Theory of Operation
BATTERY
The ZT 8809A contains a socket for an optional 1 Amp-hour 3.9 V
lithium battery. As described above, the real-time clock and 32 Kbyte
static RAM are protected by the battery if jumper W12 is installed.
Jumpers allow the RAM sockets and the configurable RAM/EPROM
socket (when configured for RAM) also to be battery-backed. Jumper
W35B battery backs the RAM sockets and W38A battery backs the
RAM/EPROM socket.
Battery life depends on the current requirement per device powered
by the battery. Battery life is calculated for both typical and worst-
case situations in the following equations, assuming the STD bus
system is powered for 8 hours a day. These equations show how
battery life decreases with a greater current load. This illustrates how
important it is to battery back only critical devices in the system.
Keep in mind that the following minimum battery life times are seen
only at the highest temperatures (65˚ C).
1.
Real-time clock and 32 Kbyte static RAM
a)
Typical Data Retention Time:
Total Current Drain = Clock + RAM + Buffer
=
1 uA + 1 uA + 8.0 uA
= 10 uA
Battery Life = 1 AHr x 1 Day
x 1
-----
---
(24-8)Hr
10 uA
= 6250 Days (17 years)
Note: The typical 10-year shelf life of the battery would
limit this figure.
3-23
