Encoding for simultaneous switching output noise reduction

Abstract

This invention reduces the Delta I noise on an integrated circuit chip by reducing the changes in current supply required for transitions in logic states of the input/output devices. The invention uses a 3/6 binary code for communicating between integrated circuit chips. This code uses six bits to represent the 16 hex code digits typically used for computer instructions. Three of the six bits are in a high logic state and three of the six bits are in a low logic state for all 16 hex code representations. Therefore, changing from any one logic state to another, does not change the overall current supply required by the six input/output devices. Groups of six input/output devices (corresponding to the 3/6 code) are located relatively close to each other with respect to the power supply pins which supply current to the six input/output devices. As a result, there is a high to low transition for every low to high transition over similar parasitic impedances on the input/output devices. This leads to low Delta I noise because the noise created by individual transitions cancel each other out when viewed as a six device group. The high overhead pin count for a six digit code instead of the normal four digit code for hex representation is compensated for because input/output pins normally replaced by extra power supply pins (to reduce Delta I noise) can be used as input/output pins. Therefore, the 3/6 code frees up extra input/output pins when the Delta I noise would normally force those pins to be used for power supply.