Electronic digital logic circuitry – Signal sensitivity or transmission integrity – Bus or line termination
Reexamination Certificate
2000-09-18
2002-04-09
Tokar, Michael (Department: 2819)
Electronic digital logic circuitry
Signal sensitivity or transmission integrity
Bus or line termination
C326S086000, C326S090000
Reexamination Certificate
active
06369605
ABSTRACT:
FIELD
The invention relates to a method and apparatus for preventing signal reflection of transmissions between electronic devices by employing a configurable, self-terminated driver.
GENERAL BACKGROUND
As faster and more complex electrical circuits are designed, one challenge is to fit an ever increasing number of components into smaller form-factor devices. The increasing number of components also causes the price of high density circuits to increase. Therefore, it is desirable to make the most effective use of every component utilized without affecting the operation of the circuit.
Presently, electronic devices, such as semiconductor integrated circuits, are typically mounted on a circuit substrate and electrically interconnected to perform a practical function. As transmission speeds between such devices increase, it is desirable that a transmission path have evenly matched impedance. A problem that occurs if impedance is not evenly matched along a transmission path is signal reflection. Signal reflection occurs where a transmission signal crosses from a first transmission medium to a second transmission medium having different characteristic impedance. As transmission frequencies increase, the disruption of signal waveforms caused by such reflection becomes more problematic.
Typically, signal transmission without reflection is achieved by coupling a termination resistor equal to the characteristic impedance of the signal transmission line at either the receiving end, the transmitting end, or both. This technique is also commonly known as “impedance matching”. The termination resistor may be coupled to a voltage source (“pulled up”) or coupled to ground (pulled down).
As shown in
FIG. 1
, the transmission path between two devices
102
&
104
, such as two semiconductor integrated circuits, is made up of a conductor
110
which has a distributed characteristic impedance Z
0
. However, the node
106
where such conductor couples to the device pins can be a source of signal reflection. Therefore, a termination resistor
108
, with a matching impedance Z
0
, is coupled to the receiving end of the transmission path
106
at one end and to ground at the other end. This causes the transmitted signal to be absorbed by the termination resistor
108
at the receiving end, producing no reflection. The receiving device thus receives the transmitted signal without disruption of the signal waveforms.
In preventing signal reflection, it is essential that the resistance of the termination resistor
108
exactly match the characteristic impedance Z
0
of the signal transmission path or conductor
110
. Because such high accuracy is required when matching a resistor to the transmission line, terminating resistors are not typically placed within semiconductor integrated circuits. That is, since the characteristic impedance of transmission lines, or conductors, can vary from application to application, placing terminating resistors within an integrated circuit is not advisable because they may not be a good match with the transmission lines utilized. Placing termination resistors within integrated circuits also increases the cost of such devices. Therefore, termination resistors are commonly mounted outside the semiconductor integrated circuit as shown in FIG.
1
.
One disadvantage of utilizing external termination resistors is that as the number of signal paths between devices increases, the number of termination resistors required also increases, requiring an increased area for their mounting. This presents a problem in achieving higher density, small form-factor devices such as computer memory modules.
Computer memory devices is one area where impedance matching is necessary but additional components are undesirable. Computer memory architectures commonly require impedance matching to minimize reflections at higher frequencies.
FIG. 2
is a high-level schematic of a conventional computer memory architecture. A controller
202
manages communications with a plurality of memory modules
204
. Note that, typically, communications between a controller and memory devices are carried out over a bus
206
. The bus
206
consists of a plurality of individual transmission lines.
FIG. 3
provides more details as to the typical placement of termination resistors in the computer memory architecture of
FIG. 2. A
single memory module
204
and a single transmission line
206
is shown in
FIG. 3
for simplicity. Termination resistors
210
are coupled to each transmission line or conductor
206
near the node where the transmission line couples with an electronic device
202
,
212
&
214
. Electronic devices typically found in memory modules include memory components to store data, such as dynamic random access memory (DRAM)
214
, and buffers
212
that act as an interface between the bus and the memory devices. At each node where a transmission line couples to a device, a termination resistor
210
is necessary to avoid signal reflection. Termination resistors
210
thus add to the cost of memory modules
204
and make it more difficult to design memory modules with small form factors.
REFERENCES:
patent: 5528168 (1996-06-01), Kleveland
patent: 5592509 (1997-01-01), McClear et al.
patent: 5936429 (1999-08-01), Tomita
patent: 5955894 (1999-09-01), Vishwanthaiah et al.
patent: 6169418 (2001-01-01), Wagner
Bonella Randy
Halbert John
Blakely , Sokoloff, Taylor & Zafman LLP
Tan Vibol
Tokar Michael
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