Electronic digital logic circuitry – Signal sensitivity or transmission integrity – Bus or line termination
Reexamination Certificate
2000-04-20
2004-07-27
Cho, James H (Department: 2819)
Electronic digital logic circuitry
Signal sensitivity or transmission integrity
Bus or line termination
C326S033000
Reexamination Certificate
active
06768334
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus for transmitting/receiving signals between appliances or chips, and more specifically the present invention is suitable for a signal transmitting/receiving apparatus which requires a stable data transmission/reception using cables and flexible substrates even if supply voltages and ground voltages are different between a transmitting apparatus and a receiving apparatus such as in the case where signals are transmitted/received between devices (e.g., LSI or IC) mounted on a board, between different boards in an appliance, or between different appliances.
2. Description of the Related Art
In conventional signal transmission/reception, e.g., differential transmission, waveform irregularities such as reflection is prevented by impedance match between transmission paths, as in a signal transmitting/receiving apparatus
1000
shown in FIG.
9
A. In order to achieve this impedance match, a receiving device
130
is provided with: a terminating resistor
105
for short circulating a pair of differential lines
103
A and
103
C (i.e., data lines); and a bias generating circuit
102
for determining an intermediate potential between differential potentials, where the output of the bias generating circuit
102
is connected at a midpoint of the terminating resistor
105
. This will set the intermediate potential of the pair of differential lines
103
A and
103
C to Vcm, which is a bias voltage output from the bias generating circuit
102
, whereby the problem of waveform irregularities such as the reflection between the pair of differential lines
103
A and
103
C is solved. In the case where the difference between a supply voltage VCC
1
of a transmitting device
120
and a supply voltage VCC
2
of a receiving device
130
and the difference between a ground voltage GND
1
of the transmitting device
120
and a ground voltage GND
2
of the receiving device
130
are not large, the intermediate potential between the pair of the differential lines
103
A and
103
C of a transmitting device
120
is also around Vcm.
The amplitude potential of the pair of differential lines
103
A and
103
C to determined by a value of a current flowing through the differential lines
103
A and
103
C, and by a value of the terminating resistor
105
Since the impedance of the differential lines
103
A and
103
C is usually
110
&OHgr;, the value of the terminating resistor
105
is also set to
110
&OHgr; for Impedance matching. Thus, when a driver circuit
101
of the transmitting device
120
applies a 2 mA current to the transmission path
110
, the amplitude voltage of the differential lines
103
A and
103
C will be 220 mV. If the bias potential is 2.0 V, the higher potential of the differential lines
103
A and
103
C will be 2.11 V (2.0 V+220 mV/2), and the lower potential of the differential lines
103
A and
103
C will be 1.89 V (2.0 V−220 mV/2).
Therefore, if the driver circuit
101
of the transmitting device
120
applies a stable 2 mA current to the higher output terminal (2.11 V) of output terminals A and C, data can be transmitted efficiently at a high-speed of 400 MHz or greater in the form of a small amplitude transmission of 220 mV. If the supply potential VCC
1
of the driver circuit
101
is sufficiently higher than the potential of the higher output terminal (the potential corresponding to Vd of the driver circuit
101
in
FIG. 11
is 2.11 V), a current can be applied from a PMOS transistor
1101
in a driver circuit
101
(as shown in
FIG. 11
) to the output terminal A or C. Therefore, data can be transmitted efficiently at a high-speed of 400 MHz or greater in the form of a small amplitude transmission of 220 mV, as mentioned above.
However, in the case where the difference between the supply voltage VCC
1
of the transmitting device
120
and the supply voltage VCC
2
of the receiving device
130
, and the difference between the ground voltage GND
1
of the transmitting device
120
and the ground voltage GND
2
of the receiving device
130
are relatively large, the potentials of the output terminals A and C of the driver circuit
101
of the transmitting device
120
(i.e., the potential of the transmission paths
110
) may become infinitely close to the supply voltage VCC
1
of the driver circuit
101
, or even higher than the supply voltage VCC
1
of the driver circuit
101
, thereby making it difficult or impossible to apply a current from the driver circuit
101
to the transmission path
110
. In other words, such a state causes a problem of not being able to transmit data.
FIG. 9B
illustrates the problem caused by the difference between the ground potential GND
1
of the transmitting device
120
and the ground potential GND
2
of the receiving device
130
in the signal transmitting/receiving circuit
1000
shown in FIG.
9
A.
FIG. 10B
illustrates the problem caused by the difference between a supply voltage VCC
1
of a transmitting device
220
and a supply voltage VCC
2
of a receiving device
230
in a signal transmitting/receiving circuit
2000
as shown in FIG.
10
A. These problems will now be more specifically described in reference to
FIGS. 9A through 10B
.
FIGS. 9A and 9B
show the case where the ground potential GND
1
of the transmitting device
120
and the ground potential GND
2
of the receiving device
130
are different. More specifically, It is assumed that the ground potential GND
2
of the receiving device
130
is higher than the ground potential GND
1
of the transmitting device
120
. In this case, as shown in
FIG. 93
, if the intermediate potential Vcm of the pair of differential lines
103
A and
103
C becomes higher than the supply voltage VCC
1
of the driver circuit
101
of the transmitting device
120
, it is impossible to apply a current. This difference between the ground potentials (GND
2
−GND
1
) is prone to occur when data is transmitted/received between different appliances grounded at different sites. A typical example of this is the case where the transmitting device
120
is a floor model VCR whose power is supplied from an outlet. In such a case, the ground potential GND
1
is determined by the ground potential of the outlet. If the corresponding receiving device
130
is a video camera operating on an internal battery, the ground of the video camera is only connected to the housing of the video camera. Therefore, the ground of the camera will be a ground potential GND
2
, which may inevitably be different from the ground potential of the outlet. In the case where the power is supplied from such a floor model VCR to such a video camera via a cable (esp. IEEE 1394 and the like), the ground potential GND
2
of the video camera may become about 0.5 V to 1.0 V higher than the ground potential GND
1
of the floor model VCR (i.e., GND
2
=GND
1
+0.5 V to 1.0 V) due to the cable resistance.
In this case, the intermediate potential Vcm generated by the receiving device
130
appears higher (e.g., 0.5 V to 1.0 V) than the ground potential GND
1
of the transmitting device
120
, with a general tendency as shown in FIG.
9
B. For example, if the intermediate potential is set at 2.0 V in the receiving device
130
, it will become 2.5 V to 3.0 V in the transmitting device
120
. If the supply voltage VCC
1
of the driver circuit
101
in the transmitting device
120
is set at 2.5 V, the potential Vd shown in
FIG. 11
will be, for example, 2.61 V to 3.11 V, which means VCC
1
≦Vd. Therefore, a problem exists when the PMOS transistor
1101
shown in
FIG. 11
is not able to apply a current to the output terminals A and C.
FIG. 10A
shows the case where the supply voltage VCC
1
of the transmitting device
220
and the supply voltage VCC
2
of the receiving device
230
are different. More specifically, it is assumed that the supply voltage VCC
2
of the receiving device
230
is higher than the supply voltage VCC
1
of the transmitting device
220
. In this case, as shown in
FIG. 10B
, the intermediate poten
Yamauchi Hiroyuki
Yoshida Tadahiro
Cho James H
Snell & Wilmer LLP
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