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TELCOM SEMICONDUCTOR, INC.

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PRELIMINARY INFORMATION

TCM809

TCM810

3-PIN

from RESET to ground to discharge stray capacitances and

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

not critical, should be chosen such that it does not apprecia-

bly load RESET under normal operation (100k

Ω will be

suitable for most applications). Similarly, a pull-up resistor to

Processors With Bidirectional I/O Pins

Some

µP's (such as Motorola 68HC11) have bidirec-

tional reset pins. Depending on the current drive capability

of the processor pin, an indeterminate logic level may result

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

4.7k resistor in series with the output of the TCM809/810

(Figure 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

µP, the buffer should

be connected as shown with the solid line.

APPLICATIONS INFORMATION

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

out/sag conditions, and rejects negative-going transients

(glitches) on the power supply line. Figure 1 shows the

maximum transient duration vs. maximum negative excur-

sion (overdrive) for glitch rejection. Any combination of

duration and overdrive which lies 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 capacitor in close proximity to

RESET Signal Integrity During Power-Down

Below this voltage the output becomes an "open circuit" and

does not sink current. This means CMOS logic inputs to the

µP will be floating at an undetermined voltage. Most digital

systems are completely shutdown well above this voltage.

However, in situations where RESET must be maintained

Figure 1. Maximum Transient Duration vs.

Overdrive for Glitch Rejection at 25

Figure 3. Interfacing to Bidirectional Reset I/O

RESET COMPARATOR OVERDRIVE,

(VTH - VCC (mV)

400

240

160

320

80

0

1

10

100

1000

Duration

Overdrive

TCM8xxLM

TCM8xxR/S/T

TCM809

R1

100k

RESET

GND

TCM809

RESET

GND

RESET

GND

BUFFERED RESET

TO OTHER SYSTEM

COMPONENTS

BUFFER

µP

4.7k