ST10F273E datasheet(98/179 Pages) STMICROELECTRONICS

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System reset

ST10F273E

98/179

To ensure correct power-up reset with controlled supply current consumption, specially if

clock signal requires a long period of time to stabilize, an asynchronous hardware reset is

required during power-up. For this reason, it is recommended to connect the external R0-C0

circuit shown in Figure 28 to the RPD pin. On power-up, the logical low level on RPD pin

forces an asynchronous hardware reset when RSTIN is asserted low. The external pull-up

R0 will then charge the capacitor C0. Note that an internal pull-down device on RPD pin is

turned on when RSTIN pin is low, and causes the external capacitor (C0) to begin

discharging at a typical rate of 100-200

µA. With this mechanism, after power-up reset, short

low pulses applied on RSTIN produce synchronous hardware reset. If RSTIN is asserted

longer than the time needed for C0 to be discharged by the internal pull-down device, then

the device is forced in an asynchronous reset. This mechanism insures recovery from very

catastrophic failure.

Figure 28.

Minimum external reset circuitry

The minimum reset circuit of

Figure 28 is not adequate when the RSTIN pin is driven from

the ST10F273E itself during software or watchdog triggered resets, because of the

capacitor C1 that will keep the voltage on RSTIN pin above VIL after the end of the internal

reset sequence, and thus will trigger an asynchronous reset sequence.

Figure 29 shows an example of a reset circuit. In this example, R1-C1 external circuit is only

used to generate power-up or manual reset, and R0-C0 circuit on RPD is used for power-up

reset and to exit from Power down mode. Diode D1 creates a wired-OR gate connection to

the reset pin and may be replaced by open-collector Schmitt trigger buffer. Diode D2

provides a faster cycle time for repetitive power-on resets.

R2 is an optional pull-up for faster recovery and correct biasing of TTL Open Collector

drivers.

RSTOUT

RPD

RSTIN

a) Hardware

VCC

b) For power-up

(and interruptible

power down

External hardware

reset

mode)

reset

ST10F273

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