ST6369 datasheet(23/71 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 20. Internal Reset Circuit

RESET

The ST6369 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

ST6369 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 bufferedby 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

oscillator is also disabled in order to reduce current

consumption. The MCU is configured in the Reset

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

The processing of the program is stopped and the

standard Input/Output ports (port A, port B and port

C) are in the input state. As soon as the level on the

reset pin becomes high, the initialization sequenceis

executed. Refer to the MCU initialization sequence

for additional information.

Watchdog Reset

The ST6369 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.

ST6369

19/67

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