Battery-Backed Memory; Battery-Backed Memory Block Diagram - NEC Advanced Personal Computer System Reference Manual

Processor PCB
2-28
2.6.2 Battery-Backed Memory
The BBM is composed of4K(two 2K X 8-bit chips) of Complementary Metal Oxide
Semiconductor (CMOS) static RAM and is addressable from AOOOO through COOOO
hexadecimal (HEX) in 32 4K visible sections of memory, each section containing
the same da ta. The APe uses partial address decoding of this 128 KB of memory,
which results in 4 KB of BBM. The BBM contains system information and is
protected from loss for at least two years by the battery that plugs into the Processor
PCB.
As shown in Figure 2-19, the BBM read/write operation is identical to that of the
main memory except for a BBM write protect circuit that safeguards the BBM from
unintentional data manipulation.
(HIGH) D8 TO DIS
DATA BUS _ _ _ _ _ _ _ _ _ _ _ _
--' - -_...J.f _____ _
BBM
F/F COMMAND
MW
+5 Vde
BATTERY
ADDRESS I TO 11
BHE
AO
L-..,.
(LOW) DO TO D7
WRT
I
BBM
WRITE
PROTECT
CIRCUIT
CS
CONTROL
CIRCUIT
VOLTAGE
CONTROl"
CIRCUIT
Figure 2-19 Battery-Backed Memory Block Diagram
~
2K X 8
J
L-.....,. 2K X 8 OUT
CMOS
STATIC
MEMORY
~
WR(LOW)
.----
CS Vee