An Overview of High Frequency Processor-System Interconnects

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Bus Issues

It Takes Time for Signals to Propagate from Point A to Point B

In Figure 3, we show that the source at point A drives a signal onto the interconnect, and after some time, the signal reaches the destination at point B.


Figure 3: Signal propagation from Point A to Point B

An Idealistic figure of signal propagation speed may be obtained by using an approximation of 0.66c, or roughly two-thirds of the speed of light. With this approximation, we find that it would take 1 ns for the signal to travel a distance of 20 cm. However, traces on modern 4 layer PCB boards and through module interconnects are far away from ideal lossless transmission lines. Effects of non-ideal transmission lines, especially mismatched load and line impedance characteristics often combine to reduce actual signal propagation speed.

Propagation delay on modern computer system boards manufactured on multi-layer PCB boards are strong functions of the dielectric material used on the board. The propagation delay may be approximated by computing the wave velocity of a strip line transmission line. For more detailed discussions, please consult the relevant text. Quick discussions on this topic may be found online from Rambus Inc. and lecture notes from Stanford Computer Systems Laboratory. These discussions are cited in the reference section.

Signal Waves Bounce Back and Forth on Transmission Lines with Mismatched Load Impedance

In Figure 4, we illustrate that a transmission line will have a given characteristic impedance, and on an ideal transmission line, a signal wave can travel from one end of the transmission line to the other end very rapidly.


Figure 4: Wave Reflections with Mismatched Load Impedances

However, when the wavefront reaches the end of the transmission line, if the input impedance of the load does not perfectly match the characteristic impedance of the transmission line, then some portion of the input signal wave will reflect back onto the transmission line. The phase and magnitude of the reflected wave are functions of the mismatches between the characteristic impedance of the transmission line and the impedance of the load. In figure 4, we show a reflected wavefront that travels from the destination back to the source with a reduced magnitude. When the reflected wavefront reaches the source driver of the initial signal, if the impedance of the source driver also does not match the characteristic impedance of the transmission line, then another reflected wave will once again reflect from the source to the destination. The voltage on the transmission line will then be a summation of the wavefronts as they reflect back and forth. Theoretically, poorly matched load and source impedances can ensure that a signal wave reflect back and forth for a long time before settling to the final signal value. For this reason, properly matched and terminated signal paths are absolute necessities for high frequency signaling. (“High Frequency” is with respect to the length of the transmission line. For a transmission line that is 100 meters in length, 10 MHz is a very high frequency) For signal paths and loads that are not nearly perfectly matched in the impedance characteristic, the worse case cycle time on the signal bus must be computed to allow the signal reflections time to settle. For a wave pipelined interconnect, where new signal wavefronts are placed by the source driver onto the interconnect even before the previous signal wave front reaches the destination, the reflections on each interface must be negligible by design.

Multi-Drop Busses Introduce Non-Ideal Discontinuities in Signal Paths

In Figure 5, we show that each load on a multi-drop bus becomes a discontinuity on the transmission line. Each discontinuity on the transmission line creates an interface where signal waves can reflect.


Figure 5: Each Load Introduces Discontinuity on the Transmission Line

In Figure 6, we show that if we represent each load as a capacitive element, with a larger number of loads, rise time of the signal decreases, signal velocity decreases, and signal ringing continues for a longer period of time.


Figure 6: More Loads, Slower Signal Transmission, More Reflections

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