ATmega128 datasheet(145/386 Pages) ATMEL

145 / 386 page

145

2467S–AVR–07/09

ATmega128

Each half period of the external clock applied must be longer than one system clock cycle to

ensure correct sampling. The external clock must be guaranteed to have less than half the sys-

tem clock frequency (f

ExtClk < fclk_I/O/2) given a 50/50% duty cycle. Since the edge detector uses

sampling, the maximum frequency of an external clock it can detect is half the sampling fre-

quency (Nyquist sampling theorem). However, due to variation of the system clock frequency

and duty cycle caused by Oscillator source (crystal, resonator, and capacitors) tolerances, it is

recommended that maximum frequency of an external clock source is less than f

clk_I/O/2.5.

An external clock source can not be prescaled.

Figure 60. Prescaler for Timer/Counter1, Timer/Counter2, and Timer/Counter3

Note:

The synchronization logic on the input pins (

T3/T2/T1) is shown in Figure 59.

Special Function IO

• 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 con-

figuration. When the TSM bit is written to zero, the PSR0 and PSR321 bits are cleared by

hardware, and the Timer/Counters start counting simultaneously.

• Bit 0 – PSR321: Prescaler Reset Timer/Counter3, Timer/Counter2, and Timer/Counter1

When this bit is one, the Timer/Counter3, Timer/Counter1, and Timer/Counter2 prescaler will be

reset. This bit is normally cleared immediately by hardware, except if the TSM bit is set. Note

that Timer/Counter3, Timer/Counter1, and Timer/Counter2 share the same prescaler and a

reset of this prescaler will affect all three timers.

PSR321

Clear

clk

TIMER/COUNTER2 CLOCK SOURCE

CS20

CS21

CS22

clk

TIMER/COUNTER1 CLOCK SOURCE

CS10

CS11

CS12

clk

TIMER/COUNTER3 CLOCK SOURCE

CS30

CS31

CS32

10-BIT T/C PRESCALER

Bit

TSM

–

ACME

PUD

PSR0

PSR321

SFIOR

Read/Write

R/W

Initial Value

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