MAX817L/M, MAX818L/M, MAX819L/M*
+5V Microprocessor Supervisory Circuits
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_______________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
power-down with typical V
CC
rise and fall times.
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.
RESET is guaranteed to be a logic low for 0V < V
CC
<
V
RST
if V
BATT
is greater than 1V. Without a backup bat-
tery (V
BATT
= GND) RESET is guaranteed valid for
V
CC
≥ 1V. Once V
CC
exceeds the reset threshold an
internal timer keeps RESET low for the reset timeout
period, t
RP
. After this interval RESET returns high
(Figure 2).
If a brownout condition occurs (V
CC
drops below the
reset threshold), RESET goes low. Each time RESET is
asserted it stays low for at least the reset timeout peri-
od. Any time V
CC
goes below the reset threshold the
internal timer clears. The reset timer starts when V
CC
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
MR is low, and for t
RP
(200ms) after it returns high.
During the reset timeout period (t
RP
), MR’s state is
ignored if the battery freshness seal is enabled. MR has
an internal 63kΩ 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.
3) Bring V
CC
above the reset threshold and hold it
there until reset is deasserted following the reset
timeout period.
4) Bring V
CC
down again (Figure 3).
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
remains enabled until V
CC
is brought above V
RST
.
Figure 2. Power-Up and Power-Down Timing
