ST52T400 datasheet(43/94 Pages) STMICROELECTRONICS

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5.2 Reset

There are four sources of Reset:

- RESET pin (external source)

- WATCHDOG (internal source)

- POWER ON Reset (Internal source)

- BROWN OUT Reset (Internal source)

When a Reset event happens, all the registers are

set to the reset value and the user program

restarts from the beginning.

5.2.1 External Reset.

The Reset pin is an input pin. An internal reset

does not affect this pin.

A Reset signal originated by external sources is

recognized istantaneously. The RESET pin may

be used to ensure Vdd has risen to a point where

the MCU can operate correctly before the user

program is run. In working mode the Reset must

be set to ‘1’ (see Table 1.1)

5.2.2 Reset Operation.

The duration of a RESET condition is fixed at

1.000.000 internal CPU clock cycles (or 4096 in

case of BOD).

Following a Power-On Reset event, or after exit-

ing Halt Mode, a 1.000.000 CPU clock cycle delay

period is initiated in order to allow the oscillator to

stabilize and to ensure that recovery has taken

place from the Reset state.

A Pull up resistor of 100 K

Ω guarantees that

RESET pin is at level “1” when no HALT or

Power-On events occurred.

If an external resistor is connected to the RESET

pin a minimum value of 10K

Ω must be used.

After a RESET procedure is completed, the core

reads the instruction stored in the first 3 bytes of

the EPROM, which contains a JUMP instruction to

the EPROM address containing the first instruc-

tion of the user program. The Assembler tool auto-

matically generates this Jump instruction with the

first instruction address.

5.2.3 Power-on Reset (POR).

A Power-On Reset is generated by an on-chip

detection circuit. This circuit ensures that the

device is not started until Vdd has reached the

nominal level of 2.3V and allows the clock oscilla-

tor to stabilize.

Once 2.3V are reached, the Power-On circuit gen-

erates an internal RST signal that releases the

internal reset to the CPU and invokes a delay

counter of 1.000.000 CPU clock cycles, during

which the device is kept in RESET after Vdd has

risen.

A correct operation of Power-on detector is guar-

anteed if the slew rate of Vdd is 0.05 V/ms.

Note:

The power supply must fall below 0 V for

the internal POR circuit to detect the next rise of

Vdd.

At power on the POR is enabled by default.

POR is designed exclusively to cope with power-

up conditions and should not be used to detect a

drop in the power supply voltage, for which the

Brown-out Detector can be used instead.

Figure 5.2 Reset Block Diagram

POWER-ON

RESET

Vdd

RESET

COUNTER x

1.000.000

BROWN-

OUT

Vdd

COUNTER x

4096

WATCHDOG

INTERNALRESET

WATCHDOG RESET

BROWN-OUT RESET

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