1. Field of the Invention
The present invention relates to a level converting circuit, and particularly to a signal-level converting circuit for bidirectionally transmitted logic signals.
2. Description of the Related Art
When a signal is transferred between circuits having different signal levels, a signal level converting circuit must be provided between the circuits. Conventionally, when only signal level conversion (hereinafter referred to as "level conversion") is intended between circuits, level conversion has been accomplished by an IC for level conversion provided between the two circuits. Such an IC, however, is capable of carrying out level conversion in only one direction of signal transfer between the circuits of differing signal levels. Consequently, in order to carry out level conversion bidirectionally between circuits such as bus lines which handle bidirectional signals, two types of level conversion ICs are necessary. Furthermore, switching between the bidirectional signals necessitates a number of switches corresponding to the signal lines making up the bus line as well as a control circuit for controlling switching of these switches. Hereinafter, level conversion in one direction of signal transfer between circuits of differing signal levels is referred to as one directional conversion, and level conversion in both directions of signal transfer between circuits of differing signal levels is referred to as bidirectional conversion.
Now, an example of the above-described conventional bidirectional conversion circuit will be explained with reference to FIG. 1. FIG. 1 shows a block diagram of an example of a conventional bidirectional conversion circuit which bidirectionally converts a logic signal having a signal level of 5 volts (hereinafter referred to as a "5-V logic signal") and a logic signal having a signal level of 3 volts (hereinafter referred to as a "3-V logic signal").
The bidirectional conversion circuit is equipped with a first switch 32 to which each of 5-V input/output terminals ({1}-{8})31 is connected, a 5-to-3 level converter 33 that converts signal level from 5 volts to 3 volts, a second switch 34, 3-V input/output terminals ({1}-{8}) 35 connected to the second switch 34, a control circuit 36 for controlling switching of the first and second switches 32, 34, a control terminal 37 connected to the control circuit 36, and a 3-to-5 level converter 38 that converts signal level from 3 volts to 5 volts. The 3-volt power source terminal 40 and 5-V power source terminal 41 are connected to 5-to-3 level converter 33 and 3-to-5 level converter 38, respectively.
First will be explained the operation of level conversion from 5 volts to 3 volts (hereinafter referred to as "5-to-3 conversion"). A 5-V logic signal is applied at 5-V input/output terminals 31, and after passing through the first switch 32, the 5-V logic signal is converted to a 3-V logic signal at 5-to-3 level converter 33. This logic signal is delivered to 3-V input/output terminals 35 by way of the second switch 34.
Next will be explained the operation of level-conversion from 3 volts to 5 volts (hereinafter referred to as "3-to-5 conversion").
Control circuit 36 is employed for 3-to-5 conversion. In this case, a synchronous clock signal, control data (information describing which switches to turn on and which switches to turn off), and a strobe signal that controls switching timing are supplied to control terminals 37. In response to the strobe signal, control circuit 36 outputs switching control signals corresponding to the control data. By this switching control signal, each switch of the first and second switches 32, 34 is switched, in this example, in the direction opposite that shown in the figure. The 3-V logic signals are received at 3-V input/output terminals 35, and after passing through the second switch 34, are converted to 5-V logic signals at 3-to-5 level converter 38. Subsequently, the level-converted signals pass through the first switch 32 and are delivered to the 5-V input/output terminals 31.
Regarding the prior art signal level conversion, a circuit is disclosed in Japanese Patent Laid-open 227317 (92). This circuit is an output buffer circuit that converts a CMOS logic level to an ECL logic level. In this circuit, ECL voltage signals of high and low voltage levels are generated in advance by a reference voltage circuit. Signal level conversion from a CMOS level to a ECL level is effected by a switch that operates to connect either of said ECL voltage signals to an ECL load in accordance with the CMOS logic states.
Because the above-described bidirectional conversion circuit of the prior art can execute level conversion using an IC for one directional conversion, level conversion between circuits such as bus lines that handle bidirectional signals necessitates at least two types of level converters as well as switches in a number equal to the number of signal lines making up the bus lines and a control circuit unit for controlling the switches. Consequently, this type of bidirectional conversion circuit of the prior art has the drawback that a large number of parts and connectors are needed, resulting in complex circuit construction.