Decimation Filter (Accumulate-and-Dump)

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Decimation Filter (Accumulate-and-Dump). Howard Chen Keng-Han Chuang Ed Stricker William Tsoi Advisor: Dave Parent May 11, 2005. Abstract. The circuit adds four 8-bit words, then latches to the output the 8 most significant bits of the 10-bit result.

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Decimation Filter(Accumulate-and-Dump)

Howard Chen

Keng-Han Chuang

Ed Stricker

William Tsoi

May 11, 2005

Abstract
• The circuit adds four 8-bit words, then latches to the output the 8 most significant bits of the 10-bit result.
• We designed an 10-bit CLA adder, 2 sets of DFF latches, a divide by 4 frequency divider and a reset circuit. The operation frequency is 200 MHz, it uses 80mW of Power and occupies an area of 205x158mm2
Introduction

In an ADC sometimes there is a need to average the output to simplify the circuitry after and lower power consumption.

A set of latches accumulates K inputs from the adder. The result of K inputs is then dumped into the output latches. At the same time the first set of latches are reset to zero to start the process over.

Introduction

• The word size for this circuit is 8-bit, after K add cycles only the 8 MSB bits are sent to the output so that the result is an average of the K inputs.
• Our clock speed is 200 MHz, and the K value is 4.

8

10

8

8

Input

Latches_2

Output

10

Latches_1

Clock

Reset

divide by 4

Block Diagram
Project Summary
• DFFs were used for latching the outputs from the adder as well as the final output.
• A circuit composed of a NAND gate and inverters is used to implement the reset function.
Project Details
• TSPC (True Single-Phase Clock) DFFs were used exclusively (text p.399) for the latches and frequency divider.
• A CMOS pair was inserted to implement reset function
• These are fast and only use 11 to 13 transistors
• The reset circuit consists of a three input NAND gate, an inverter for the output, an inverter for the fs/2 input, and a double inverter to buffer the clock input.
• The circuit outputs a high once every 4 clock cycles when all three inputs are high
• The three inputs are system clock, fs/2 and fs/4
Project Details
• The add portion consists of two 4-bit and one 2-bit CLA adder chained together.
• The first 4-bit is designed for carry out only, no carry in.
• The second 4-bit has both carry in and carry out.
• The carry out of the 2-bit adder is the tenth bit.
Cost Analysis
• Estimate how much time you spent on each phase of the project
• verifying logic 60hr.
• verifying timing 20hr.
• Layout 60hr.
• post extracted timing 20hr.
Lessons Learned
• The circuit worked at 200MHz in schematic but was slower in layout because of interconnection lengths. Better planning for layout could have helped maintain speed.
• The size of this project may be too ambitious for a 166 class.
Summary
• This project was a good experience for learning circuit design with CAD tools as well as project planning and execution.