ST72F324LJ2T5 datasheet(25/154 Pages) STMICROELECTRONICS

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ST72324Lxx

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6.3 RESET SEQUENCE MANAGER (RSM)

6.3.1 Introduction

The reset sequence manager includes two RE-

SET sources as shown in Figure 13:

■ External RESET source pulse

■ 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 12:

■ Active Phase depending on the RESET source

■ 256 or 4096 CPU clock cycle delay (selected by

option byte)

■ RESET vector fetch

The 256 or 4096 CPU clock cycle delay allows the

oscillator to stabilise and ensures that recovery

has taken place from the Reset state. The shorter

or longer clock cycle delay should be selected by

option 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 12. 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. This detection is asynchro-

nous and therefore the MCU can enter reset state

even in HALT mode.

The RESET pin is an asynchronous signal which

plays a major role in EMS performance. In a noisy

environment, it is recommended to follow the

guidelines mentioned in the electrical characteris-

tics section.

6.3.3 External Power-On RESET

To start up the microcontroller correctly, the user

must ensure by means of an external reset circuit

that the reset signal is held low until VDD is over

the minimum level specified for the selected fOSC

frequency.

A proper reset signal for a slow rising VDD supply

can generally be provided by an external RC net-

work connected to the RESET pin.

6.3.4 Internal Watchdog RESET

Starting from the Watchdog counter underflow, the

device RESET pin acts as an output that is pulled

low during at least tw(RSTL)out.

Figure 13. Reset Block Diagram

RESET

Active Phase

INTERNAL RESET

256 or 4096 CLOCK CYCLES

FETCH

VECTOR

RESET

RON

VDD

WATCHDOG RESET

INTERNAL

RESET

PULSE

GENERATOR

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