ST72F321BAR9TCRE datasheet(47/247 Pages) STMICROELECTRONICS

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ST72321Bxxx-Auto

Supply, reset and clock management

47/247

The RESET service routine vector is fixed at addresses FFFEh-FFFFh in the ST7 memory

map.

The basic RESET sequence consists of three 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

Caution:

When the ST7 is unprogrammed or fully erased, the Flash is blank and the RESET vector is

not programmed. For this reason, it is recommended to keep the RESET pin in low state

until programming mode is entered, in order to avoid unwanted behavior.

The 256 or 4096 CPU clock cycle delay allows the oscillator to stabilize 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 (see Section 21.1.1: Flash configuration on page 228).

The RESET vector fetch phase duration is 2 clock cycles.

Figure 11.

Reset block diagram

Figure 12.

RESET sequence phases

6.5.2

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 accordance with the input voltage. It

can be pulled low by external circuitry to reset the device. See Section 19.9: Control pin

characteristics on page 211 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 detection is asynchronous and

therefore the MCU can enter reset state even in Halt mode.

RESET

RON

VDD

WATCHDOG RESET

LVD RESET

INTERNAL

RESET

PULSE

GENERATOR

Filter

RESET

ACTIVE PHASE

INTERNAL RESET

256 or 4096 CLOCK CYCLES

FETCH

VECTOR

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