5-Pin µP Supervisory Circuits with

Watchdog and Manual Reset

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7

Bidirectional

R

RE

ES

SE

ET

T Output

The MAX6316M/MAX6318MH/MAX6319MH are designed

to interface with µPs that have bidirectional reset pins,

such as the Motorola 68HC11. Like an open-drain output,

these devices allow the µP or other devices to pull the

bidirectional reset (

RESET) low and assert a reset condi-

tion. However, unlike a standard open-drain output, it

includes the commonly specified 4.7k

Ω pullup resistor

with a P-channel active pullup in parallel.

This configuration allows the MAX6316M/MAX6318MH/

MAX6319MH to solve a problem associated with µPs

that have bidirectional reset pins in systems where sev-

eral devices connect to

RESET (Figure 3). These µPs

can often determine if a reset was asserted by an exter-

nal device (i.e., the supervisor IC) or by the µP itself

(due to a watchdog fault, clock error, or other source),

and then jump to a vector appropriate for the source of

the reset. However, if the µP does assert reset, it does

not retain the information, but must determine the

cause after the reset has occurred.

The following procedure describes how this is done in

the Motorola 68HC11. In all cases of reset, the µP pulls

RESET low for about four external-clock cycles. It then

releases

RESET, waits for two external-clock cycles,

then checks

RESET’s state. If RESET is still low, the µP

concludes that the source of the reset was external

and, when

RESET eventually reaches the high state, it

jumps to the normal reset vector. In this case, stored-

state information is erased and processing begins from

scratch. If, on the other hand,

RESET is high after a

delay of two external-clock cycles, the processor

knows that it caused the reset itself and can jump to a

different vector and use stored-state information to

determine what caused the reset.

A problem occurs with faster µPs; two external-clock

cycles are only 500ns at 4MHz. When there are several

devices on the reset line, and only a passive pullup resis-

tor is used, the input capacitance and stray capacitance

can prevent

RESET from reaching the logic high state (0.8

be interpreted as external. The µP output stage is guaran-

teed to sink 1.6mA, so the rise time can not be reduced

considerably by decreasing the 4.7k

Ω internal pullup

resistance. See Bidirectional Pullup Characteristics in the

Typical Operating Characteristics.

The MAX6316M/MAX6318MH/MAX6319MH overcome

this problem with an active pullup FET in parallel with the

4.7k

Ω resistor (Figures 4 and 5). The pullup transistor

holds

RESET high until the µP reset I/O or the supervisory

circuit itself forces the line low. Once

RESET goes below

cating that the next transition for

RESET will be low to

high. When

RESET is released, the 4.7k

Ω resistor pulls

turns on. Once

2µs one-shot times out, the active pullup turns off. The

parallel combination of the 4.7k

Ω pullup and the

4.7k

Ω

MR**

RESET

RESET

CIRCUITRY

RESET***

WDI*

MAX6316M

MAX6318MH

MAX6319MH

68HC11

RESET

CIRCUITRY

RESET

OTHER DEVICES

RESET

MAX6316M/MAX6318MH

MAX6316M/MAX6319MH

ACTIVE-HIGH PUSH/PULL MAX6318MH/MAX6319MH

*

**

***

Figure 3. MAX6316M/MAX6318MH/MAX6319MH Supports Additional Devices on the Reset Bus