AT90SCR100 datasheet(34/433 Pages) ATMEL

34 / 433 page

TPR0327AY–SMS–30Jan09

AT90SCR100

delay options are shown in Table 7-2. The frequency of the Watchdog Oscillator is voltage

dependent.

The main purpose of the delay is to keep the AVR in reset until it is supplied with the minimum

Vcc. The delay will not monitor the actual voltage thus it will be required to select a delay longer

than the Vcc rise time. If this is not possible, an internal or external Brown-Out Detection circuit

should be used. A BOD circuit will ensure sufficient Vcc before it releases the reset, thus the

time-out delay can be disabled. Disabling the time-out delay without utilizing a Brown-Out Detec-

tion circuit is not recommended.

The oscillator is required to oscillate for a minimum number of cycles before the clock is consid-

ered stable. An internal ripple counter monitors the oscillator output clock, and keeps the internal

reset active for a given number of clock cycles. The reset is then released and the device will

start to execute. The recommended oscillator start-up time is dependent on the clock type, and

varies from 6 cycles for an externally applied clock to 32K cycles for a low frequency crystal.

The start-up sequence for the clock includes both the time-out delay and the start-up time when

the device starts up from reset. When starting up from Power-save or Power-down mode, Vcc is

assumed to be at a sufficient level and only the start-up time is included.

7.2.3

Clock Source Connections

The pins XTAL1 and XTAL2 are the input and output, respectively, of an inverting amplifier

which can be configured for use as an On-chip Oscillator, as shown in Figure 7-2. Either a

quartz crystal or a ceramic resonator may be used.

C1 and C2 should always be equal for both crystals and resonators. For ceramic resonators, the

capacitor values given by the manufacturer should be used.

Figure 7-2.

Crystal Oscillator Connections

7.2.3.1

Low Power Crystal Oscillator

This Crystal Oscillator is a low power oscillator, with reduced voltage swing on the XTAL2 out-

put. It gives the lowest power consumption, but is not capable of driving other clock inputs.

The only crystal supported by the AT90SCR100 is an 8Mhz crystal.

Table 7-2.

Number of Watchdog Oscillator Cycles

Typ Time-out (V

CC = 5.0V)

Typ Time-out (V

CC = 3.0V)

Number of Cycles

0 ms

4.1 ms

4.3 ms

512

65 ms

69 ms

8K (8,192)

C1 = 47pF

C2 = 47pF

8Mhz

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