FIG. 11 shows a configuration of a conventional data transmission device 200. The data transmission device 200 includes a driver 210 for sending data, a receiver 220 for receiving the data sent from the driver 210, and a transmission line path 230 for connecting between the driver 210 and the receiver 220. Data is transmitted via the transmission line path 230 from the driver 210 to the receiver 220.
The driver 210 includes an output buffer 212 for outputting data onto the transmission line path 230. The output buffer 212 is connected via a pad 214 to the transmission line path 230.
The receiver 220 includes an input buffer 222 for receiving data from the transmission line path 230. One input terminal of the input buffer 222 is connected via a pad 224 and a stub resistor 232 to the transmission line path 230.
An end of a terminator resistor 240 is connected to an end on the receiver 220 side of the transmission line path 230. The other end of the terminator resistor 240 is connected to a terminator potential V.sub.term.
The amplitude of a data signal on the transmission line path 230 is determined by the resistance of the terminator resistor 240 and the output impedance of the driver 210. Therefore, with an appropriate setting of the resistance of the terminator resistor 240 and the output impedance of the driver 210, the amplitude of the data signal on the transmission line path 230 can be limited to a sufficiently small value.
The resistance of the terminator resistor 240 is typically set so as to be substantially equal to the characteristic impedance Z of the transmission line path 230. This prevents data sent from the driver 210 from being reflected at the end on the receiver 220 side of the transmission line path 230.
However, the use of the terminator resistor 240 for terminating the transmission line path 230 causes a problem such that there is power consumption in the absence of data transmission on the transmission line path 230. This is because when data is held at a HIGH level, a direct current (I.sub.sink) flows from the terminator potential V.sub.term to the driver 210 via the terminator resistor 240; and when data is held at a LOW level, a direct current (I.sub.source) flows from the driver 210 to the terminator potential V.sub.term via the terminator resistor 240.
Also, in the presence of data transmission, since a direct current flows via the terminator resistor 240, the slopes of a waveform showing the transition of the potential of the transmission line path 230 becomes mild as the potential difference between the potential of the transmission line path 230 and the terminal potential V.sub.term is increased (see FIG. 12). This often causes skew.
Further, the output impedance of the driver 210 when the driver 210 outputs data of the HIGH level is not always in agreement with the output impedance of the driver 210 when the driver 210 outputs data of the LOW level. When these are not in agreement with each other, the absolute value of the direct current (I.sub.source) flowing from the driver 210 to the terminal potential V.sub.term is not identical to the absolute value of the direct current (I.sub.sink) flowing from the terminal potential V.sub.term to the driver 210. Therefore, the value of the potential amplitude of the transmission line path 230 from the terminal potential V.sub.term when the driver 210 outputs the HIGH level data is different from the value of the potential amplitude of the transmission line path 230 from the terminal potential V.sub.term when the driver 210 outputs the LOW level data.
This means that the terminal potential V.sub.term is shifted from a middle value between a potential (Hi-potential) corresponding to the HIGH level data and a potential (Lo-potential) corresponding to the LOW level data. For instance, in an example shown in FIG. 12, the terminal potential V.sub.term is 1.1 V; the Hi-potential is 1.5 V; and the Lo-potential is 0.8 V.
The receiver 220 determines whether data on the transmission line path 230 has the HIGH level or the LOW level using the terminal potential V.sub.term as a reference potential. Therefore, when the terminal potential V.sub.term is shifted from the middle value of the Hi-potential and the Lo-potential, the time which it takes data to transit from the LOW level to the HIGH level is different from the time which it takes data to transit from the HIGH level to the LOW level. This is responsible for skew occurring when the receiver 220 latches data on the transmission line path 230 in synchronization with a predetermined clock signal.
An object of the present invention is to provide a data transmission device in which power consumption is reduced.
Another object of the present invention is to provide a data transmission device in which occurrence of skew is prevented.