Theory of Operation
System Battery Fail
For systems whose power is generated entirely from a large battery,
the AC power-fail detection circuit may be useful to generate an early
warning of battery failure. This warning should take place before the
system battery voltage to the ZT 8809A falls below 4.75 V, at which
time the system would reset.
The values of R1 and R2 must be changed so the input voltage to the
power monitor chip, the DS 1231-35, is above 2.5 V when power is
good. The following equation may be used to assign the resistor
values:
V
Sense
=
R1 + R2
x 2.15 V
R1
where V
Sense
=
battery voltage minimum allowed (4.8 V) minus
the diode drop (0.7 V) of CR1 or CR2
Example:
R1 is a variable 10 k
Ω
resistor. If we assume it
is set to a nominal 5 k
Ω
, and the diode CR1 is
replaced by a wire, then for V
Sense
of 4.8 V, R2
becomes > 6.16 k
Ω
. Choose 6.19 k
Ω
, the closest
standard value.
The battery may then be monitored at pin 1 of connector J5, leaving
pin 2 open. A non-maskable interrupt is generated upon detection of
low system voltage, at which time the interrupt service routine
indicates low battery and goes into an idle loop. Refer to Chapter 4,
"Application Examples," for an example using this feature.
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