68HC11/Bidirectional-Compatible

µP Reset Circuit

6

_______________________________________________________________________________________

MR has an internal 63k

Ω pull-up resistor, so it can be

left open if not used. Connect a normally open momen-

tary switch from MR to GND to create a manual reset

function; external debounce circuitry is not required. If

MR is driven from long cables or if the device is used in

a noisy environment, connecting a 0.1µF capacitor from

MR to ground provides additional noise immunity.

__________Applications Information

In addition to issuing a reset to the µP during power-up,

power-down, and brownout conditions, these devices

are relatively immune to short-duration negative-going

transients (glitches).

The Typical Operating Character-

istics show the Maximum Transient Duration vs. Reset

Threshold Overdrive, for which reset pulses are not

generated. The graph was produced using negative-

the programmed reset threshold by the magnitude

indicated (reset threshold overdrive). The graph shows

transient may typically have without causing a reset

pulse to be issued. As the amplitude of the transient

increases (i.e., goes farther below the reset threshold),

the maximum allowable pulse width decreases. A 0.1µF

tional transient immunity.

Ensuring a Valid

RESET Output

current—it becomes an open circuit. Therefore, high-

impedance CMOS-logic inputs connected to RESET

can drift to undetermined voltages. This presents no

problem in most applications, since most µP and other

applications where RESET must be valid down to

cause any stray leakage currents to flow to ground,

holding RESET low (Figure 3). R1’s value is not critical;

100k

Ω is large enough not to load RESET and small

enough to pull RESET to ground.

Figure 2. MAX6314 Supports Additional Devices on the Reset Bus

4.7k

MR

RESET

RESET

CIRCUITRY

MAX6314

68HC11

RESET

CIRCUITRY

RESET

OTHER DEVICES

RESET

MAX6314

GND

RESET

R1