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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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
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.
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.
divide by 4
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.
A CLA (Carry Look Ahead) adder is used for addition.
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
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.
Adder Block Diagram
Estimate how much time you spent on each phase of the project
verifying logic 60hr.
verifying timing 20hr.
post extracted timing 20hr.
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.
This project was a good experience for learning circuit design with CAD tools as well as project planning and execution.