PROGRAMMABLE ARRAY LOGIC
The PAL device is a special case of PLA which
has a programmable AND arrayand a fixed OR array. The basic structure of Rom is
same as PLA. It is cheap comparedto PLA as only the AND array is programmable.
It is also easy to program a PALcompared to PLA as only AND must be programmed.
The figure 1 below shows a segment of an
unprogrammed PAL. The input bufferwith non inverted and inverted outputs is
used, since each PAL must drive many ANDGates inputs. When the PAL is
programmed, the fusible links (F1, F2, F3…F8)
areselectively blown to leave the desired connections to the AND Gate inputs.
Connectionsto the AND Gate inputs in a PAL are represented byXs, as shown here:
Figure 1: segment of an unprogrammed and
As an example, we will use the PAL segment of
figure 1 to realize the function I1I2‘+I1I2.
theXs indicate that the I1 and I2‘ lines are
connected to the first AND Gate, and the I1‘ and I2
lines are connected to the other Gate.
Typical combinational PAL have 10 to 20 inputs
and from 2 to 10 outputs with 2to 8 AND gates driving each OR gate. PALs are
also available which contain D flip-flopswith inputs driven from the
programming array logic. Such PAL provides a convenientway of realizing
sequential networks. Figure 2 below shows a segment of a sequentialPAL. The D
flip-flop is driven from the OR gate, which is fed by two AND gates.
Theflip-flop output is fed back to the programmable AND array through a buffer.
Thus theAND gate inputs can be connected to A, A‘, B, B‘, Q, or Q‘. The Xs on the diagramshow the realization of
the next-state equation.
Q+ = D = A‘BQ‘ + AB‘Q
The flip-flop output is connected to an inverting
tristate buffer, which is enabled when
EN = 1
Figure 3 below shows a logic diagram for a
typical sequential PAL, the 16R4.This PAL has an AND gate array with 16 input
variables, and it has 4 D flip-flops. Eachflip-flop output goes through a
tristate-inverting buffer (output pins 14-17). One input(pin 11) is used to
enable these buffers. The rising edge of a common clock (pin 1) causesthe
flip-flops to change the state. Each D flip-flop input is driven from an OR
gate, andeach OR gate is fed from 8 AND gates. The AND gate inputs can come
from the externalPAL inputs (pins2-9) or from the flip-flop outputs, which are
fed back internally. Inaddition there are four input/output (i/o) terminals
(pins 12,13,18 and 19), which can beused as either network outputs or as inputs
to the AND gates. Thus each AND gate canhave a maximum of 16 inputs (8 external
inputs, 4 inputs fed back from the flip-flopoutputs, and 4 inputs from the i/o
terminals). When used as an output, each I/O terminalis driven from an
inverting tristate buffer. Each of these buffers is fed from an OR gateand each
OR gate is fed from 7 AND gates. An eighth AND gate is used to enable the