Epson PX-8 - 5.6 The BIOS Interface

REV.-A
2.9.2 Battery Voltage Detector Circuit
The
battery voltage delection circuit detects two specific signals from low voltage (approximately
4.7V) and a recharge start voltage (approximately 5V)
When these voltage are sensed, following sequence is initialed.
•
Low voltage .......................... approximately +4.7V
When this voltage is detected, the 7508 forces the current computer operation to an end at an
appropriate point (a point at which the terminated operation can be properly resumed) and
switch the main battery to the auxiliary battery.
• Recharge
start voltage ............ approximately +5V
When this voltage is detected while the AC adaptor is connected, the 7508 switches the
charge from trickle to normal mode.
2.9.2.1 Circuit Operations
Fig.
2-90 shows the battery voltage detector circuit. The battery voltage VB is fed to Ithe divider
circuit, which consists of R69 and R57, through the fuse F1 ,and the transistor 032. The divided
voltage is supplied to channel AN1 of the A-D converter. The voltage drop across F1 and 032 is
negligible and
the voltage at the AN 1 terminal VAN1 is given as follows:
VB·R57
AN1= R69 + R59
...... AN 1 ~ 0.36VB
• Low voltage: The converted digital value, which is equivalent to the low voltage of +4.7V, is
D9(H)
at a reference voltage of +2.OV (the digital equivalent is FF(H)). This value is approximate-
ly equivalent to 1.69V. Thus, the VB voltage of 4.7V should generate a potential of 1.75V at
terminal AN 1 as determined by the following expression:
VB
1.69
(x) = 0.36
• Recharge start voltage: The recharge start voltage of +5V is converted to a digital value of
E6(H). Thus, the potential at terminal AN 1 should be 1.8V.
6
Sub-CPU
Fig. 2-90
2-88