ST7263 datasheet(18/109 Pages) STMICROELECTRONICS

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ST7263

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3.2 RESET

The Reset procedure is used to provide an orderly

software start-up or to exit low power modes.

Three reset modes are provided: a low voltage

(LVD) reset, a watchdog reset and an external re-

set at the RESET pin.

A reset causes the reset vector to be fetched from

addresses FFFEh and FFFFh in order to be loaded

into the PC and with program execution starting

from this point.

An internal circuitry provides a 4096 CPU clock cy-

cle delay from the time that the oscillator becomes

active.

3.2.1 Low Voltage Detector (LVD)

Low voltage reset circuitry generates a reset when

VDD is:

below VIT+ when VDD is rising,

below VIT- when VDD is falling.

During low voltage reset, the RESET pin is held low,

thus permitting the MCU to reset other devices.

The Low Voltage Detector can be disabled by set-

ting the LVD bit of the Miscellaneous Register.

3.2.2 Watchdog Reset

When a watchdog reset occurs, the RESET pin is

pulled low permitting the MCU to reset other devic-

es in the same way as the low voltage reset (Fig-

ure 11).

3.2.3 External Reset

The external reset is an active low input signal ap-

plied to the RESET pin of the MCU.

As shown in Figure 14, the RESET signal must

stay low for a minimum of one and a half CPU

clock cycles.

An internal Schmitt trigger at the RESET pin is pro-

vided to improve noise immunity.

Table 6. List of sections affected by RESET, WAIT and HALT (Refer to 3.5 for Wait and Halt Modes)

Section

RESET

WAIT

HALT

CPU clock running at 8 MHz

Timer Prescaler reset to zero

Timer Counter set to FFFCh

All Timer enable bit set to 0 (disable)

Data Direction Registers set to 0 (as Inputs)

Set Stack Pointer to 013Fh

Force Internal Address Bus to restart vector FFFEh,FFFFh

Set Interrupt Mask Bit (I-Bit, CCR) to 1 (Interrupt Disable)

Set Interrupt Mask Bit (I-Bit, CCR) to 0 (Interrupt Enable)

Reset HALT latch

Reset WAIT latch

Disable Oscillator (for 4096 cycles)

Set Timer Clock to 0

Watchdog counter reset

Watchdog register reset

Port data registers reset

Other on-chip peripherals: registers reset

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