AT89LP51 datasheet(33/113 Pages) ATMEL

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3709C–MICRO–5/11

AT89LP51/52 - Preliminary

7.2

Brown-out Reset

The AT89LP51/52 has an on-chip Brown-out Detection (BOD) circuit for monitoring the V

DD level

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

BOD for the BOD is

nominally 2.0V. The purpose of the BOD is to ensure that if V

DD 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 7-2. When V

DD decreases to a value below the

trigger level V

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

trigger level plus about 200 mV of hysteresis, the start-up timer releases the internal reset after

the specified time-out period has expired (Table 7-1).

Figure 7-2.

Brown-out Detector Reset

The AT89LP51/52 allows for a wide V

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

cient to prevent incorrect execution if V

BOD is lower than the minimum required VDD range, such

as when a 5.0V supply is coupled with high frequency operation. In such cases an external

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

7.3

External Reset

The RST pin of the AT89LP51/52 can function as either an active-low reset input or as an active-

high reset input. The polarity of the RST pin is selectable using the POL pin (formerly EA). When

POL is high the RST pin is active high with an on-chip pull-down resistor tied to GND. When

POL is low the RST pin is active low with an on-chip pull-up resistor tied to V

DD. The RST pin

structure is shown in Figure 7-3. In Compatibility mode the reset pin is sampled every six clock

cycles and must be held active for at least twelve clock cycles to trigger the internal reset. In

Fast mode the reset pin is sampled every clock cycle and must be held active for at least two

clock cycles to trigger the internal 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

512

Crystal Oscillator

2048

Internal RC/External Clock

1024

Crystal Oscillator

4096

Internal RC/External Clock

4096

Crystal Oscillator

16384

VDD

Time-out

VPOR

Internal

Reset

tSUT

VBOD

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