Simple I/O Decoding and Interfacing Using PLDs:
Programmable logic is particularly useful for decoding the addresses and
control lines from a processor, because it can be used to activate the chip
enable signals for the various memories and I/O chips in a system. PLDs are
more flexible than standard logic for several reasons. Each of the PLD outputs
can be programmed to go active when the inputs are in a particular state, such
as a particular address or range of addresses. The same functions that can be
decoded in a PLD would generally take several standard logic chips. This is
because many of the inputs, such as address lines, are common to several of
the output logic functions. Also, because the devices are programmable, the
decoding logic can be changed without changing the wiring on the printed
circuit board. These characteristics have made PLDs very popular, which has
in turn brought their prices down to levels that are comparable to standard
logic solutions. The only disadvantage to using PLDs is that they require
software to “compile” the logic into binary patterns and an instrument,
equivalent to a PROM programmer, which can program the device with those
patterns. Each type of device requires a special programming procedure,
which may be unique to the manufacturer of the PLD. Generic compilers and
programmers are available, but there are devices that can only be programmed
using the manufacturers proprietary software or programmer.
Programmable logic is particularly useful for decoding the addresses and
control lines from a processor, because it can be used to activate the chip
enable signals for the various memories and I/O chips in a system. PLDs are
more flexible than standard logic for several reasons. Each of the PLD outputs
can be programmed to go active when the inputs are in a particular state, such
as a particular address or range of addresses. The same functions that can be
decoded in a PLD would generally take several standard logic chips. This is
because many of the inputs, such as address lines, are common to several of
the output logic functions. Also, because the devices are programmable, the
decoding logic can be changed without changing the wiring on the printed
circuit board. These characteristics have made PLDs very popular, which has
in turn brought their prices down to levels that are comparable to standard
logic solutions. The only disadvantage to using PLDs is that they require
software to “compile” the logic into binary patterns and an instrument,
equivalent to a PROM programmer, which can program the device with those
patterns. Each type of device requires a special programming procedure,
which may be unique to the manufacturer of the PLD. Generic compilers and
programmers are available, but there are devices that can only be programmed
using the manufacturers proprietary software or programmer.
