AT89LP428 datasheet(25/149 Pages) ATMEL

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3654A–MICRO–8/09

AT89LP428/828

7.2

Brown-out Reset

The AT89LP428/828 has an on-chip Brown-out Detector (BOD) circuit for monitoring the V

level during operation by comparing it to a fixed trigger level. The trigger level V

BOD for the BOD

is nominally 2.2V. The purpose of the BOD is to ensure that if V

CC fails or dips while executing at

speed, the system will gracefully enter reset without the possibility of errors induced by incorrect

execution. A BOD sequence is shown in Figure 7-3. When V

CC decreases to a value below the

trigger level V

BOD, the internal reset is immediately activated. When VCC increases above the

trigger level, the start-up timer releases the internal reset after the specified time-out period has

expired (Table 7-1). Note that there is approximately 200 mV of hysteresis between the rising

and falling V

BOD thresholds. The Brown-out Detector must be enabled by setting the BOD

Enable Fuse. (See “User Configuration Fuses” on page 121.)

The AT89LP428/828 allows for a wide V

CC operating range. The on-chip BOD may not be suffi-

cient to prevent incorrect execution if V

BOD is lower than the minimum required VCC range, such

as when a 5V supply is coupled with high frequency operation. In such cases an external Brown-

out Reset circuit connected to the RST pin may be required.

Figure 7-3.

Brown-out Detector Reset

Table 7-1.

Start-up Timer Settings

SUT Fuse 1

SUT Fuse 0

Clock Source

SUT (± 5%) µs

Internal RC/External Clock

Crystal Oscillator

1024

Internal RC/External Clock

N/A

Crystal Oscillator

2048

Internal RC/External Clock

1024

Crystal Oscillator

4096

Internal RC/External Clock

N/A

Crystal Oscillator

16384

VCC

Time-out

VPOR

Internal

Reset

tSUT

VBOD

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