1:0: Compare Match Output A Mode

These bits control the Output Compare pin (OC2A) behavior. If one or both of the COM2A1:0

bits are set, the OC2A output overrides the normal port functionality of the I/O pin it is connected

to. However, note that the Data Direction Register (DDR) bit corresponding to the OC2A pin

must be set in order to enable the output driver.

When OC2A is connected to the pin, the function of the COM2A1:0 bits depends on the

WGM22:0 bit setting. Table 16-2 shows the COM2A1:0 bit functionality when the WGM22:0 bits

are set to a normal or CTC mode (non-PWM).

Table 16-3 on page 154 shows the COM2A1:0 bit functionality when the WGM21:0 bits are set

to fast PWM mode.

Bit

210

(0xB0)

COM2A1

COM2A0

COM2B1

COM2B0

–

WGM21

WGM20

TCCR2A

Read/Write

R/W

Initial Value

Table 16-2.

Compare Output Mode, non-PWM Mode

COM2A1

COM2A0

Description

Normal port operation, OC2A disconnected.

Toggle OC2A on Compare Match

Clear OC2A on Compare Match

Set OC2A on Compare Match

ATMEGA48 Datasheet(PDF) 153 Page - ATMEL Corporation

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