ST6391 datasheet(25/68 Pages) STMICROELECTRONICS

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VA000200

TO ST6

RESET

ST6

INTERNAL RESET

OSCILLATOR

SIG NAL

WATCHDOG RESET

VDD

300 k

RESET

(ACTIVE LOW)

COUNTER

1.0k

Figure 22. Internal Reset Circuit

RESET

The ST639x devices can be reset in two ways: by the

external reset input (RESET) tied low and by the

hardware activated digital watchdog peripheral.

RESET Input

The external active low reset pin is used to reset the

ST638x devices and provide an orderly software

startup procedure. The activation of the Reset pin

may occur at any time in the RUN or WAIT mode.

Even short pulses at the reset pin will be accepted

since the reset signal is latched internally and is only

cleared after 2048 clocks at the oscillator pin. The

clocks from the oscillator pin to the reset circuitry are

buffered by a schmitt trigger so that an oscillator in

start-up conditions will not give spurious clocks.

When the reset pin is held low, the external crystal os-

cillator is also disabled in order to reduce current con-

sumption. The MCU is configured in the Reset mode

as long as the signal of the RESET pin is low. The

processingof theprogramis stoppedandthe standard

Input/Outputports (port A, port B and port C) are in the

input state. As soon as the level on the reset pin be-

comes high, the initialization sequence is executed.

Refer to the MCU initialization sequencefor additional

information.

Watchdog Reset

The ST639x devices are provided with an on-chip

hardware activated digital watchdog function in or-

der to provide a graceful recovery from a software

upset. If the watchdog register is not refreshed and

the end-of-count is reached, then the reset state

will be latched into the MCU and an internal circuit

pulls down the reset pin. This also resets the

watchdog which subsequently turns off the pull-

down and activates the pull-up device at the reset

pin. This causes the positive transition at the reset

pin. The MCU will then exit the reset state after

2048 clocks on the oscillator pin.

Application Notes

An external resistor between VDD and the reset pin

is not required because an internal pull-up device

is provided. The user may prefer to add an external

pull-up resistor.

An internal Power-on device does not guarantee

that the MCU will exit the reset state when VDD is

above 4.5V and therefore the RESET pin should

be externally controlled.

ST6391,92,93,95,97,99

21/64

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