Microprocessor Supervisory Circuit
______________________________________________________________Pin Description
PIN
1
2
3
4
5
NAME
VBATT
V
OUT
V
CC
GND
BATT ON
–—
—–
PFO
PFI
FUNCTION
Backup-Battery Input. Connect to external battery or capacitor and charging circuit.
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.
Input Supply Voltage—+5V input
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. PFO 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,
–—
—–
PFO 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 1.6sec 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.
Manual-Reset Input. This input can be tied to an external momentary pushbutton switch, or to a logic gate out-
–——
— —–
–—
–
–—
–
put. RESET remains low as long as MR is held low and for 200ms after MR returns high.
– ——— –
— ——
LOW LINE 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, WDO goes low. WDO 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 Output. CE OUT goes low only when CE IN is low and V
CC
is above the reset threshold. If CE IN is
––
—
––
—
low when reset is asserted, CE OUT will stay low for 15µs or until CE IN goes high, whichever occurs first.
Chip-Enable Input. The input to chip-enable gating circuit. Connect to GND or V
OUT
if not used.
–— –
—
Watchdog Output. WDO goes low if WDI remains— – high or low longer than the watchdog—
–— –
—
– either
—
–—timeout period.
–
WDO returns high on the next transition at WDI. WDO remains high if WDI is unconnected. WDO is also high
–——
— —–
when RESET is asserted.
–——
— —–
–——
— —–
RESET Output goes low whenever V
CC
falls below the reset threshold. RESET 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, WDPO will pulse low for a minimum of 1ms.
–— —
—–
–— –
—
WDPO precedes WDO by 70ns.
MAX791
6
7
8
SWT
9
–—
–
MR
– ——— –
— ——
LOW LINE
10
11
WDI
12
13
14
––
—
CE OUT
––
—
CE IN
–— –
—
WDO
–——
— —–
RESET
–— —
—–
WDPO
15
16
_______________________________________________________________________________________
7
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
