Microprocessor and Non-Volatile

Memory Supervisory Circuits

8

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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) can connect directly to a

switch without an external pull-up resistor or debounc-

ing network. MR internally connects to a 1.30V com-

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

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,

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

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

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

Calculate the values for the resistor voltage divider in

Figure 4b using the following equations:

First choose the desired maximum current through the

contributes to the overall supply current for the circuit, so

you would generally make it as small as possible.

adversely affect the programmed threshold voltages; 5µA

is often appropriate. Determine R3 after you have chosen

R2 and R3 to determine R1.

For example, to program a 4.75V low-line threshold and a

R3 = (1.30 x 5.5)/(4.75 x 5E-6) = 301.05k

Ω.

301k

Ω is the nearest standard 0.1% value. Substitute

into equation 2:

R2 = [(1.30 x 5.5)/(4.4 x 5E-6)] - 301k

Ω = 23.95kΩ.

The nearest 0.1% resistor value is 23.7k

Ω. Finally, sub-

stitute into equation 3:

R1 = (5.5/5E-6) - (23.7k

Ω + 301kΩ) = 775kΩ.

The nearest 0.1% value resistor is 787k

Ω. 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.