ST7L09 datasheet(27/104 Pages) STMICROELECTRONICS

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ST7L05, ST7L09

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SUPPLY, RESET AND CLOCK MANAGEMENT (Cont’d)

7.4.2 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 ac-

cordance with the input voltage. It is pulled low by

external circuitry to reset the device. See Electrical

Characteristics section 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. This detection is asynchro-

nous and therefore the MCU can enter reset state

even in HALT mode.

The RESET pin is an asynchronous signal which

plays a major role in EMS performance. In a noisy

environment, it is recommended to follow the

guidelines mentioned in the electrical characteris-

tics section.

7.4.3 External Power-On RESET

To start up the microcontroller correctly, the user

must ensure by means of an external reset circuit

that the reset signal is held low until VDD is over

the minimum level specified for the selected fOSC

frequency.

A proper reset signal for a slow rising VDD supply

can generally be provided by an external RC net-

work connected to the RESET pin.

7.4.4 Internal Watchdog RESET

A RESET sequence is generated by a internal

Watchdog counter overflow.

Starting from the Watchdog counter underflow, the

device RESET pin acts as an output that is pulled

low during at least tw(RSTL)out.

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