ST63156 datasheet(24/86 Pages) STMICROELECTRONICS

24 / 86 page

Figure 20. Internal Reset Circuit

The ST631xx device can be reset in two ways: by

the external reset input (RESET ) tied low, by

power-on reset and by the digital Watchdog pe-

ripheral.

RESET Input

The external active low RESET pin is used to reset

the ST631xx devices and provide an orderly soft-

ware startup procedure. The activation of the RE-

SET pin may occur 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 cir-

cuitry are buffered by a Schmitt Trigger so that an

oscillator in start-up conditions will not give spuri-

ous clocks. 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/Outputports (port A, port B and port

C) are in the input state (except PC2). As soon as

the level on the RESET pin becomes high, the in-

itialization sequence is executed.

Watchdog Reset

The ST631xx devices are provided with an on-

chip hardware activated digital watchdog function

in order to provide a graceful recovery from a soft-

ware upset. If the watchdog register is not re-

freshed and the end-of-count is reached, then the

reset state will be latched into the MCU and an in-

ternal 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 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.

RESET

ST63140,142,126,156

20/82

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