Chapter 6 Digital System Design 242-208 Digital Systems and Logic Designs
Feb 24, 2016
Chapter 6Digital System Design
242-208 Digital Systems and Logic Designs
Content•Programmable Logic Devices (PLDs)•PLD programming•Combinational PLDs•Sequential PLDs•Field programmable gate arrays (FPGAs)
•Systematic Design•ASM methods and charts•Controller and data processor designs
•References
PLD: Why we need ?
Design is more economical to implement using a fewLarge chips than many small chips.
Design and fabrication time for VLSI chips are extremely long
PLD can be mage in large volume and PROGRAMMED toimplement large numbers of different low-volume designs
What is PLD ?An IC that contains large numbers of gates, flip-flops, etc. that can be configured by the user to perform different functions
Two PLD types which are : SPLD (Simple PLD) CPLSs (Complex PLD)
PLD programming methods: Antifuse technology Two metal layers sandwich a layer of non-conductive, amorphous silicon. When voltage is applied to this middle layer, the amorphous silicon is turned into polysilicon, which is conductive.
After
PLD programming methods: Floating gate
Used in EPROM device !!
High voltage applied to the Drain, electrons jump to gate 1
Current cannot passed since no channel for carrier charge. Potential energy is still high !!
UV light strikes electrons,causes more enough energy for electrons to jump back to the channel,The transistor starts conducting !!
PLD programming methods: SRAM
Combinational PLD:PROM Programmable Read Only Memory (PROM) :A memory device that stores data at specific locationsthat can be addressed through a set of address pins. Another view : a large array of AND gates followed by a large array of OR gates
Combinational PLD:PLA Programmable Logic Array (PLA) : Both arrays of logic AND and OR are programmable
Combinational PLD:PAL Programmable Array Logic (PAL) consists of a programmable array of AND gates that connect to a fixedarray of OR gates
The PAL structure allows any SOP expression with a defined number ofvariables to be implemented
Combinational PLD:PAL
Combinational PLD:GAL A PAL that can be reprogrammed.
Sequential PLD: Consists of combinational PLDs with a set of FFs.See the diagram of IC PAL16R8.
Sequential PLD: Implement three-bit Gray code counter using PAL16R8
Complex PLD
•Programmable PLD blocks•Programmable interconnects•Electrically erasable links
Field Programmable Gate ArrayWhy we need FPGA ?
Large complex functions(millions gates)Customised designExpensive to design (in small quantities)Hard to design and long design cyclesNot reprogrammableHigh risks
Limited complexityThousands of gatesCheap and easy to designReprogrammable
Field Programmable Gate ArrayWhy we need FPGA ?
InexpensiveEasy and rapid design PrototypingReprogrammable
Field Programmable Gate Array
Simple programmable Logic blocksMassive of programmable interconnects
FPGA architecture
FPGA CLB
Look up tableFF, registers, clock storage elementsMUX
CLBFPGA architecture
FPGA CLB with Look Up Table LUT contains memory cells to implement logic functionEach cell holds ‘0’ or ‘1’Programmed with outputs of truth tableInputs select content of one of the cells as output
What is in a LUT ?4 input – 16 outputs LUT requires 16 storage elementswith 16 latches.
FPGA CLBLarger logic functions built by connecting many CLBstogether
Programmable routingConnections routing signals between CLBsDetermined by SRAM cells
Programmable routing
Programmable routing Switch matrix
FPGA : I/O Blocks
Systematic DesignFor circuit design:
small circuit : gate-level design (truth tables, K maps, etc) large circuit : block-level design (ICs)
Larger digital systems need more abstract and systematic design techniques.Systematic design methodology :
Top-down approach Partitioning Developing overall architecture Detailing hardware.
Systematic Design : Top-down approach
Starting from original problem and gradually refine it towards solution.Steps for a top-down design procedure:
Specify the problem clearly (at global/top level without unnecessary details).
Break the problem into smaller sub-problems.Repeat the process until sub-problems are small
enough to be solved directly.
Systematic Design : Top-down approach
Relevant to goal-directed approach
State goal, then find sub-goals to solve main goal.Repeat until sub-goals are directly solvable.
Systematic Design : PartitioningA digital system consists of two components
A control algorithm (Controller)An architecture (Data processor)
Control unit (Controller)
Data Processor (Architecture)
Commands
Input data
External command
Status condition
Output data
ASM Algorithmic State Machine (ASM) Chart is a high-level flowchart-like notation to specify the hardware algorithms in digital systems to obtain “control” and “data processor” units.Major differences from flowcharts are:
only three box types:- 1) state box (similar to operation box), 2) decision box 3) conditional box
contains exact (or precise) timing information while flowcharts impose a relative timing order for the operations.
Components of ASM chart State box Rectangular shape One entry point and one exit point Used to specify one or more operations which could be simultaneously completed in one clock cycle.
one or more operations
statebinary code
Components of ASM chart Decision box Diamond in shapeOne entry point but multiple exit points Used to specify a number of alternative paths that can be followed.
deciding factors
deciding factors
Components of ASM chart Conditional box Rectangle with rounded cornersAlways follows a decision box and contains one or more conditional operations that are only invoked when the path containing the conditional box is selected by the decision box.
conditional operations
Example of using ASM chartInitial state
S
A 0F 0
A A + 1
A2
E 0 E 1
A3
F 1
0
0
0
1
1
1
T2
T1
T0
Init S=0 if S equal 1 { A = 0 F = 0 } increase A if A2 equal 1 { E =1 if A3 equal 1 { F = 1 } }
Register operationRegisters present in the data processor for storing and processing data. Flip-flops (1-bit registers) and memories (set of registers) are also considered as registers.The register operations are specified in either the state and/or conditional boxes, and are written in the form:
destination register function(other registers)
Timing in ASM chartsInitial state
S
A 0F 0
A A + 1
A2
E 0 E 1
A3
F 1
0
0
0
1
1
1
T2
T1
T0Precise timing is implicitly presented in ASM charts.Each state box, together with its immediately following decision and conditional boxes, occurs within one clock cycle.A group of boxes which occur within a single clock cycle is called an ASM block.
Timing in ASM chartsOperations of ASM can be illustrated through a timing diagram.Two factors which must be considered are
operations in an ASM block occur at the same time in one clock cycledecision boxes are dependent on the status of the previous clock cycle (that is, they do not depend on operations of current block)
Timing in ASM charts
A = A4A3A2A1
Initial state
S
A 0F 0
A A + 1
A2
E 0 E 1
A3
F 1
0
0
0
1
1
1
T2
T1
T0
clock 1 2 3 4 5 6states T0 T0 T1 T1 T1 T1input S=0 S=1 S=0
registervalues
A=0F=0
A=1
E=0
A=2
E=0
A=3
E=1
OperationsA0F0
AA+1E0
AA+1E0
AA+1E1
AA+1E1
AA+1E0
AA+1E0
AA+1E1
F1
clock 7 8 9 10 11 12 13states T1 T1 T1 T2 T0 T0 T0input
registervalues
A=4
E=1
A=5
E=0
A=6
E=0
A=7
E=1F=1
Operations
Digital system synthesisFrom ASM chart, we can synthesize
Controller logic (via State Table/Diagram)Architecture/Data Processor
Design of controller is determined from the decision boxes and the required state transitions.Design requirements of data processor can be obtained from the operations specified with the state and conditional boxes.
Controller synthesis procedureStep 1: Identify all states and assign suitable codes.Step 2: Draw state diagram.Step 3: Formulate state table usingState from state boxesInputs from decision boxesOutputs from operations of state/conditional boxes.Step 4: Obtain state/output equations and draw circuit.
Controller synthesisInitial state
S
A 0F 0
A A + 1
A2
E 0 E 1
A3
F 1
0
0
0
1
1
1
T2
T1
T0 T0
T1
T2
Assign codes to states: T0 = 00 T1 = 01 T2 = 11
Presentstate inputs
Nextstate outputs
G1 G0 S A2 A3 G1+ G0
+ T0 T1 T20 0 0 X X 0 0 1 0 00 0 1 X X 0 1 1 0 00 1 X 0 X 0 1 0 1 00 1 X 1 0 0 1 0 1 00 1 X 1 1 1 1 0 1 01 1 X X X 0 0 0 0 1
Inputs from conditions in decision boxes. Outputs = present state of controller.
Controller synthesisDecoder + D flip-flops
-suitable for moderately large controllers.- procedure: use decoder to obtain individual states; from the state table, obtain the next-state functions by inspection.
The flip-flop input functions can be obtained directly from the state table by inspection.This is because for the D flip-flops,
the next state = flip-flop D input.Decoder is then used to provide signals to represent different states.
Controller synthesisD Q
D Q
2x4 decoder
T0
T1
T2
unused
G1
G0
?
?
clock
Given the state table:
Presentstate inputs
Nextstate outputs
G1 G0 S A2 A3 G1+ G0
+ T0 T1 T20 0 0 X X 0 0 1 0 00 0 1 X X 0 1 1 0 00 1 X 0 X 0 1 0 1 00 1 X 1 0 0 1 0 1 00 1 X 1 1 1 1 0 1 01 1 X X X 0 0 0 0 1
The inputs of the D flip-flops for G1 and G0 are
DG1 = T1.A2.A3 DG0 = T0.S + T1
Controller synthesis
D Q
D Q
2x4 decoder
T0
T1
T2
unused
G1
G0
clock
A2
A3
S
Data processor synthesisArchitecture is more difficult to design than controller.Nevertheless, it can be deduced from the ASM chart. In particular, the operations from the ASM chart determine:
What registers to useHow they can be connectedWhat operations to supportHow these operations are activated.
Guidelines:always use high-level unitssimplest architecture possible.
Data processor synthesisInitial state
S
A 0F 0
A A + 1
A2
E 0 E 1
A3
F 1
0
0
0
1
1
1
T2
T1
T0
Various operations are:Counter incremented (A A + 1) when state = T1.Counter cleared (A 0) when state = T0 and S = 1.E is set (E 1) when state = T1 and A2 = 1.E is cleared (E 0) when state = T1 and A2 = 0.F is set (F 1) when state = T2.
Deduce:One 4-bit register A (e.g.: 4-bit synchronous counter with clear/increment).Two flip-flops needed for E and F (e.g.: JK flip-flops).
Data processor synthesis
Controller
K
J Q
K
J Q
Clk
4-bit syn. counter A
A2
A1A2
A3
A3A4
start S
E
F
clock
CP
count
clear
T2
T1
T0
References[1] A. T. T. Choy, Lecture notes on CS1104-Computer Organization.[2] M. M. Mano, Digital Design 3rd , Prentice-Hall.[3] John Coughlan, Lecture note on Introduction to Programmable Logic Device.[4] P. Cheung, Lecture note on Programmable Logic Devices.[5] F. Floyd, Digital Fundamentals 9th Edition, Prentice-Hall.