Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Parameter related to the reproduction or fidelity of a...
Reexamination Certificate
2000-04-29
2001-11-13
Brown, Glenn W. (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Parameter related to the reproduction or fidelity of a...
C324S1540PB, C324S076350, C702S125000
Reexamination Certificate
active
06316944
ABSTRACT:
BACKGROUND
Printed circuit boards (PCBs) connect chip devices together, as well as couple the devices to external components such as keyboards and storage devices. PCBs use wires or nets for the connections. Some devices have multiple wires that send and/or receive data bits substantially simultaneously. The time that it takes for data to traverse the wires is known as propagation delay. Note that differences in the wire lengths will causes differences in the time of transit of the data bits. These differences arc known as skew. Acceptable skew is generally less than one clock cycle. Thus, signals arriving within one clock cycle are treated as having arrived simultaneously by the receiving device. Signals arriving with a time differential of greater than one clock cycle are treated as having arrived in different clock cycles. If a signal was to arrive in the wrong clock cycle, then the skew can cause the system to crash.
FIG. 1
depicts some of the nets on a PCB board
100
. The board includes drivers
101
that transmit signals and receivers
102
that receive the signals sent by the drivers. Connecting wiring
103
connects the drivers to the receivers. The wiring is also known as the net. Net length is the length of the wires. As shown in
FIG. 1
, some drivers are located closer to their associated receivers, than other driver/receiver pairs. Also boards have features, e.g. mounting holes
104
, surface mounted devices, etc., such that the wiring must be routed around the features. Thus, the propagation delay for pair
101
B/
102
B will be less than delay for pair
101
A/
102
A if straight wiring is used. To increase the path length, the wiring is formed in a square wave or sawtooth pattern
105
, such that the additional path length added by the pattern
105
should increase the total path length for the
101
B/
102
B pair such that it equals the path length for the
101
A/
102
A pair.
A problem with this arrangement is that the physical length added by the sawtooth pattern, namely the vertical portions of the pattern, does not equal the intended propagation delay. Physically the pattern should produce the intended propagation delay, however, the actual propagation delay is less than expected. In other words, the current path is shorter than the predicted physical path. The predicted propagation delay is formed by a CAD tool, which measures distance from the center line of the wire. The predicted delay equals the physical length times a constant (delay per length). But the real current path does not follow the center line, the real current path follows a shorter path and cuts the corners of the bends of the pattern. Note that this differential is very small, but is multiplied by the number of bends. Thus, for wires with a few sawteeth, the wire behaves as is predicted, but as the number of teeth increases, the resulting propagation delay begins to vary from the predicted or intended value. In the past, this problem has been ignored as its effects were not important because of slower computer speeds. However, as computer speeds have increased, this problem has become more important.
One prior art solution is to use delay lines
201
, as shown in FIG.
2
. The delay line is a manufactured wire, that has a built in delay. However, the delay lines are not accurate. In other words, two delay lines with the same rating will have different delays, i.e. they are unpredictable. Also, the delay wires require attachment vias which uses board space. Moreover, the delay wires arc predefined, and a particular delay may not be attainable with the predefined delay lines.
Another prior art solution is to use few teeth, but with longer vertical distances, as shown in FIG.
3
. This is known as a trombone pattern
301
. Such a pattern would have the same amount of vertical space as the sawtooth pattern
105
, but with fewer bends. Since there are fewer bends, then the current path is closer to the predicted path and the actual delay is closer to the predicted delay. However, this arrangement requires a great deal of board space. Also, since the wires are close together for relatively long distances, the wires can have cross-talk, that would change the timing of the signal travel. In other words, the signal may couple energy onto one or more of the vertical paths and arrive sooner than expected to the receiver.
SUMMARY OF THE INVENTION
These and other objects, features and technical advantages are achieved by a system and method that accurately determines propagation delay for a sawtooth pattern.
Through measurement, the actual delays added per bend in the sawtooth pattern are determined. These values are then used in a CAD tool. The CAD tool can then accurately determine the propagation delay for a sawtooth pattern. Note that other patterns can be used as well, for example, a 45 degree bend pattern or triangle tooth pattern.
Therefore, it is a technical advantage of the present invention to be able to add a known amount of propagation delay to a wire length.
It is another a technical advantage of the present invention to be able to add a known amount of propagation delay to a wire length without using a large amount of board space.
It is a further technical advantage of the present invention to be able to route net wires close together in a smaller amount of board space.
It is a still further technical advantage of the present invention to be able to add a propagation delay by using a repeatable structure.
It is a still further technical advantage of the present invention to be able to test the predicted delay values and add more teeth if needed.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.
REFERENCES:
patent: 4587480 (1986-05-01), Zasio
patent: 4939677 (1990-07-01), Otuji et al.
patent: 5208213 (1993-05-01), Ruby
patent: 5926589 (1999-07-01), Gaeta
Barnette Christopher M.
Fail Douglas B.
Morris Terrel L.
Ross Marvin D.
Brown Glenn W.
Hewlett -Packard Company
Nguyen Vincent Q.
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