STM1810 datasheet(7/20 Pages) STMICROELECTRONICS

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STM1810/1811/1812/1813/1815/1816/1817/1818

OPERATION

Reset Output

The STM181x asserts a reset signal to the Micro-

controller (MCU) whenever VCC goes below the

reset threshold (VRST), and is guaranteed valid

down to VCC = 1.0V (0° to 105°C). A microcontrol-

ler’s (MCU) reset input starts the MCU in a known

state. The STM1810 - STM1813/ STM1815 -

STM1818 Low Power Reset circuits assert reset to

prevent code-execution errors during power-up,

power-down, and brownout conditions (Figure

8., page 7).

During power-up, once VCC exceeds the reset

threshold an internal timer keeps RST low for the

reset time-out period, trec. After this interval, RST

returns high.

If VCC drops below the reset threshold, RST goes

low. Each time RST is asserted, it stays low for at

least the reset time-out period. Any time VCC goes

below the reset threshold, the internal timer clears.

The reset timer starts when VCC returns above the

reset threshold. Reset trec is also triggered by an

externally initiated rising edge on the RST pin

(STM1813/STM1818), following a low signal of

1.5µs minimum duration.

Push-Button Detect Reset (STM1813/1818)

Many systems require push-button reset capability

(Figure 9., page 8), allowing the user or external

logic circuitry to initiate reset. On the STM1813/

STM1818, a logic low on RST held for greater than

1.5µs asserts a reset. RST deasserts following a

100ms minimum reset time-out delay (trec). A

manual reset input shorter than 1.5µs may release

RST without the 100ms minimum reset time-out

delay. To facilitate use with mechanical switches,

the

STM1813/STM1818

contain

internal

de-

bounce circuitry. A debounced waveform is shown

in Figure 10., page 8 The RST output has an inter-

nal 5.5k

Ω pull-up resistor.

Interfacing to Bidirectional Microcontrollers

(MCU’s)

As the RST output on the STM1811/STM1816 is

open drain, these devices interface easily with

MCU’s that have bidirectional reset pins. Connect-

ing the µP supervisor’s reset (RST) output directly

to the microcontroller’s reset (RST) pin allows ei-

ther device to assert reset (Figure 11., page 8). No

external pull-up resistor is required, as it is within

the STM1811/STM1816.

Negative Going VCC Transients

The STM181x are relatively immune to negative-

going

VCC

transients

(glitches).

Figure

20., page 13 shows typical transient duration ver-

sus reset comparator overdrive (for which the

STM181x will NOT generate a reset pulse). The

graph was generated using a negative pulse ap-

plied to VCC, starting at 0.5V above the actual re-

set threshold

and

ending

below it by the

magnitude indicated (comparator overdrive). The

graph indicates the maximum pulse width a nega-

tive VCC transient can have without causing a re-

set pulse. As the magnitude of the transient

increases (further below the threshold), the maxi-

mum allowable pulse width decreases. Any com-

bination of duration and overdrive which lies under

the curve will NOT generate a reset signal. Typi-

cally, a VCC transient that goes 100mV below the

reset threshold and lasts 20µs or less will not

cause a reset pulse. A 0.1µF bypass capacitor

mounted as close as possible to the VCC pin pro-

vides additional transient immunity.

Valid RST Output Down to VCC = 0V

When VCC falls below 1V, the RST output no long-

er sinks current, but becomes an open circuit. In

most systems this is not a problem, as most MCUs

do not operate below 1V. However, in applications

where RST output must be valid down to 0V, a

pull-down resistor may be added to hold the RST

output low (see Figure 12., page 9). This resistor

must be large enough to not load the RST output,

and still be small enough to pull the output to

ground. A 100k

Ω resistor is recommended.

Note: The same situation applies for the active-

high RST of the STM1810/1812. A 100k

Ω pull-up

resistor to VCC should be used if RST must remain

valid for VCC < 1.0V.

Figure 8. Reset Timing Diagram

Note: 1. RST for STM1812 and STM1817

AI09653

RST

(1)

VCC

VRST

VCC (min)

trec

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