ATmega128 datasheet(110/386 Pages) ATMEL

110 / 386 page

110

2467S–AVR–07/09

ATmega128

Timer/Counter Prescaler

Figure 45. Prescaler for Timer/Counter0

The clock source for Timer/Counter0 is named clk

T0. clkT0 is by default connected to the

main system clock clk

I/O. By setting the AS0 bit in ASSR, Timer/Counter0 is asynchro-

nously clocked from the TOSC1 pin. This enables use of Timer/Counter0 as a Real

Time Counter (RTC). When AS0 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/Counter0. The Oscillator is optimized for use

with a 32.768 kHz crystal. Applying an external clock source to TOSC1 is not

recommended.

For Timer/Counter0, the possible prescaled selections are: clk

T0S/8, clkT0S/32, clkT0S/64,

clk

T0S/128, clkT0S/256, and clkT0S/1024. Additionally, clkT0S as well as 0 (stop) may be

selected. Setting the PSR0 bit in SFIOR resets the prescaler. This allows the user to

operate with a predictable prescaler.

Special Function IO Register –

SFIOR

• Bit 7 – TSM: Timer/Counter Synchronization Mode

Writing the TSM bit to one activates the Timer/Counter Synchronization mode. In this

mode, the value that is written to the PSR0 and PSR321 bits is kept, hence keeping the

corresponding prescaler reset signals asserted. This ensures that the corresponding

Timer/Counters are halted and can be configured to the same value without the risk of

one of them advancing during configuration. When the TSM bit is written to zero, the

PSR0 and PSR321 bits are cleared by hardware, and the Timer/Counters start counting

simultaneously.

10-BIT T/C PRESCALER

TIMER/COUNTER0 CLOCK SOURCE

clk

I/O

clk

T0S

TOSC1

AS0

CS00

CS01

CS02

PSR0

Clear

clk

Bit

TSM

–

ACME

PUD

PSR0

PSR321

SFIOR

Read/Write

R/W

Initial Value

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