Microprocessor and Non-Volatile

Memory Supervisory Circuits

14

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Alternative Chip-Enable Gating

Using memory devices with both CE and CE inputs

allows the MAX792/MAX820 CE propagation delay

to be bypassed. To do this, connect CE IN to ground,

input of each memory device (Figure 11). The CE input

of each memory device then connects directly to the

chip-select logic, which does not have to be gated by

the MAX792/MAX820.

Interfacing to µPs with Bidirectional

Reset Inputs

µPs with bidirectional reset pins, such as the Motorola

68HC11 series, can contend with the MAX792/MAX820

RESET output. If, for example, the MAX792/MAX820 RESET

output is asserted high and the µP wants to pull it low,

indeterminate logic levels may result. To avoid this,

connect a 4.7k

Ω resistor between the MAX792/MAX820

RESET output and the µP reset I/O, as in Figure 12.

Buffer the MAX792/MAX820 RESET output to other sys-

tem components.

While issuing resets to the µP during power-up, power-

down, and brownout conditions, these supervisors are

transients (glitches). It is usually undesirable to reset

Figure 13 shows maximum transient duration vs. reset-

comparator overdrive, for which reset pulses are not

generated. The graph was produced using negative-

reset threshold by the magnitude indicated (reset-

comparator overdrive). The graph shows the maximum

cally have without causing a reset pulse to be issued.

As the amplitude of the transient increases (i.e., goes

farther below the reset threshold), the maximum allow-

that goes 100mV below the reset threshold and lasts for

30µs or less will not cause a reset pulse to be issued.

pin provides additional transient immunity.

MAX792

MAX820

13

14

12

+5V

GND

3

CE IN

CE OUT

CE

CE

CE

CE

CE

CE

CE

CE

RAM 1

RAM 2

RAM 3

RAM 4

ACTIVE-HIGH CE

LINES FROM LOGIC

THE NUMBER OF RAMS.

*

Figure 11. Alternate CE Gating

BUFFER

TO OTHER

SYSTEM RESET

INPUTS

4.7k

1

RESET

RESET

GND

GND

12

MAX792

MAX820

µP

3

Figure 12. Interfacing to µPs with Bidirectional RESET Pins

Figure 13. Maximum Transient Duration without Causing a

Reset Pulse vs. Reset-Comparator Overdrive

100

0

10

100

10,000

40

20

80

60

1000