Datasheet

MAX791
Microprocessor Supervisory Circuit
_______________________________________________________________________________________ 7
Pin Description
PIN NAME FUNCTION
1 VBATT
2V
OUT
Output Supply Voltage. V
OUT
connects to V
CC
when V
CC
is greater than VBATT and V
CC
is above the reset
threshold. When V
CC
falls below VBATT and V
CC
is below the reset threshold, V
OUT
connects to VBATT.
Connect a 0.1µF capacitor from V
OUT
to GND.
3V
CC
Input Supply Voltage—+5V input
4 GND
5 BATT ON
6
–
P
—
F
—
O
–
7 PFI
8 SWT
9
–
M
—
R
–
10
–
L
—
O
—
W
— —
L
—
I
—
N
—
E
–
11 WDI
12
–
C
—
E
–
OUT
Chip-Enable Output.
–
C
—
E
–
OUT goes low only when
––
C
—
E
–
IN is low and V
CC
is above the reset threshold. If
–
C
—
E
–
IN is
low when reset is asserted,
–
C
—
E
–
OUT will stay low for 15µs or until
–
C
—
E
–
IN goes high, whichever occurs first.
13
––
C
—
E
–
IN
Backup-Battery Input. Connect to external battery or capacitor and charging circuit.
Ground. 0V reference for all signals.
Battery-On Output. Goes high when V
OUT
switches to VBATT. Goes low when V
OUT
switches to V
CC
. Connect
the base of a PNP through a current-limiting resistor to BATT ON for V
OUT
current requirements greater than
250mA.
Power-Fail Output. This is the output of the power-fail comparator.
–
P
—
F
—
O
–
goes low when PFI is less than 1.25V.
This is an uncommitted comparator, and has no effect on any other internal circuitry.
Power-Fail Input. This is the noninverting input to the power-fail comparator. When PFI is less than 1.25V,
–
P
—
F
—
O
–
goes low. Connect PFI to GND or V
OUT
when not used.
Set Watchdog-Timeout Input. Connect this input to V
OUT
to select the default 1.6s watchdog-timeout period.
Connect a capacitor between this input and GND to select another watchdog-timeout period. Watchdog-timeout
period = 2.1 x (capacitor value in nF) ms.
16
–
W
—
D
—
P
—
O
–
15
–
R
—
E
—
S
—
E
—
T
–
14
–
W
—
D
—
O
–
Manual-Reset Input. This input can be tied to an external momentary pushbutton switch, or to a logic gate out-
put.
–
R
—
E
—
S
—
E
—
T
–
remains low as long as
–
M
—
R
–
is held low and for 200ms after
–
M
—
R
–
returns high.
–
L
—
O
—
W
— —
L
—
I
—
N
—
E
–
Output goes low when V
CC
falls to 150mV above the reset threshold. The output can be used to gen-
erate an NMI if the unregulated supply is inaccessible.
Watchdog Input. WDI is a three-level input. If WDI remains either high or low for longer than the watchdog time-
out period,
–
W
—
D
—
O
–
goes low.
–
W
—
D
—
O
–
remains low until the next transition at WDI. Leaving WDI unconnected disables
the watchdog function. WDI connects to an internal voltage-divider between V
OUT
and GND, which sets it to mid-
supply when left unconnected.
Chip-Enable Input. The input to chip-enable gating circuit. Connect to GND or V
OUT
if not used.
Watchdog Output.
–
W
—
D
—
O
–
goes low if WDI remains either high or low longer than the watchdog-timeout period.
–
W
—
D
—
O
–
returns high on the next transition at WDI.
–
W
—
D
—
O
–
remains high if WDI is unconnected.
–
W
—
D
—
O
–
is also high
when
–
R
—
E
—
S
—
E
—
T
–
is asserted.
–
R
—
E
—
S
—
E
—
T
–
Output goes low whenever V
CC
falls below the reset threshold.
–
R
—
E
—
S
—
E
—
T
–
will remain low for typically 200ms
after V
CC
crosses the reset threshold on power-up.
Watchdog-Pulse Output. Upon the absence of a transition at WDI,
–
W
—
D
—
P
—
O
–
will pulse low for a minimum of 1ms.
–
W
—
D
—
P
—
O
–
precedes
––
W
—
D
—
O
–
by 70ns.