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Priority encoder

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					Priority encoder

• Priority encoder- theoretic view • Other implementations • The chosen implementationsimulations • Calculations and comparisons

Overview

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

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)

basic concepts of priority encoders
• The i-th output bit EPi = Di

* Pi

Di- the input data Pi- the priority 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

Different implementations
For 4 bit priority encoder

Sum of minterms, the straight-forward implementation

matrix

Because of a minimal distance needed between the lines the layout is large and complicated.

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

Then, by chaining the units we construct the output

In this implementation we save silicon area, but pay in propagation delay

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

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

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

8-bit macro cell

Diagram of 32-bit chain designed encoder

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

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 )

Block diagram of a 32-bit priority encoder with folding

64 bit priority encoder with first level folding

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

• • •

64-bit priority encoder with 2 levels of folding

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

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

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

The output

The simulation:

ahead it

he put

he ock

The 4 – bit unit
The 4 bit unit circuit:

The input signals:

The outputs:
Lookahead

outputs

When the lookahead high all the outputs equals zero

The 8-bit unit

The output signals
v3

Not valid

v0

The next lookahead

v7
v4

The 32-bit chain encoder

The results

The problem we encountered
“glitches”

The “glitch”

the glitch starts after clock rising

clock rising

The widest glitch comes at higher bits

clock

Bit #60

32 bit-folding

64 bit first level folding

64 bit second level folding

64 bit second level folding with one critical path

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.

Results – 32 bit

Results – 64 bit

Results – 64 bit (high)

80% high pulse

The vhdl simulation

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

Compare table
unit Area [mm²] Power
Time [ns]
[10^-11fw]

matrix 0.076 173.4
75

tree 0.043 112.8
18

folding 0.053 127.5
8

0.076 149.6
241.2

250

200 150

unit matrix tree folding

100 50 0

pd

80000 70000 60000 50000 40000 30000 20000 10000 0

180 160

unit matrix tree folding

140 120 100 80 60 40 20 0

unit matrix tree folding

area

power


				
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posted:12/9/2009
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