TCM811/TCM812

DS21615B-page 4

 2002 Microchip Technology Inc.

2.0

APPLICATIONS INFORMATION

The TCM811/TCM812 provides accurate V

ing and reset timing during power-up, power-down, and

brownout/sag conditions. These devices also reject

negative-going transients (glitches) on the power sup-

ply line. Figure 2-1 shows the maximum transient dura-

tion vs. maximum negative excursion (overdrive) for

glitch rejection. Any combination of duration and over-

drive that is under the curve will not generate a reset

signal. Combinations above the curve are detected as

a brownout or power-down. Transient immunity can be

improved by adding a 0.1 µF capacitor in close proxim-

ity to the V

FIGURE 2-1:

Maximum

Transient

Duration

vs.

Overdrive for Glitch Rejection at 25°C.

2.1

RESET Signal Integrity During Power-

Down

The TCM811 RESET push-pull output is valid to V

1.0V. Below this voltage the output becomes an "open

circuit" and does not sink current. This means CMOS

logic inputs to the microcontroller will be floating at an

undetermined voltage. Most digital systems are com-

pletely shutdown well above this voltage. However, in

situations where RESET must be maintained valid to

V

RESET to ground to discharge stray capacitances and

hold the output low (Figure 2-2). This resistor value,

though not critical, should be chosen such that it does

not appreciably load RESET under normal operation

(100 k

Ω will be suitable for most applications). Simi-

larly, a pull-up resistor to V

TCM812 to ensure a valid high RESET for V

1.1V.

FIGURE 2-2:

output of the TCM811 ensures that the RESET output

is valid to V

2.2

Controllers and Processors With

Bidirectional I/O Pins

Some microcontrollers have bi-directional reset pins.

Depending on the current drive capability of the control-

ler pin, an indeterminate logic level may result if there

is a logic conflict. This can be avoided by adding a

4.7 k

Ω resistor in series with the output of the TCM811/

TCM812 (Figure 2-3). If there are other components in

the system which require a reset signal, they should be

buffered so as not to load the reset line. If the other

components are required to follow the reset I/O of the

microcontroller, the buffer should be connected as

shown with the solid line.

FIGURE 2-3:

Interfacing the TCM811 to a Bi-

directional Reset I/O.

RESET COMPARATOR OVERDRIVE,

VTH - VDD (mV)

400

240

160

320

80

0

1

10

100

1000

Duration

Overdrive =

TCM81xL/M

TCM81xR/S/T

TCM811

V

DD

V

DD

100 k

Ω

RESET

GND

4

2

0.1µF

1

TCM811

V

DD

RESET

GND

RESET

GND

Buffered RESET

To Other System

Components

BUFFER

Micro

4.7 k

Ω

controller

0.1µF

4

2

1