Transcript Logical Circuits

Logical Circuits
Philip Gebhardt
3/15/2011
Logic Circuits
• Negative, Positive, and Complimentary circuits
• Logic Gates
• Programmable Logic Devices
Logic Circuits are built from Transistors
• Transistors act as the switches in circuits
• Controlled by a logical signal from another logic circuit
• The most popular transistor used today
• The metal oxide semiconductor field-effect transistor
(MOSFET)
• Two different types: n-channel (NMOS) and p-channel
(PMOS)
Vx
Vx
X = LOW
X = HIGH
X = HIGH
X = LOW
Negative (NMOS)
Positive (PMOS)
NMOS Realization of a NOT gate
VDD
VDD
Vf = 0
Vf
Vx
VDD
on
Vf = VDD
off
X
Vf
on
off
off
on
NMOS Realization of the NAND gate
VDD
Vf
Vy
Vx
Vx
Vy
Vf
0
0
1
0
1
1
1
0
1
1
1
0
NMOS Realization of the AND gate
VDD
Vf
VDD
Vf ’
Vy
Vx
Vx
Vy
Vf’
Vf
0
0
1
0
0
1
1
0
1
0
1
0
1
1
0
1
NMOS Realization of the AND gate
VDD
VDD
Vf
Vy
NOT Gate
Vx
NAND Gate
NMOS Realization of the NOR gate
VDD
Vf
Vx
Vy
NMOS Realization of the OR gate
VDD
VDD
Vf
Vx
Vy
Complementary MOS (CMOS)
VDD
VDD
Vy
Vf
Vx
Vy
Vf
Vx
CMOS
NAND gate
CMOS
NOR gate
Logic Devices
VDD
Vf
Vy
• 7400 Series Standard Chips
• Programmable Logic Devices
Vx
• Programmable Logic Array (PLA)
• Programmable Array Logic (PAL)
• Complex Programmable Logic Devices (CPLDs)
Logic Devices
• 7400 Series Standard Chips and all other Logic
Devices measured in NAND equivalency gates
Logic Devices
• 7400 Series Standard Chips and all other Logic
Devices measured in NAND equivalency gates
• Ex: The macrocells in PALs are often about 20
equivalent NAND gates
7400 Series Standard Chips
• External pins used as input/ouput
• Two pins used for VDD and GND leads
• Multiple chips with different logical gates can
be connected to realize logical functions
F = X1X2 + X2’X3
VDD
X1
X2
X3
F
• Not very efficient for operations with large
logical capacity
• Mostly used today as buffers from other
logical circuits
Programmable Logic Devices
• PLDs are general-purpose computer chips
designed to implement logical circuits.
• Devices with a lot of circuitry and logical gates
• Unlike 7400 series chips, has Non-fixed structure
• Can be considered a black box with wide
range of logical combinations
• Programmable Logic Array (PLA)
• Programmable Array Logic (PAL)
PLD as a Black Box
Input Variables
.
.
.
Logic Gates
And Switches
.
.
.
Output Values
(function values)
Programmable Logic Arrays (PLAs)
• PLAs can be broken down into two planes of
wires which pass logic gates
• AND plane: logic variables (and their compliments)
pass through the AND gates
• OR plane: the resultants from the AND gates then pass
through the OR gates
• Switches programmed by the user determine
which variable pass through which gates
Generic PLA
Xn
.
.
.
Input
(buffers & inverters)
.
.
.
AND plane
.
.
.
OR plane
…
fm
PLA Implementation
X1, X’1, … Xn, X’n
f1 … f m
Programmable Array Logic
• Unlike PLAs, PALs are not fixed on both logical
planes
• Generally only the AND plane fixed
• Less flexibility but more cost efficient and
better performance
• Uses extra circuitry after OR gates for more versatility
• Macrocells
• Flip-flops, MUXs, more buffers
PAL Structure
Programmable
switches determine
AND gate results…
Fixed OR gates
connect to
macrocells
Example of macrocell
MUX
2:1
PAL block
D
Q
CLK
Back to AND plane
fi
Benefits Limitations of PLAs and PALs
• PLAs
• Efficient for the area needed for implementation
• Too many switches decreases performance and
increases costs for manufacturing
• PALs
• Less expensive and easier to manufacture compared to
PLAs
• Macrocells allow for implementation of logical circuits
with multiple levels or phases
• Less flexibility requires extra circuitry