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Technical Supplement
9.10.9.2 Address Decoding
The address decoding circuit is illustrated in Figure 9-16.
The CPU has a 64 Kbyte address range of 0-FFFF. RAM, EPROM, and I/O ports share this space.
The address decoding circuit splits up this space and output enable lines to the RAM, EPROM, and
I/O ports.
U30A generates the static RAMs active low enable signal, RAMEN. When address lines A13, A14,
A15 are all high, U30As output goes low, enabling the RAM. This occurs for the 8K address range of
E000-FFFF.
U30B and U28 are used to generate the input port and output port active low enable signals EXINEN
and EXOUTEN. When address lines A15, A14, A11, and A10 are high, and A13 is low, U28
becomes enabled. With U28 enabled, one of the 8 outputs is set low. The output to go low is selected
by pins A, B, and C. They form a 3-bit binary number with pin C being the most significant bit. So
when address line A12 is high, WR active (low), and RD inactive (high), a binary 5 is produced on
pins A, B, and C, forcing output Y5 (EXOUTEN) low. This enables the output port for writing.
When address line A12 is high, WR inactive, and RD active, a binary 3 is produced on pins A, B, and
C, forcing output Y3 (EXINEN) low. Note that in both previous conditions, A15, A14, A12, A11,
and A10 are high and A13 is low.
The input port and the output port both share the same 1 Kbyte address space of DC00-DFFF. When
data are written to that address, the output port enable signal EXOUTEN is activated. But when data
are read from the same address, EXINEN is activated. Because the CPU is configured to use a 16-bit
bus, except for RAM, any even address in the DC00-DFF range could be used for external port
access. In other words, reading or writing address DC00, DC02, DC04, etc., will all produce the same
9-16
ADDRESS DECODING
U28
1
WR
2
RD
3
A10
6
4
5
74HC138
A12
A13
A14
A15
U30B
A14
3
A15
4
6
A11
5
74HC10
TP71
Figure 9-16: Address Decoding Circuit
15
A
Y0
14
B
Y1
13
C
Y2
12
Y3
11
Y4
DC00-DFFF
10
G1
Y5
9
G2A
Y6
7
G2B
Y7
U30A
1
2
12
13
74HC10
U30C
9
10
8
11
74HC10
10K
R134
VCC
EXINEN
19
EXOUTEN
20
20 HDR
TO: N-20
AUX PCB
E000-FFFF
RAMEN
TP74
0000-DBFF
ROMEN
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