
PSD3XX Famly
2-11
Programmable
Address
Decoder (PAD)
The PSD3XX consists of two programmable arrays referred to as PAD A and PAD B
(Figure 3). PAD A is used to generate chip select signals derived from the input address to
the internal EPROM blocks, SRAM, I/O ports, and Track Mode signals. All its I/O functions
are listed in Table 3 and shown in Figure 3. PAD B outputs to Ports B and C for off-chip
usage.
PAD B can also be used to extend the decoding to select external devices or as a random
logic replacement. The input bus to both PAD A and PAD B is the same. By using the
PSDsoft Development Tools software, each programmable bit in the PAD array can have
one of three logic states of 0, 1, and don’t care (X). In a user’s logic design, both PADs can
share the same inputs, using the X for input signals that are not supposed to affect other
functions. The PADs use reprogrammable CMOS EPROM technology and can be
programmed and erased (if using windowed packages) by the user.
Multiplexed Address/Data
Non-Multiplexed Address/Data
8-bit Data Bus
I/O or low-order address
lines or low-order multiplexed
address/data byte
Port A
D0–D7 data bus byte
Port B
I/O and/or CS0–CS7
I/O and/or CS0–CS7
AD0/A0–AD7/A7
Low-order multiplexed
address/data byte
Low-order address bus byte
AD8/A8–AD15/A15
High-order multiplexed
address data byte
High-order address bus byte
16-bit Data Bus
I/O or low-order address
lines or low-order multiplexed
address/data byte
Port A
Low-order data bus byte
Port B
I/O and/or CS0–CS7
High-order data bus byte
AD0/A0–AD7/A7
Low-order multiplexed
address/data byte
Low-order address bus byte
AD8/A8–AD15/A15
High-order multiplexed
address/data byte
High-order address bus byte
Table 2.
PSD30X Bus
and Port
Configuration
Options
Multiplexed Address/Data
Non-Multiplexed Address/Data
8-bit Data Bus
I/O or low-order address
lines or low-order multiplexed
address/data byte
Port A
D0–D7 data bus byte
Port B
I/O and/or CS0–CS7
I/O and/or CS0–CS7
AD0/A0–AD7/A7
Low-order multiplexed
address/data byte
Low-order address bus byte
A8–A15
High-order address bus byte
High-order address bus byte
Table 2a.
PSD31X Bus
and Port
Configuration
Options