ST6380 datasheet(19/82 Pages) STMICROELECTRONICS

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ST6380, ST6381, ST6382, ST6383, ST6388, ST6389

3.2 RESETS

The MCU can be reset in three ways:

– by the external Reset input being pulled low;

– by Power-on Reset;

– by the digital Watchdog peripheral timing out.

3.2.1 RESET Input

The RESET pin may be connected to a device of

the application board in order to reset the MCU if

required. The RESET pin may be pulled low in

RUN, WAIT or STOP mode. This input can be

used to reset the MCU internal state and ensure a

correct start-up procedure. The pin is active low

and features a Schmitt trigger input. The internal

Reset signal is generated by adding a delay to the

external signal. Therefore even short pulses on

the RESET pin are acceptable, provided VDD has

completed its rising phase and that the oscillator is

running correctly (normal RUN or WAIT modes).

The MCU is kept in the Reset state as long as the

RESET pin is held low.

If RESET activation occurs in RUN or WAIT

modes, processing of the user program is stopped

(RUN mode only), the Inputs and Outputs are con-

figured as inputs with pull-up resistors if available.

When the level on the RESET pin then goes high,

the initialization sequence is executed following

expiry of the internal delay period.

If RESET pin activation occurs in the STOP mode,

the oscillator starts up and all Inputs and Outputs

are configured as inputs with pull-up resistors if

available. When the level of the RESET pin then

goes high, the initialization sequence is executed

following expiry of the internal delay period.

3.2.2 Power-on Reset

The function of the POR circuit consists in waking

up the MCU at an appropriate stage during the

power-on sequence. At the beginning of this se-

quence, the MCU is configured in the Reset state:

all I/O ports are configured as inputs with pull-up

resistors and no instruction is executed. When the

power supply voltage rises to a sufficient level, the

oscillator starts to operate, whereupon an internal

delay is initiated, in order to allow the oscillator to

fully stabilize before executing the first instruction.

The initialization sequence is executed immediate-

ly following the internal delay.

The internal delay is generated by an on-chip

counter. The internal reset line is released 2048 in-

ternal clock cycles after release of the external re-

set.

The internal POR device is a static mechanism

which forces the reset state when VDD is below a

threshold voltage in the range 3.4 to 4.2 Volts (see

Figure 13). The circuit guarantees that the MCU

will exit or enter the reset state correctly, without

spurious effects, ensuring, for example, that EEP-

ROM contents are not corrupted.

Figure 13. Power ON/OFF Reset operation

Figure 14. Reset and Interrupt Processing

VR02037

VDD

4.2

3.4

POWER

ON/OFF

Threshold

RESET

INT LATCH CLEARED

NMI MASK SET

RESET

( IF PRESENT )

SELECT

NMI MODE FLAGS

IS RESET STILL

PRESENT?

YES

PUT FFEH

ON ADDRESS BUS

FROM RESET LOCATIONS

FFE/FFF

FETCH INSTRUCTION

LOAD PC

VA000427

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