Programmable Logic Devices
305
to generate all possible minterms of the given number of input variables. The OR array is fixed and
the AND outputs are equally divided between available OR gates. For instance, a practical PAL device
may have eight input variables, 64 programmable AND gates and four fixed OR gates, with each OR
gate having 16 inputs. That is, each OR gate is fed from 16 of the 64 AND outputs. Figure 9.5 shows
the internal architecture of a PAL device that has four input lines, an array of eight AND gates at the
input and two OR gates at the output, to introduce readers to the arrangement of various building blocks
inside a PAL device and allow them a comparison between different programmable logic devices.
9.2.4 Generic Array Logic
A generic array logic (GAL) device is similar to a PAL device and was invented by
Lattice Semiconductor. It differs from a PAL device in that the programmable AND array of
D
C
B
A
Programmable AND-array
Y1
Y2
Hard-Wired OR-array
Figure 9.5 Internal architecture of a PAL device.
305
to generate all possible minterms of the given number of input variables. The OR array is fixed and
the AND outputs are equally divided between available OR gates. For instance, a practical PAL device
may have eight input variables, 64 programmable AND gates and four fixed OR gates, with each OR
gate having 16 inputs. That is, each OR gate is fed from 16 of the 64 AND outputs. Figure 9.5 shows
the internal architecture of a PAL device that has four input lines, an array of eight AND gates at the
input and two OR gates at the output, to introduce readers to the arrangement of various building blocks
inside a PAL device and allow them a comparison between different programmable logic devices.
9.2.4 Generic Array Logic
A generic array logic (GAL) device is similar to a PAL device and was invented by
Lattice Semiconductor. It differs from a PAL device in that the programmable AND array of
D
C
B
A
Programmable AND-array
Y1
Y2
Hard-Wired OR-array
Figure 9.5 Internal architecture of a PAL device.