ST72260G datasheet(23/171 Pages) STMICROELECTRONICS

23 / 171 page

ST72260G, ST72262G, ST72264G

23/171

RESET SEQUENCE MANAGER (Cont’d)

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.

6.3.3 External Power-On RESET

If the LVD is disabled by option byte, 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.

6.3.4 Internal Low Voltage Detector (LVD)

RESET

Two different RESET sequences caused by the in-

ternal LVD circuitry can be distinguished:

Power-On RESET

Voltage Drop RESET

The device RESET pin acts as an output that is

pulled low when VDD<VIT+ (rising edge) or

VDD<VIT- (falling edge) as shown in Figure 13.

The LVD filters spikes on VDD larger than tg(VDD) to

avoid parasitic resets.

6.3.5 Internal Watchdog RESET

The RESET sequence generated by a internal

Watchdog counter overflow is shown in Figure 13.

Starting from the Watchdog counter underflow, the

device RESET pin acts as an output that is pulled

low during at least tw(RSTL)out.

Figure 13. RESET Sequences

VDD

RUN

RESET PIN

EXTERNAL

WATCHDOG

ACTIVE PHASE

VIT+(LVD)

VIT-(LVD)

th(RSTL)in

RUN

WATCHDOG UNDERFLOW

tw(RSTL)out

RUN

RESET

SOURCE

EXTERNAL

RESET

LVD

RESET

WATCHDOG

RESET

INTERNAL RESET (4096 TCPU)

VECTOR FETCH

ACTIVE

PHASE

ACTIVE

PHASE

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