Noise Reduction Using Low Weight and Constant Weight Coding Techniques
Abstract
Signalling off-chip requires significant current. As a result, a chip's power-supply current changes drastically during certain output-bus transitions. These current fluctuations cause a voltage drop between the chip and circuit board due to the parasitic inductance of the power-supply package leads. Digital designers often go to great lengths reduce this transmitted noise. Cray, for instance, carefully balances output signals using a technique called differential signalling to guarantee a chip has constant output current. Transmitted-noise reduction costs Cray a factor of two in output pins and wires. Coding achieves similar results at smaller costs. In a circuit using parallel- terminated transmission lines such as the Cray-1, a chip exhibits nearly constant output current throughout a single clock cycle. However, since current for an output 1 differs greatly from that for an output 0, current can change drastically at the end of a cycle generating an intolerable amount of transmitted noise as successfully as differential signalling. Experimental results substantiate this. Information capacity of a word is greatest when the word has half 1's. Coding requires fewer output signals than differential signalling. For example, an 8-bit byte can be encoded as eleven bits with but requires a more costly coding scheme. (jhd)
Document Details
- Document Type
- Technical Report
- Publication Date
- Jun 01, 1990
- Accession Number
- ADA228346
Entities
People
- Jeff Tabor
Organizations
- Massachusetts Institute of Technology