AT89S2051 View Datasheet(PDF) - Atmel Corporation

Part Name

Description

Manufacturer

AT89S2051

8-bit Microcontroller with 2K/4K Bytes Flash

Atmel Corporation 

10. Reset

During reset, all I/O Registers are set to their initial values, the port pins are weakly pulled to

VCC, and the program starts execution from the Reset Vector, 0000H. The AT89S2051/S4051

has three sources of reset: power-on reset, brown-out reset, and external reset.

10.1

Power-On Reset

A Power-On Reset (POR) is generated by an on-chip detection circuit. The detection level is

nominally 1.4V. The POR is activated whenever VCC is below the detection level. The POR cir-

cuit can be used to trigger the start-up reset or to detect a supply voltage failure in devices

without a brown-out detector. The POR circuit ensures that the device is reset from power-on.

When VCC reaches the Power-on Reset threshold voltage, the Pierce Oscillator is enabled (if the

XTAL Oscillator Bypass fuse is OFF). Only after VCC has also reached the BOD (brown-out

detection) level (see Section 10.2 ”Brown-out Reset”), the BOD delay counter starts measuring a

2-ms delay after which the Internal Reset is deasserted and the microcontroller starts executing.

The built-in 2-ms delay allows the VCC voltage to reach the minimum 2.7V level before execut-

ing, thus guaranteeing the maximum operating clock frequency. The POR signal is activated

again, without any delay, when VCC falls below the POR threshold level. A Power-On Reset (i.e.

a cold reset) will set the POF flag in PCON. Refer to Figure 10-1 for details on the POR/BOD

behavior.

Figure 10-1. Power-up and Brown-out Detection Sequence

VCC

Min VCC Level 2.7V

BOD Level 2.3V

POR Level 1.4V

t

POR

XTAL1

2.4V

BOD

Internal

RESET

0

tPOR

(2 ms)

1.2V

tPOR

(2 ms)

t

t

t

tPOR

(2 ms)

t

8 AT89S2051/S4051

3390C–MICRO–7/05

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