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Priority encoder

Priority encoder. Overview. Priority encoder- theoretic view Other implementations The chosen implementation- simulations Calculations and comparisons. The target of the project. Building priority encoder using the multilevel lookahead and folding techniques. Uses of priority encoding.

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Priority encoder

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  1. Priority encoder

  2. Overview • Priority encoder- theoretic view • Other implementations • The chosen implementation- simulations • Calculations and comparisons

  3. The target of the project Building priority encoder using the multilevel lookahead and folding techniques

  4. Uses of priority encoding • INR - interconnection network router • design of SAE – sequential address encoder of a content associate memory (CAM) • microcontroller and microprocessor (incrementer / decrementer)

  5. basic concepts of priority encoders The i-th output bit EPi =Di * Pi Di- the input data Pi- thepriority token passed into this bit the relationship between Pi and Pi-1 Pi = Di-1 * Pi-1 the generated EPi is EPi = Di * Di-1 * Di-2 … D1 * D0

  6. Different implementations For 4 bit priority encoder

  7. matrix Sum of minterms, the straight-forward implementation Because of a minimal distance needed between the lines the layout is large and complicated.

  8. Basic units The structure is build from equal units. Each unit calculates yi and xpi for the i-th bit

  9. Then, by chaining the units we construct the output In this implementation we save silicon area, but pay in propagation delay

  10. tree Tree of multiplexers implemented by butterflies Efficient implementation in area and power, has longer propagation then the folding technique

  11. the multilevel lookahead structure The output third-level lookahead signal of the ith 8-bit macro is: LA3i|i=0~n-1 = D8i+7 + D8i+6 + D8i+5 + D8i+4 + D8i+3 + D8i+2 + D8i+1 + D8i + LA3i-1 LA3-1 = 0 n = N/8 N – number of input bits The ith 4-bit sub macros LA2i = D8i+3+D8i+2+D8i+1+D8i+LA3i-1

  12. The 8-bit macro formulas EP8i = D8i * LA3i-1 EP8i+1 = D8i+1 * D8i * LA3i-1 EP8i+2 = D8i+2 * D8i+1 * D8i * LA3i-1 EP8i+3 = D8i+3 * D8i+2 * D8i+1 * D8i * LA3i-1 EP8i+4 = D8i+4 * LA2i EP8i+5 = D8i+5 * D8i+4 * LA2i EP8i+6 = D8i+6 * D8i+5 * D8i+4 * LA2i EP8i+7 = D8i+7 * D8i+6 * D8i+5 * D8i+4 * LA2i

  13. 8-bit macro cell

  14. Diagram of 32-bit chain designed encoder

  15. The folding technique-first level folding • The LA3i that generated by the macro with the higher priority can be connected to other macros with lower priority. • Such connection canmake the critical path shorter • In this connection we’ll lose the advantage in layout arrangement and wiring complexity

  16. Folding - implementation • We’ll connect LA30 to the second and the fourth macros (not to the third) and we’ll get 2x2 matrix • in this way the fourth macro is connected to 2 neighboring macros • the number of gate delays is reduced to 4 (<log232 )

  17. Block diagram of a 32-bit priority encoder with folding

  18. 64 bit priority encoder with first level folding

  19. Multilevel folding • In order to reduce the gate delay to be less then log2N in grater priority encoders, we can apply the folding technique again & again for example : N=128 • First-Level folding : 8 gate delay • Second-Level folding : 7 gate delay • Third-Level folding :<7 gate delay

  20. 64-bit priority encoder with 2 levels of folding

  21. For 256-bit priority encoder the new design can achieve about 10 times performance while spending ½ power consumption.

  22. The implementation We decided to implement the project using bottom up architecture, starting with a 1 bit unit. Each stage will be checked separately. Moving to the next stage is only after the previous stage is finished

  23. 1 bit unit At first we implemented 1 bit unit and checked it. The circuit:

  24. The output The simulation: Lookahead bit The input The clock

  25. The 4 – bit unit The 4 bit unit circuit:

  26. The input signals:

  27. The outputs: Lookahead When the lookahead high all the outputs equals zero outputs

  28. The 8-bit unit

  29. The output signals v3 Not valid v0

  30. The next lookahead v7 v4

  31. The 32-bit chain encoder

  32. The results

  33. The problem we encountered “glitches”

  34. The “glitch” the glitch starts after clock rising clock rising

  35. The widest glitch comes at higher bits clock Bit #60

  36. 32 bit-folding

  37. 64 bit first level folding

  38. 64 bit second level folding

  39. 64 bit second level folding with one critical path

  40. Propagation delay - reduction To minimize the propagation delay of the EP we made the following changes : • Reduced the clock period from 200ns to 20ns. • Divide the clock pulse to different periods for low time and high time. Those changes made under the constrains of : • Keeping the high pulse length 80% of the base pulse. • Making sure all the requested changes and currents are stable before clock raising. • The optimum result we conclude for the clock period: 5ns for low time and 15ns high time.

  41. Results – 32 bit

  42. Results – 64 bit

  43. Results – 64 bit (high)

  44. 80% high pulse

  45. The vhdl simulation

  46. The vhdl simulation of a 32 bit priority encoder Here the lsb of input changes from 0 to 1, and the output changes

  47. Compare table

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