ATmega16 datasheet(134/357 Pages) ATMEL

134 / 357 page

134

2466S–AVR–05/09

ATmega16(L)

Timer/Counter

Prescaler

Figure 64. Prescaler for Timer/Counter2

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. Apply-

ing an external clock source to TOSC1 is not recommended.

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 PSR2 bit in SFIOR resets the prescaler. This allows the user to operate with a pre-

dictable prescaler.

Special Function IO

• Bit 1 – PSR2: Prescaler Reset Timer/Counter2

When this bit is written to one, the Timer/Counter2 prescaler will be reset. The bit will be cleared

by hardware after the operation is performed. Writing a zero to this bit will have no effect. This bit

will always be read as zero if Timer/Counter2 is clocked by the internal CPU clock. If this bit is

written when Timer/Counter2 is operating in asynchronous mode, the bit will remain one until the

prescaler has been reset.

10-BIT T/C PRESCALER

TIMER/COUNTER2 CLOCK SOURCE

clk

I/O

clk

T2S

TOSC1

AS2

CS20

CS21

CS22

PSR2

Clear

clk

Bit

ADTS2

ADTS1

ADTS0

–

ACME

PUD

PSR2

PSR10

SFIOR

Read/Write

R/W

Initial Value

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