MAX792/MAX820
Microprocessor and Nonvolatile
Memory Supervisory Circuits
8 _______________________________________________________________________________________
Detailed Description
Manual-Reset Input
Many µP-based products require manual-reset capabil-
ity, allowing the operator to initiate a reset. The manu-
al/external-reset input (
MR) ca
n connect directly to a
switch without an external pull-up resistor or debounc-
ing network.
MR
internally connects to a 1.30V com-
parator, and has a high-impedance pull-up to V
CC
, as
shown in Figure 1. The propagation delay from assert-
ing
MR
to reset asserted is typically 12µs. Pulsing
MR
low for a minimum of 25µs asserts the reset function
(see Reset Function section). The reset output remains
active as long as
MR
is held low, and the reset timeout
period begins after
MR
returns high (Figure 2). To pro-
vide extra noise immunity in high-noise environments,
pull
MR
up to V
CC
with a 100kΩ resistor.
Use
MR
as either a digital logic input or as a second low-
line comparator. Normal TTL/CMOS levels can be
wire-OR connected via pull-down diodes (Figure 3),
and open-drain/collector outputs can be wire-ORed
directly.
Monitoring the Regulated Supply
The MAX792/MAX820 offer two modes for monitoring
the regulated supply and providing reset and non-
maskable interrupt (NMI) signals to the µP: internal
threshold mode uses the factory preset low-line and
reset thresholds, and external programming mode
allows the low-line and reset thresholds to be pro-
grammed externally using a resistor voltage divider
(Figure 4).
Internal Threshold Mode
Connecting the reset-input/internal-mode select pin
(RESET IN
/INT
) to ground selects internal threshold
mode (Figure 4a). In this mode, the low-line and reset
thresholds are factory preset by an internal voltage
divider (Figure 1) to the threshold voltages specified in
the Electrical Characteristics (Reset Threshold Voltage
and Low-Line Threshold Voltage). Connect the low-line
output (
LOWLINE
) to the µP NMI pin, and connect the
active-high reset output (RESET) or active-low reset
output
(RESET
) to the µP reset input pin.
Additionally, the low-line input/reference-output pin
(LLIN/REFOUT) connects to the internal 1.30V refer-
ence in internal threshold mode. Buffer LLIN/REFOUT
with a high-impedance buffer to use it with external
circuitry. In this mode, when V
CC
is falling,
LOWLINE
is
guaranteed to be asserted prior to reset assertion.
External Programming Mode
Connecting RESET IN
/INT
to a voltage above 600mV
selects external programming mode. In this mode, the
low-line and reset comparators disconnect from the inter-
nal voltage divider and connect to LLIN/REFOUT and
RESET IN
/INT
, respectively (Figure 1). This mode allows
flexibility in determining where in the operating voltage
range the NMI and reset are generated. Set the low-line
and reset thresholds with an external resistor divider, as in
Figure 4b or Figure 4c. RESET typically remains valid for
V
CC
down to 2.5V;
RESET
is guaranteed to be valid with
V
CC
down to 1V.
Calculate the values for the resistor voltage divider in
Figure 4b using the following equations:
1) R3 = (1.30 x V
CC
MAX)/(V
LOW LINE
x I
MAX
)
2) R2 = [(1.30 x V
CC
MAX)/(V
RESET
x I
MAX
)] - R3
3) R1 = (V
CC
MAX/I
MAX
) - (R2 + R3).
First choose the desired maximum current through the
voltage divider (I
MAX
) when V
CC
is at its highest (V
CC
MAX). There are two things to consider here. First, I
MAX
contributes to the overall supply current for the circuit, so
you would generally make it as small as possible.
Second, I
MAX
cannot be too small or leakage currents will
adversely affect the programmed threshold voltages; 5µA
is often appropriate. Determine R3 after you have chosen
I
MAX
. Use the value for R3 to determine R2, then use both
R2 and R3 to determine R1.
For example, to program a 4.75V low-line threshold and a
4.4V reset threshold, first choose I
MAX
to be 5µA when
V
CC
= 5.5V and substitute into equation 1.
R3 = (1.30 x 5.5)/(4.75 x 5E-6) = 301.05kΩ.
301kΩ is the nearest standard 0.1% value. Substitute
into equation 2:
R2 = [(1.30 x 5.5)/(4.4 x 5E-6)] - 301kΩ = 23.95kΩ.
The nearest 0.1% resistor value is 23.7kΩ. Finally, sub-
stitute into equation 3:
R1 = (5.5/5E-6) - (23.7kΩ + 301kΩ) = 775kΩ.
The nearest 0.1% value resistor is 787kΩ. Determine the
actual low-line threshold by rearranging equation 1 and
plugging in the standard resistor values. The actual low-
line threshold is 4.75V and the actual reset threshold is
4.40V. An additional resistor allows the MAX792/MAX820
to monitor the unregulated supply and provide an NMI
before the regulated supply begins to fall (Figure 4c).
Both of these thresholds will vary from circuit to circuit
with resistor tolerance, reference variation, and compara-
tor offset variation. The initial thresholds for each circuit
will also vary with temperature due to reference and off-
set drift. For highest accuracy, use the MAX820.
