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PLDs

PLDs. ROM : Programmable OR array PLA : Programmable Logic Array . Programmable OR – AND arrays. PAL : Programmable Array Logic . Programmable AND array, fixed OR GAL : Generic Array Logic

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PLDs

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  1. PLDs ROM : Programmable OR array PLA : Programmable Logic Array . Programmable OR – AND arrays. PAL : Programmable Array Logic . Programmable AND array, fixed OR GAL : Generic Array Logic Can be configured to emulate many earlier PLDs including those with internal Flip-Flops CPLD : Complex PLD FPGA : Field Programmable Gate Arrays

  2. PLDs ROM : Read Only Memories Matrix of data that is accesded one row at a time Internaly a ROM contains a decoder and a storage array. Functionally the data array can be viewed as a programmable OR array. Types: Mask programmed ROM Field programmable ROM – PROM Erasable ROM – EPROM Ellectrically programmable ROM - EEROM

  3. PALs and PLAs Pre-fabricated building block of many AND/OR gates (or NOR, NAND) "Personalized" by making or breaking connections among the gates Programmable Array Block Diagram for Sum of Products Form

  4. PALs and PLAs Key to Success: Shared Product Terms Equations F0 = A + B' C' F1 = A C' + A B F2 = B' C' + A B F3 = B' C + A Example: Input Side: 1 = asserted in term 0 = negated in term - = does not participate Personality Matrix Output Side: 1 = term connected to output 0 = no connection to output

  5. PALs and PLAs Example Continued All possible connections are available before programming

  6. PALs and PLAs Example Continued Unwanted connections are "blown" Note: some array structures work by making connections rather than breaking them

  7. PALs and PLAs Alternative representation for high fan-in structures Short-hand notation so we don't have to draw all the wires! Notation for implementing F0 = A B + A' B' F1 = C D' + C' D

  8. PALs and PLAs Design Example Multiple functions of A, B, C F1 = A B C F2 = A + B + C F3 = A B C F4 = A + B + C F5 = A xor B xor C F6 = A xnor B xnor C

  9. PALs and PLAs What is difference between Programmable Array Logic (PAL) and Programmable Logic Array (PLA)? PAL concept — implemented by Monolithic Memories constrained topology of the OR Array A given column of the OR array has access to only a subset of the possible product terms PLA concept — generalized topologies in AND and OR planes The main advantage of the PAL over the Pla and the ROM is that it is faster

  10. PALs and PLAs Design Example: BCD to Gray Code Converter Truth Table K-maps Minimized Functions: W = A + B D + B C X = B C' Y = B + C Z = A'B'C'D + B C D + A D' + B' C D'

  11. PALs and PLAs Programmed PAL: 4 product terms per each OR gate

  12. PALs and PLAs Code Converter Discrete Gate Implementation 4 SSI Packages vs. 1 PLA/PAL Package!

  13. PALs and PLAs Another Example: Magnitude Comparator

  14. PALs and PLAs EXAMPLES Typical First Generation PAL : 16L8 10 input, 2 complemented output, 6 I/O pins Programmable (one AND term) 3- state outputs Seven product terms per output 20 pin chip GAL 16V8C and 20V8C ( 20 pins and 24 pins) 10 input (14 for 20V8) Programmable (one AND term) 3-state outputs Seven or eigth product terms per output Programmable output polarity Three combinational ouputs configurations: Bidir I/O , dedicated output , dedicated input. GAL 22V10C 24 pin chip, 12 input terminals and 10 I/O terminals Two of the outputs can have up to 8 product terms, two have 10, two have 12, two have 14 and two have 16, not counting the ouput buffer control. Combinational configurations.

  15. Combinational Logic Word Problems BCD to 7 Segment Display Controller Understanding the problem: input is a 4 bit bcd digit output is the control signals for the display 4 inputs A, B, C, D 7 outputs C0 — C6 Block Diagram

  16. BCD to 7 Segment Display Controller C0 = A + B D + C + B' D' C1 = A + C' D' + C D + B' C2 = A + B + C' + D C3 = B' D' + C D' + B C' D + B' C C4 = B' D' + C D C5 = A + C' D' + B D' + B C' C6 = A + C D' + B C' + B' C 14 Unique Product Terms

  17. BCD to 7 Segment Display Controller 16H8PAL Can Implement the function

  18. BCD to 7 Segment Display Controller 14H8PAL Cannot Implement the function

  19. BCD to 7 Segment Display Controller PLA Implementation

  20. BCD to7 Segment Display Controller Multilevel Implementation X = C' + D' Y = B' C' C0 = C3 + A' B X' + A D Y C1 = Y + A' C5' + C' D' C6 C2 = C5 + A' B' D + A' C D C3 = C4 + B D C5 + A' B' X' C4 = D' Y + A' C D' C5 = C' C4 + A Y + A' B X C6 = A C4 + C C5 + C4' C5 + A' B' C 52 literals 33 gates Ineffective use of don't cares

  21. Implementation Strategies More Advanced PAL Architectures Registered PAL Architecture Buffered Input or product term Negative Logic Feedback D2 = Q2 • Q0 + Q2 • Q0 D1 = X • Q2 • Q1 • Q0 + X • Q2 + X • Q0 + Q2 • Q0 + Q1 • Q0 D0 = Q0 Z = X • Q1 + X • Q1

  22. Implementation Strategies Advanced PAL Architectures Programmable Output Polarity/XOR PALs Buried Registers: decouple FF from the output pin Advantage of XOR PALs: Parity and Arithmetic Operations

  23. Implementation Strategies Example of XOR PAL Example of Registered PAL

  24. Implementation Strategies FSM Design with More Sophisticated PLDs CPLDs Increasing the size of a conventional PAL or GAL is not an effective way to increase complexity. Several PALs on one chip. Xilinx 9500-series : PLDs have 18 output macrocells, CPLDs have from 2 to 16 PLDs Packages have from 44 to 352 pins Programmable Logic Devices = PLD PALs, PLAs = 10 - 100 Gate Equivalents Field Programmable Gate Arrays = FPGAs (CLB:Configurable Logic Block) • Altera MAX Family • Actel Programmable Gate Array • Xilinx Logical Cell Array 100 - 1000(s) of Gate Equivalents!

  25. Implementation Strategies Design with More Sophisticated PLDs Altera EPLD (Erasable Programmable Logic Devices) Historical Perspective: PALs – same technology as programmed once bipolar PROM EPLDs — CMOS erasable programmable ROM (EPROM) erased by UV light Altera building block = MACROCELL 8 Product Term AND-OR Array + Programmable MUX's I/O Pin Seq. Logic Block Programmable polarity Programmable feedback

  26. Implementation Strategies Design with More Sophisticated PLDs Altera EPLDs contain 8 to 48 independently programmed macrocells Personalized by EPROM bits: Flipflop controlled by global clock signal local signal computes output enable Flipflop controlled by locally generated clock signal + Seq Logic: could be D, T positive or negative edge triggered + product term to implement clear function

  27. Implementation Strategies Design with More Sophisticated PLDs AND-OR structures are relatively limited Cannot share signals/product terms among macrocells Altera solution: Multiple Array Matrix (MAX) Logic Array Blocks (similar to macrocells) Global Routing: Programmable Interconnect Array EPM5128: 8 Fixed Inputs 52 I/O Pins 8 LABs 16 Macrocells/LAB 32 Expanders/LAB

  28. Implementation Strategies Design with More Sophisticated PLDs LAB Architecture Expander Terms shared among all macrocells within the LAB

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