Z-World Fox LP3500, User Manual

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User’s Manual 121
Parallel Port PF7 on the Rabbit 3000 chip controls whether the linear regulator is on or off.
Parallel Port PB0 senses whether there is an output from the linear regulator, and shuts off
the RS-232 (except RxE, which is used to “listen”), RS-485, A/D converter, and relay sec-
tions via Parallel Ports PB6, PG0, PG1, PG4, and PG5 to conserve power.
E.2 Batteries and External Battery Connections
The SRAM and the real-time clock have battery backup. Power to the SRAM and the real-
time clock (VRAM) is provided by two different sources, depending on whether the main
part of the LP3500 is powered or not. When the LP3500 is powered normally, and Vcc is
within operating limits, the SRAM and the real-time clock are powered from Vcc. If power
to the board is lost or falls below 2.75 V, the VRAM and real-time clock power will come
from either the onboard or the external battery. The reset generator circuit controls the source
of power by way of its
/RESET output signal.
A replaceable onboard 265 mA·h lithium battery provides power to the real-time clock
and SRAM when all external power is removed from the circuit board and the LP3500
processor is off. The drain on the battery is typically 46 µA under these worst-case condi-
tions, and so the expected life of the onboard battery is
The drain on the battery is typically less than 4 µA when external power is applied, and so
the expected LP3500 battery in-service life is
The primary role of the onboard battery is to keep the SRAM and the real-time clock func-
tional when the LP3500 processor is off. Even though there is limited capacity in the
onboard battery, and its circuit is in parallel with that of the external battery, Z-World
strongly recommends against relying on the onboard battery for any role besides that of
keeping the SRAM and the real-time clock functional when the LP3500 processor is off.
The short in-service life of the onboard battery highlights the importance of an external
battery, especially if the LP3500 is to operate in the power-save mode. A 2.8–3.3 V exter-
nal battery or equivalent “regulated” voltage is recommended.
There is provision for an external battery connection via pins 14 and 15 on header J2 on
the LP3500, and the LP3500 Prototyping Board has a plug-in header at J6 that can be used
to connect an external battery to the LP3500/Prototyping Board combination.
265 mA·h
46 µA
------------------------ 240 days. =
265 mA·h
4 µA
------------------------ 7.5 years. =