Equalization of mid-frequency power supply noise via a spectrum-shaping encoder for parallel buses

Noise from the simultaneous switching of outputs (SSO) creates a significant problem for high-speed parallel buses that use single-ended signaling. Further exacerbating this problem is the fact that many single-ended interfaces share the return path of numerous signals with the same power delivery network (PDN) used to power the I/O transceiver circuitry. These single-ended interfaces share power and ground conductors in an effort to reduce the cost associated with package pin count and chip pad area. In addition to creating low-cost interfaces, there is a need for performance enhancing techniques that are ldquointerface compatiblerdquo, which helps to ease their adoption. This paper introduces a new concept in bus coding that shapes the spectrum of aggregate bus current in an effort to ldquoequalizerdquo the induced voltage response of a power supply. The single-ended buses discussed in this paper are those used in the industry standard interfaces of today´s graphics systems, and possibly the interfaces that may be used in future generations of main memory.