Acorn BBC Master 128, Руководство по обслуживанию

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Master Series Service Manual
Real time clock with RAN
Battery back-up
A 146818 RTC and RAM chip is provided with battery backed supply. The
chip operation is described in the manufacturers data sheet. In its
fully charged state, the internal lithium manganese dioxide battery
will provide over one year of back up power. Alternatively, components
may be fitted to the circuit board which provide for the fitting of a
rechargeable battery on the keyboard. This battery, like its support
components are not fitted as standard. If these are fitted by the user,
the warranty may be invalidated.
The keyboard mounted battery is charged whilst the computer is running
from the mains supply. The proportion of charge accummulated during
operation is shown in figure 6 for an initially flat battery. This is
an approximation dependent on conditions of use, as the actual charge
accumulated will vary with temperature and duty cycle (i.e. ratio of
time on charge, to time on discharge).
Figure 6
An over-charge prevention circuit is provided with the following
action:
a) Upon switch on, charging current of about 30mA is applied.
b) After approximately 15 minutes the charging current falls to 1mA.
c) "Trickle" charging continues at 1mA for as long as mains power is
applied.
The minimum charge burst is designed to provide battery back-up over a
weekend after just a few minutes operation.
21 Issue 1