Method of increasing bus performance to reduce signal...

Details Method of increasing bus performance to reduce signal... Method of increasing bus performance to reduce signal...

2000-10-27

2002-08-20

Pascal, Robert (Department: 2817)

Wave transmission lines and networks

Plural channel systems

Having branched circuits

C361S788000

Reexamination Certificate

active

06437660

ABSTRACT:

BACKGROUND OF THE INVENTION
Modern electronic systems contain electronic backplanes, also referred to as motherboards, that serve as a communication medium for the exchange of electronic signals between a plurality of daughter cards. The daughter cards generate and receive communication signals, for example data signals, address signals, and control signals which are distributed to daughter card connectors mounted on one or both sides of each daughter card. The daughter cards register with a corresponding set of backplane connectors on the backplane, which in turn distribute the signals between daughter cards along various communication paths. These paths, collectively, are referred to in the art as a bus.
In a standard backplane, for example a VME configuration, the connectors are configured in card slots which are relatively closely spaced, for example 0.8 inches apart. The conductive paths, or “traces”, are commonly formed on the motherboard along the shortest distance between adjacent connectors. A group of traces is referred to in the art as a bus. The characteristic impedance of a trace in such a configuration is generally in the 50-65 ohm range. When daughter cards are inserted in the various connectors (with associated traces and capacitive loads on the daughter cards), the signal propagation of the bus is affected, lowering the effective characteristic impedance, for example, to approximately 24 ohms. If the bus is driven from a middle slot, the characteristic impedance is divided further (24 ohms to the left and 24 ohms to the right of the middle slot), resulting in a 12 ohm effective impedance as seen by the bus driver in the middle slot. Standard drivers provided on modern daughter cards are incapable of driving such a low impedance. This leads to ringing and other anomalies on the backplane bus, potentially rendering the transmitted signal undecipherable.
A common technique for mitigating this effect is referred to as “star bus” topology as disclosed in U.S. Pat. Nos. 5,696,667, and 5,930,119, issued Dec. 9, 1997 and Jul. 27, 1999 respectively, both being incorporated herein by reference. In this topology, all backplane slots are connected to a common node at the center slot.
The star bus topology is a viable alternative to the standard bus structure. The advantages of such a topology include a lower propagation time from one end of the bus to the other, as compared to the conventional bus topology. This is due to the fact that the individual transmission lines do not experience the reduced propagation velocity experienced by conventional busses. Furthermore, the large capacitance at the common node slows the rise time, resulting in reduced crosstalk, and reducing ripple by operating as a low-pass filter. However, the total conductor path length on the backplane of the star bus is much longer than the standard bus configuration. For example, for a 21 slot VME backplane, the conductor path is at least 88 inches in the star bus topology configuration, as compared with 16 inches for the standard VME bus configuration. The extra path length translates directly to additional capacitance and inductance that must be charged or discharged during a change of bus state. This results in slower rise and fall times, limiting performance. Strong drivers are required to supply adequate current to rapidly charge or discharge the capacitance, and power dissipation in the drivers is very high. Simulations demonstrate that a standard ABTE driver, well known in the art, is capable of driving the star bus, however in this configuration, the ABTE driver supplies more than its rated current. The output transistor of the driver is thus pulled out of saturation, increasing power dissipation in the transistor. Therefore, drivers with higher current rating and higher power ratings should be used with the star bus. Furthermore, the additional conductor path length on the star bus renders mechanical layout of the backplane more difficult, and requires additional board layers, increasing cost. The resonant circuit formed by the inductance of the driving line and the equivalent capacitance at the common node tends to cause overshoot, and the rise and fall times are determined, at least in part, by this resonance. However, simulation indicates that the overshoot can be controlled by biased Schottky diodes which are required at the common node.
A conventional bus architecture, for example VME, experiences limiting factors that strain the operating frequency. A first limitation arises from the fact that the bus is not terminated at or near its effective characteristic impedance. The effective characteristic impedance of a fully loaded VME backplane with 60-ohm trace impedance is approximately 24.6 ohms. The standard VME terminating impedance of the bus is 190 ohms, resulting in a reflection coefficient {grave over (&Ggr;)} of:
{grave over (&Ggr;)}=(
Z
T
−Z
0
)/(
Z
T
+Z
0
)=(190−24.6)/(190+24.6)=0.77
This means that the magnitude of the reflected signal is 77% of the magnitude of the incident signal. The voltage at the end of the bus is the sum of the incident and reflected voltages causing the output voltage to jump to 77% above the voltage of the incident wave.
To overcome this obstacle, others have provided active clipping diodes connected to both ends of the bus as an effective technique for reducing the signals reflected from the terminations. The idea is to connect a diode clipping circuit that clips at a voltage just above the incident wave voltage to each end of the bus. When an incident wave reaches the end of the bus, the voltage at the termination normally would become the sum of the incident and reflected wave voltages. If the diode clipper limits the terminating voltage to precisely the incident wave voltage, it forces the magnitude of the reflected wave to zero. In practice it is not possible to clip at exactly the incident wave voltage, and thus there will be some reflected signal, but the reflected wave can be significantly reduced.
An additional limitation of conventional backplane architecture lies in the fact that the weak process driver is inadequate for driving the bus. Consider a 21-slot bus being driven from the bus center, for example middle slot
11
. The driver is connected to the bus through a stub and connector. If the propagation delay through the stub and connector is short compared to the rise time, the driver essentially sees the effective characteristic impedance of two busses in parallel. If the effective characteristic impedance of the bus is 24 ohms, the driver initially sees approximately half of this, or 12 ohms. This impedance remains approximately constant for the time it takes a signal to propagate to the end of the bus and back to the center. For example, assuming a 3.0 volt pulse source driver with a source resistance of 12 ohms, the initial voltage at the input to the bus is 1.5 volts and the driver supplies 125 mA of current. This is a realistic example because the weak-process ABTE driver is capable of sinking about 120 mA of current. As a result, the voltage waveform at the bus input will exhibit a ‘shelf’ or ‘shoulder’ where the voltage remains approximately constant at about 1.5 volts for the roughly 8 ns propagation delay of the bus. This shelf was present in all simulations that use the weak-process ABTE driver at the center of the bus. Except for a very small delay, the output voltage at the slot nearest the driver exhibits a shelf that is very similar to the shelf on the driver output. The effect of the shelf is an uncertainty of as much as 8 ns in the switching time of the receiver, depending upon whether the shelf falls just below or just above the switching threshold of the receiver. When the bus is driven from the end, less current is required from the driver because the driver sees a higher impedance than it sees when driven from the center slot. The resulting impedance for this condition consists of the effective bus impedance in parallel with the 190 ohm terminating resistor.
SUMMARY OF THE INVEN

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