ST72260G datasheet(22/171 Pages) STMICROELECTRONICS

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6.3 RESET SEQUENCE MANAGER (RSM)

6.3.1 Introduction

The reset sequence manager includes three RE-

SET sources as shown in Figure 12:

External RESET source pulse

Internal LVD RESET (Low Voltage Detection)

Internal WATCHDOG RESET

These sources act on the RESET pin and it is al-

ways kept low during the delay phase.

The RESET service routine vector is fixed at ad-

dresses FFFEh-FFFFh in the ST7 memory map.

The basic RESET sequence consists of 3 phases

as shown in Figure 11:

Active Phase depending on the RESET source

4096 CPU clock cycle delay (selected by option

byte)

RESET vector fetch

The 4096 CPU clock cycle delay allows the oscil-

lator to stabilise and ensures that recovery has

taken place from the Reset state. The shorter or

longer clock cycle delay should be selected by op-

tion byte to correspond to the stabilization time of

the external oscillator used in the application.

The RESET vector fetch phase duration is 2 clock

cycles.

Figure 11. RESET Sequence Phases

6.3.2 Asynchronous External RESET pin

The RESET pin is both an input and an open-drain

output with integrated RON weak pull-up resistor.

This pull-up has no fixed value but varies in ac-

cordance with the input voltage. It can be pulled

low by external circuitry to reset the device. See

Electrical Characteristic section for more details.

A RESET signal originating from an external

source must have a duration of at least th(RSTL)in in

order to be recognized (see Figure 13). This de-

tection is asynchronous and therefore the MCU

can enter reset state even in HALT mode.

Figure 12. Reset Block Diagram

RESET

Active Phase

INTERNAL RESET

4096 CLOCK CYCLES

FETCH

VECTOR

RESET

RON

VDD

WATCHDOG RESET

LVD RESET

INTERNAL

RESET

PULSE

GENERATOR

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