3. Overview
The various design options for digital systems
3
Design options
ASIC
Full
Custom
Cell-
based
Gate-
array-
based
Field Programmable
PLDs
ROM PAL PLA
CPLDs FPGAs
4. Hierarchical System Design
Digital system designers widely use a hierarchical structure
to design digital systems
The essence is to partition the system into smaller
independent sub-structures which combine to perform the
same functionality as the original system
The design hierarchy can be classified into four level:
System level
Register transfer level
Logic gate level
Circuit level
4
5. Hierarchical System Design
Differences in the various hierarchies
5
Design
levels
Operand size Processing
time range(s)
Logic devices
System Bytes/byte blocks 10−3
𝑎𝑛𝑑 10−6
Microprocessors/
microcomputers,
memory devices,
timers
RTL Bits/bytes 10−8
𝑎𝑛𝑑 10−9
Decoders, encoders,
multiplexers,
registers, counters
Logic gate Bits 10−9
𝑎𝑛𝑑 10−11
Basic logic gates,
flip-flops, latches
circuit Bits 10−10
𝑎𝑛𝑑 10−12
MOSFETs, BJTs
SB
RB
Q
QB
S
G
D
G
S
D
register
register
Combinational
circuit
mux
CLK A CLK B
X F
𝑉𝐷𝐷
RAM
ROM
Parallel
I/O port
DMA
controller
Clock
microP
SYSTEM LEVEL
RTL LEVEL
LOGIC LEVEL
CIRCUIT LEVEL
6. Design Options for Digital Systems
A variety of options are available for designing a given
digital system
These options are used to:
Reduce hardware size
Reduce power consumption
Increase system performance
6
Design options of digital systems
ASIC
Full-custom Cell-based
Cell library
Compiled macros
(RAM/ROM/PLA)
Platform IP
Gate array
microP/DSP Field-programmable devices
PLD
ROM PAL PLA
FPGA CPLD
7. Design Options for Digital Systems
Comparison of various design options in terms of time to
market and cost
7
PLDs
FPGAs, GAs
Cell-based ASICs
Full-custom
ASICs
Timetomarket,Cost
Design flexibility, Process complexity,
Density, speed, NRE cost
8. Design Options for Digital Systems
Comparison of design options in terms of cost and design
flexibility
8
cost
Product volume
FPGAs
ASICs
k
GAs
FPGAs
CPLDs
Cell-based
ASICs
Full-custom
ASICs
PLDs
Designflexibility
Easy to use
Cost = NRE + Fixed + recurring
NRE +
fixed cost
k = division of product volume
9. ASIC Designs: Full-Custom Design
In full custom design, each transistor and its layout is
designed carefully in order to achieve the best
performance
Full custom offers highest performance and reduced area
9
10. ASIC Designs: Cell-Based Design
Cell based design is composed of a set of predefined cells
with layouts known as standard cells
The basic cell types of a typical cell library is as below
10
Standard cell types Variations
Inverter/buffer/tristate buffer 1X, 2X, 4X, 8X, 16X
NAND/AND gate 2 – 8 inputs
NOR/OR gate 2 – 8 inputs
XOR/XNOR gate 2 – 8 inputs
MUX/deMUX 2 – 8 inputs
Encoder/Decoder 2 – 16 inputs
Schmitt trigger circuit Inverted/non-inverted output
Latch/register/counter D/JK (sync/async clear/reset)
I/O pad circuits I/O (tristate/bidirectional)
11. ASIC Designs:
Full-custom vs Cell-based
11
Full Custom Cell Based
Design from the scratch Predefined cells with layouts
Sizes are customized Size of each cell is standard
Space is reduced Requires much more space
Performance is higher Performance is lower
Productivity is reduced Improve productivity
12. ASIC: Gate-Array-Based
Designer uses a library of standard cells
The design is mapped onto an array of transistors which is
already created on a wafer
12
13. ASIC Designs: Comparisons
Full-custom vs cell-based vs gate-array
13
Full-Custom Cell-Based Gate-Array
Density Highest Medium Low
Performance Highest Medium Low
Design time Long Medium Short
Chip dev.
Cost
High Medium Low
testability Difficult Less difficult Easy
14. Field Programmable devices: FPGA
The basic structure of an FPGA is composed of
configurable logic blocks (CLBs) and interconnections as
well as input/output blocks
14
15. PLDs
Any combination of logic can be implemented with sum of
product which is AND-OR implementation
PLD as a black box:
15
Inputs
(logic variables)
Outputs
(logic functions)
Logic gates
and
Programmable
switches
16. PLDs
General structure of PLDs:
Functionality table
16
Device AND-array OR-array
ROM Fixed Programmable
PAL Programmable Fixed
PLA Programmable Programmable
Input buffer
and inverters
AND
Plane
OR
Plane
X1
X1 X1’ Xn Xn’
Xn
Pk
P1 F1
Fk
17. PLDs
To examine the essential difference among the various
PLD devices, an implementation example is given
Implement the table below using ROM, PLA and PAL
Simplifying the table gives:
𝐹1 = 𝑥𝑦′
+ 𝑥′
𝑧
𝐹2 = 𝑦′
+ 𝑥′
𝑧
𝐹3 = 𝑥𝑦 + 𝑦′ 𝑧 17
x y z F1 F2 F3
0 0 0 0 1 0
0 0 1 1 1 1
0 1 0 0 0 0
0 1 1 1 1 0
1 0 0 1 1 0
1 0 1 1 1 1
1 1 0 0 0 1
0 1 1 0 0 1
18. PLDs: ROM
Requirements:
18
Product
term
Inputs
x y z
Output function
F1 F2 F3
x’y’z’ P1 0 0 0 - 1 -
x’y’z P2 0 0 1 1 1 1
x’yz’ P3 0 1 0 - - -
x’yz P4 0 1 1 1 1 -
xy’z’ P5 1 0 0 1 1 -
xy’z P6 1 0 1 1 1 1
xyz’ P7 1 1 0 - - 1
xyz P8 1 1 1 - - 1
X Y Z
The AND array generates all
minterms of the inputs and hence
is fixed but the OR array is
programmable to implement the
required function
P1
P8
P7
P6
P5
P4
P3
P2
F1 F3F2Fixed AND array
Programmable OR array
19. PLDs: PLA
Requirements:
19
Product term Inputs
X y z
output
xy’ P1 1 0 - F1
x’z P2 0 - 1 F1, F2
y’ P3 - 0 - F2
xy P4 1 1 - F3
y’z P5 - 0 1 F3
X Y Z
Common terms in the
outputs are combined
P1
P2
P3
P4
P5
F1 F2 F3
Programmable AND
array
Programmable OR
array
20. PLDs: PAL
Requirements:
20
Product term Inputs
X y z
Output
xy’ P1
x’z P2
1 0 -
0 - 1
F1
y’ P3
x’z P4
- 0 -
0 - 1
F2
xy P5
y’z P6
1 1 -
- 0 1
F3
X Y Z
F1 F2 F3
P1
P2
P3
P4
P5
P6
Programmable AND
array
Fixed OR array
OR gates do not share
product terms
21. Modeling ROM
Code and truth table for a 22
∗ 4 𝑅𝑂𝑀
21
Input
A1 A0
Output
O1 O2
0 0 0 1
0 1 1 0
1 0 0 0
1 1 1 1
