1. Technical Field
This invention relates to the field of data transmission, and in particular to systems and methods for reliably determining the voltage level of data signals transmitted at high speeds.
2. Background Art
Computers and other data processing devices operate at ever increasing frequencies. In order to avoid bottlenecks, data must be provided to these devices at high speeds and without distortion. However, as transmission frequencies exceed 150 MHz, it becomes more difficult to ensure that transmitted data signals have sufficient time to settle before they are sampled and the next data transition begins. This problem is compounded by the need to operate computers at lower system voltages, to limit the power dissipated by high frequency processors.
High frequency operation of buses, which transfer data signals to different parts of the computer system, poses especially difficult problems. At frequencies of 100 MHz or more, bus traces behave like transmission lines. In particular, the inductive and capacitive properties of bus traces distort data signals, increasing the time it takes for signal voltages to settle sufficiently to be sampled reliably.
For example, at high frequencies, the capacitance of bus traces requires signal transitions to be driven with higher currents, and for 32-bit and wider buses, the number of signal transitions being driven results in substantial current pulses on the supply voltage lines. These current pulses couple with the inductance of the bus traces to generate voltage glitches. In addition, impedance mismatches between the bus and the circuits driving the data signal creates reflections on the bus. Data signals are distorted by reflected signals generated by voltage glitches and signal voltages. One result of these distortions is that a data signal in a given clock cycle may appear to be in either a high or low voltage state, depending on when in the clock cycle it is sampled by a receiver.
Conventional strategies for handling high speed data signals employ circuitry to guarantee a window of relatively clean signal transitions, and a single reference voltage is selected within this voltage window. There are a number of drawbacks to this approach. Noise is generated differently for high-going and low-going transitions, and the resulting asymmetry in the signal distortion must be accounted for by the circuitry. In addition, the circuitry used to provide a clean voltage window becomes more complex and less effective as signal frequencies increase.