+5V Microprocessor Supervisory Circuits

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9

_______________Detailed Description

General Timing Characteristics

Designed for 5V systems, the MAX817/MAX818/

MAX819 provide a number of microprocessor (µP)

supervisory functions (see the

Selector Guide on the

first page). Figure 2 shows the typical timing relation-

ships of the various outputs during power-up and

RESET Output

A µP’s reset input starts the µP in a known state. The

MAX817/MAX818/MAX819 µP supervisory circuits

assert a reset to prevent code-execution errors during

power-up, power-down, and brownout conditions.

internal timer keeps

RESET low for the reset timeout

(Figure 2).

reset threshold),

RESET goes low. Each time RESET is

asserted it stays low for at least the reset timeout peri-

returns above the reset threshold.

RESET both sources

and sinks current.

Manual Reset Input (MAX819)

Many µP-based products require manual reset capabil-

ity, allowing the operator, a test technician, or external

logic circuitry to initiate a reset. On the MAX819, a logic

low on

MR asserts reset. Reset remains asserted while

ignored if the battery freshness seal is enabled.

MR has

an internal 63k

Ω pull-up resistor, so it can be left open

if not used. This input can be driven with TTL/CMOS-

logic levels or with open-drain/collector outputs.

Connect a normally open momentary 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 the device is used in a noisy environ-

ment, connect a 0.1µF capacitor from

MR to GND to

provide additional noise immunity.

Note that

MR must be high or open to enable the bat-

tery freshness seal. Once the battery freshness seal is

enabled its operation is unaffected by

MR.

Battery Freshness Seal

The MAX817/MAX818/MAX819 battery freshness seal

disconnects the backup battery from internal circuitry

and OUT until it is needed. This allows an OEM to

ensure that the backup battery connected to BATT will

be fresh when the final product is put to use. To enable

the freshness seal on the MAX817 and MAX819:

1) Connect a battery to BATT.

2) Ground

PFO.

there until reset is deasserted following the reset

timeout period.

Use the same procedure for the MAX818, but ground

CE OUT instead of PFO. Once the battery freshness

seal is enabled (disconnecting the backup battery from

internal circuitry and anything connected to OUT), it

Figure 2. Power-Up and Power-Down Timing

CE OUT FOLLOWS CE IN

*MAX817/MAX819 ONLY.

RESET TO

CE OUT

DELAY**

PFO FOLLOWS PFI

** MAX818 ONLY.

Figure 3. Battery Freshness Seal Timing

RESET

CE OUT

(MAX818)

(EXTERNALLY HELD AT 0V)

CE OUT STATE LATCHED

FRESHNESS SEAL ENABLED

PFO

(MAX817/MAX819)

(EXTERNALLY HELD AT 0V)

PFO STATE LATCHED

FRESHNESS SEAL ENABLED