Coded data generation or conversion – Analog to or from digital conversion – Analog to digital conversion
Reexamination Certificate
2002-05-01
2003-09-30
Williams, Howard L. (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Analog to digital conversion
C326S030000, C333S017300, C333S033000
Reexamination Certificate
active
06628223
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a resistance changeable device used for a data transmission system having a plurality of data transmission channels, and more particularly to a resistance changeable device used for a receiving end device which is used for a data transmission system requiring of the adjustment of an input impedance. Also, the present invention relates to a resistance changeable device usable for a differential output circuit such as a low voltage differential signaling (LVDS) system which is used for a notebook-sized personal computer and is characterized by data transmission at high speed, low voltage and low noise.
2. Description of Related Art
FIG. 6
is a view of an equivalent circuit of a conventional data transmission system, and
FIG. 7
is a view of an equivalent circuit of another conventional data transmission system. In FIG.
6
and
FIG. 7
,
1
indicates a transmission line formed of a single data bus,
2
indicates a transmitting end device, and
3
indicates a receiving end device. Also,
4
indicates a transmission line formed of two data buses,
5
indicates a transmitting end device, and
6
indicates a receiving end device in which noise is reduced by performing differential amplification.
FIG. 8
is a view of an equivalent circuit of a conventional data transmission system in which impedance matching is performed. In
FIG. 8
,
11
indicates a plurality of transmission lines formed of a plurality of data buses respectively,
12
indicates a transmitting end device,
13
indicates a receiving end device, and
14
indicates a termination resistor connected to each transmission line
11
on the outside of the corresponding receiving end device
13
.
Next, operations of the conventional data transmission systems will be described below.
In general, in cases where the transmission delay time in the transmission line
1
or
4
is longer than a rise time or a fall time of a transmitted signal, the transmission line
1
or
4
is regarded as a distributed constant line, and it is required to consider influence of noise such as reflection of the transmitted signal. In particular, in case of a high speed data transmission system, the transmission delay time in a transmission line is often longer than a rise time or a fall time of a transmitted signal, and it is important to protect the data transmission system from noise such as reflection of the signal caused by the impedance mismatching in the transmission line.
Also, a termination resistor is generally used to obtain the impedance matching in the transmission line. As shown in
FIG. 8
, in cases where the impedance matching among an output impedance Z
1
of the transmitting end device
12
, a characteristic impedance Z
0
of the transmission line
11
and a resistance value ZL of the termination resistor
14
is obtained, no reflection of the signal occurs fundamentally. In other words, it is important to match the resistance value ZL of the termination resistor
14
with the characteristic impedance Z
0
of the transmission line
11
.
However, in a general high speed data transmission system, a multi-bit bus such as an 8-bit bus or a 16-bit bus is often disposed as a plurality of data buses respectively connecting the transmitting end device
12
and the receiving end device
13
. Therefore, in cases where a plurality of termination resistors
14
disposed on the outside of the receiving end device
13
are connected to all input terminals of the receiving end device
13
respectively, problems have arisen that the number of constituent elements disposed on a substrate (or a semiconductor chip), on which the receiving end device is disposed, is increased, the manufacturing cost of the data transmission system is increased and an element disposing area of the substrate is increased.
In contrast, there is a case where the termination resistors
14
are disposed on the inside of the receiving end device
13
to reduce both the manufacturing cost of the data transmission system and the element disposing area of the substrate. However, in this case, because it is difficult to uniformly form the termination resistors
14
in the actual manufacturing so as to set the termination resistors
14
to the same resistance value as each other, there is high probability that the impedance mismatching occurs between the transmitting end device
12
and the receiving end device
13
. Also, in cases where the termination resistors
14
are disposed on the inside of the receiving end device
13
, each termination resistor
14
is fixed to a resistance value near to a desired resistance value. Therefore, a problem has arisen that the receiving end device
13
having one termination resistor
14
cannot be used for another data transmission system in which a characteristic impedance of a transmission line differs from the characteristic impedance Z
0
of the transmission line
11
originally connected to the receiving end device
13
. Therefore, in the prior art, in a viewpoint of data transmission characteristics, it is advantageous that a plurality of termination resistors adjusted according to the characteristic impedances Z
0
of the transmission line
11
respectively are disposed on the outside of the receiving end device
13
.
SUMMARY OF THE INVENTION
An object of the present invention is to provide, with due consideration to the drawbacks of the conventional data transmission systems, a resistance changeable device in which a resistance value of a termination resistor disposed on the inside of a semiconductor chip is adjustable so as to reduce both a manufacturing cost of a data transmission system and an element disposing area of the semiconductor chip and so as to keep the impedance matching of the data transmission system to a degree of the impedance matching obtained on the assumption that the termination resistor is disposed on the outside of the semiconductor chip.
The object is achieved by the provision of a resistance changing device comprising a pull-up resistor unit having a plurality of resistive elements of which first ends are connected to a high electric potential source, a switching circuit unit having a plurality of switches of which first ends are connected to second ends of the resistive elements of the pull-up resistor unit respectively and second ends are connected to a signal input terminal, a mode switch of which a first end is connected to the signal input terminal and which is switched on in response to a significant mode signal input to a mode setting terminal and is switched off in response to an insignificant mode signal input to the mode setting terminal, a reference voltage producing circuit for producing a reference voltage, a voltage-to-current converting unit, which has an NMOS transistor having a drain connected to a second end of the mode switch, an external resistor disposed outside a chip and having both an end connected to a source of the NMOS transistor and another end connected to a low electric potential source and an operational amplifier having a first input terminal connected to the reference voltage producing circuit, a second input terminal connected to a node between the NMOS transistor and the external resistor and an output terminal connected to a gate of the NMOS transistor, for converting the reference voltage received by the operational amplifier from the reference voltage producing circuit into a current flowing through the NMOS transistor according to a resistance value of the external resistor, an analog-to-digital converting circuit for converting a node voltage at a node between the mode switch and the NMOS transistor of the voltage-to-current converting unit into a digital measured value code, an expected value holding unit for holding a digital expected value code denoting an expected value of a composite resistance of the pull-up resistor unit, a decoding circuit for producing a correction code according to both the digital measured value code obtained by the analog-to-digital converting circuit a
Burns Doane , Swecker, Mathis LLP
Mitsubishi Denki & Kabushiki Kaisha
Williams Howard L.
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