Selector circuit, equalizer circuit, and semiconductor integrated circuit

A first P-channel transistor to a gate of which a first input signal is inputted and a second P-channel transistor to a gate of which a selection signal is inputted are provided in series between a power supply line and an output node. A first N-channel transistor to a gate of which a second input signal is inputted and a second N-channel transistor to a gate of which the selection signal is inputted are provided in series between a ground line and the output node. A third P-channel transistor to a gate of which the second input signal is inputted is provided between the gate of the second P-channel transistor and the output node, and a third N-channel transistor to a gate of which the first input signal is inputted is provided between the gate of the second N-channel transistor and the output node.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2015-103191, filed on May 20, 2015, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are directed to a selector circuit, an equalizer circuit, and a semiconductor integrated circuit.

BACKGROUND

A selector circuit selectively outputs one input signal from among a plurality of input signals in correspondence with a selection signal. The selector circuit is one of basic elements of a CMOS logic circuit. For example, as illustrated in a truth table inFIG. 13, a two-input selector circuit, for two input signals Q0, Q1, outputs the input signal Q0as an output signal Z when a selection signal S1is “0” and outputs the input signal Q1as the output signal Z when the selection signal S1is “1”.

As a selector circuit which realizes a function of the truth table illustrated inFIG. 13, there are ones illustrated inFIG. 14AtoFIG. 14C, for example.FIG. 14AtoFIG. 14Care diagrams illustrating configuration examples of a conventional two-input selector circuit.

In the selector circuit illustrated inFIG. 14A, an input signal Q1and a selection signal S1are inputted to a NAND (negative logical product) gate41, and an input signal Q0and the selection signal S1inverted by an inverter44are inputted to a NAND gate42. Outputs of the NAND gates41,42are inputted to a NAND gate43. An output of the NAND gate43is outputted as an output signal Z. The selector circuit illustrated inFIG. 14Aoutputs a value of the input signal Q0as the output signal Z via the NAND gates42,43when the selector signal S1is “0”, and outputs a value of the input signal Q1as the output signal Z via the NAND gates41,43when the selection signal S1is “1”.

In the selector circuit illustrated inFIG. 14B, an input signal Q1is inputted to a transfer gate48composed of a P-channel MOS transistor MP21and an N-channel MOS transistor MN21, and an input signal Q0is inputted to a transfer gate49composed of a P-channel MOS transistor MP22and an N-channel MOS transistor MN22. The two transfer gates48,49are controlled to come to be in ON states (continuity states) exclusively, by a selection signal S1and the selection signal S1inverted by the inverter45. In the selector circuit illustrated inFIG. 14B, the transfer gate49comes to be in an ON state when the selection signal S1is “0”, a value of the input signal Q0being outputted as an output signal Z, and the transfer gate48comes to be in an ON state when the selection signal S1is “1”, a value of the input signal Q1being outputted as the output signal Z.

The selector circuit illustrated inFIG. 14Cis a selector circuit by a dynamic logic circuit. In the selector circuit illustrated inFIG. 14C, a precharge period and an evaluation period are repeated alternately and a selector function is realized in the evaluation period. An input node of an inverter47which outputs an output signal Z is connected to a power supply line via a 2-channel MOS transistor MP23to a gate of which a precharge signal φpcis inputted. The input node of the inverter47is connected to a ground line via N-channel MOS transistors MN23, MN24to gates of which an input signal Q1and a selection signal S1are inputted respectively, and connected to a ground line via N-channel MOS transistors MN25, MN26to gates of which an input signal Q0and the selection signal S1inverted by an inverter46are inputted respectively.

In the selector circuit illustrated inFIG. 14C, in the precharge period (at this time, both the input signals Q0, Q1are “0”) during which the precharge signal φpcis “0”, the input node of the inverter47is reset to be “1” and the output signal Z is reset to be “0”. In the evaluation period during which the precharge signal φpcis “1”, the input node of the precharged inverter47transits to “0” and the output signal Z transits to “1” when the selection signal S1and the input signal Q1are “1” simultaneously or when the inversion signal of the selection signal S1and the input signal Q0are “1” simultaneously, whereby the selector function is realized.

In the two-input selector circuits illustrated inFIG. 14AtoFIG. 14C, the selector function is each realized by the inversion signal of the selection signal S1obtained by inverting the selection signal S1by the inverters44,45,46. In contrast, as illustrated inFIG. 15A, there is suggested a two-input selector circuit which realizes a selector function by connecting P-channel MOS transistors MP31to MP34, N-channel MOS transistors MN31to MN34, and an inverter51, without using an inversion signal of a selection signal S1(see Patent Document 1).

When the selection signal S1is “1”, the P-channel MOS transistors MP32, MP34come to be in OFF states and the N-channel MOS transistor MN33comes to be in an ON state, and thus a signal path of the selector circuit illustrated inFIG. 15Abecomes as illustrated inFIG. 15B. When it is assumed that the N-channel MOS transistor MN31is almost in an ON state, a circuit illustrated inFIG. 15Bperforms an operation of an inverter whose input is an input signal Q1, so that an output signal Z is the same value as that of the input signal Q1, which means that the input signal Q1is selected.

When the selection signal S1is “0”, the P-channel MOS transistor MP32comes to be in an ON state and the N-channel MOS transistor MN33comes to be in an OFF state, and thus the signal path of the selector circuit illustrated inFIG. 15Abecomes as illustrated inFIG. 15C. When it is assumed that the P-channel MOS transistor MP34is almost in an ON state, a circuit illustrated inFIG. 15Cperforms an operation of an inverter whose input is an input signal Q0, so that the output signal Z is the same value as that of the input signal Q0, which means that the input signal Q0is selected. As described above, the selector circuit illustrated inFIG. 15Aoperates as a selector circuit which outputs the value of the input signal Q1as the output signal Z when the selection signal S1is “1”, and which outputs the value of the input signal Q0as the output signal Z when the selection signal S1is “0”.

As one of circuits in which a selector circuit is used, there is a decision feedback equalizer (DFE) used for a receiver of a serializer/de-serializer (SerDes).FIG. 16Ais a diagram illustrating an application example of the selector circuit in the decision feedback equalizer. A selector circuit61selects, in correspondence with a selection signal S1, an input signal from input signals Q0, Q1being decision results at reference voltages corresponding to cases where previous data is “0” and “1” respectively and outputs as an output signal Z. A flip-flop62latches the output signal Z of the selector circuit61in synchronization with a clock signal CK, and outputs the latched signal as an output signal OUT and outputs the latched signal to the selector circuit61as the selection signal S1related to the next data.

In a decision feedback equalizer such as illustrated inFIG. 16A, an output signal of a selector circuit is feedbacked as a selection signal of the selector circuit. In a case where the decision feedback equalizer deals with a high-speed signal whose data rate or clock is of high speed, generation of the selection signal (a loop part in which the selection signal S1is generated through the flip-flop62in the example of the drawing) sometimes becomes a bottleneck of a circuit operation. In this case, if an inversion signal of the selection signal is generated from the selection signal by using an inverter as in the selector circuits illustrated inFIG. 14AtoFIG. 14C, there is a problem that an operating frequency of the decision feedback equalizer becomes low due to delay of the above. For example, as illustrated inFIG. 16B, as a result that an entire circuit is constituted with a differential circuit by using two selectors61A,61B and two flip-flops62A,62B, it becomes possible to generate an inversion signal of a selection signal without delay. However, if the entire circuit is constituted with the differential circuit, a circuit scale becomes twofold, and a power consumption and a circuit area also become twofold.

Though the selector circuit illustrated inFIG. 15Arealizes the selector function without using the inversion signal of the selection signal S1, there is a case where a high-speed operation is not performed as described below. For example, in a case where the input signal Q1is “0” and the input signal Q0is “1” when the selection signal S1is “1”, a potential of an input node N of the inverter51rises quickly to a potential lowered from a power supply voltage by a threshold voltage of the N-channel MOS transistor MN31. However, in order for rising to a power supply voltage level thereafter, it is necessary to wait for a leak of the N-channel MOS transistor MN31, so that a high-speed operation is not performed in a recent circuit which operates at a low voltage. Similarly, in a case where the input signal Q0is “1” when the selection signal S1is “0”, for example, the potential of the input node N of the inverter51lowers quickly to a potential raised from a ground voltage by a threshold voltage of the P-channel MOS transistor MP34. However, for lowering to a ground level thereafter, it is necessary to wait for a leak from the P-channel MOS transistor MP34, so that a high-speed operation is not be performed in a circuit operated at a low voltage.

SUMMARY

In one aspect of a selector circuit, a first P-channel transistor to a gate of which a first input signal is inputted and a second P-channel transistor to a gate of which a selection signal is inputted are provided in series between a power supply line and an output node. And a first N-channel transistor to a gate of which a second input signal is inputted and a second N-channel transistor to a gate of which the selection signal is inputted are provided in series between a ground line and the output node. A third P-channel transistor to a gate of which the second input signal is inputted is provided between the gate of the second P-channel transistor and the output node. A third N-channel transistor to a gate of which the first input signal is inputted is provided between the gate of the second N-channel transistor and the output node.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments will be described based on the drawings. It is assumed hereinafter that each signal is of positive logic, and explanation is carried out on the assumption that “1” means being at a high level (first logic level) and that “0” means being at a low level (second logic level).

FIG. 1is a diagram illustrating a configuration example of a selector circuit in a present embodiment. The selector circuit illustrated inFIG. 1includes P-channel MOS transistors MP1to MP4, N-channel MOS transistors MN1to MN4, and inverters INV1, INV2.

The first P-channel MOS transistor MP1to a gate of which a first input signal Q0is inputted and the second P-channel MOS transistor MP2to a gate of which a selection signal S1is inputted are connected in series between a power supply line which supplies a power supply voltage and an output node N. The first N-channel MOS transistor MN1to a gate of which a second input signal Q1is inputted and the second N-channel MOS transistor MN2to a gate of which the selection signal S1is inputted are connected in series between a ground line which supplies a reference voltage (ground voltage) and the output node N.

The third P-channel MOS transistor MP3to a gate of which the second input signal Q1is inputted and the fourth P-channel MOS transistor MP4to a gate of which a signal XZ with the same value as that of the output node N is inputted are connected in series between the output node N and the gate of the second P-channel MOS transistor MP2. The third N-channel MOS transistor MN3to a gate of which the first input signal Q0is inputted and the fourth N-channel MOS transistor MN4to a gate of which the single XZ with the same value as that of the output node N is inputted are connected in series between the output node N and the gate of the second N-channel MOS transistor MN2.

The inverter INV1logically inverts a signal of the output node N and outputs as an output signal Z. The inverter INV2logically inverts the output signal Z and outputs as an inversion signal XZ of the output signal Z.

In other words, in the first P-channel MOS transistor MP1, a source is connected to the power supply line, to the gate is inputted the first input signal Q0, and a drain is connected to a source of the second P-channel MOS transistor MP2. In the second P-channel MOS transistor MP2, to the gate is inputted the selection signal S1, and a drain is connected to the output node N. In the fourth P-channel MOS transistor MP4, a source is connected to the gate of the second P-channel MOS transistor MP2, to the gate is inputted the inversion signal XZ of the output signal Z, and a drain is connected to a source of the third P-channel MOS transistor MP3. In the third P-channel MOS transistor MP3, to the gate is inputted the second input signal Q1and a drain is connected to the output node N.

In the first N-channel MOS transistor MN1, a source is connected to the ground line, to the gate is inputted the second input signal Q1, and a drain is connected to a source of the second N-channel MOS transistor MN2. In the second N-channel MOS transistor MN2, to the gate is inputted the selection signal S1, and a drain is connected to the output node N. In the fourth N-channel MOS transistor MN4, a source is connected to the gate of the second N-channel MOS transistor MN2, to the gate is inputted the inversion signal XZ of the output signal Z, and a drain is connected to a source of the third N-channel MOS transistor MN3. In the third N-channel MOS transistor MN3, to the gate is inputted the first input signal Q0, and a drain is connected to the output node N.

Next, an operation of the selector circuit illustrated inFIG. 1will be described. When the selection signal S1is “1”, the P-channel MOS transistor MP2comes to be in an OFF state, and the N-channel MOS transistor MN2comes to be in an ON state, so that a signal path of the selector circuit illustrated inFIG. 1becomes as illustrated in FTG.2A. The N-channel MOS transistor MN3to which the input signal Q0is inputted exists in a path that connects the power supply voltage (selection signal S1, in practice) and the output node N, and the N-channel MOS transistor MN4to which the inversion signal XZ is inputted is inserted in series into this path.

When the input signal Q1is “1”, the P-channel MOS transistor MP3comes to be in an OFF state and the N-channel MOS transistor MN1comes to be in an ON state. In a case where the N-channel MOS transistor MN1is in the ON state, even if the N-channel MOS transistors MN3, MN4are in ON states, the N-channel MOS transistors MN3, MN4operate in a saturation region (operating similarly to a current source), whereby a potential of the output node N comes to be a potential almost equal to a ground level. As a result that the potential of the output node N becomes the potential almost equal to the ground level, the output signal Z comes to be “1” and the inversion signal XZ comes to be “0”. The N-channel MOS transistor MN4comes to be in an OFF state finally and a current in this path is interrupted, so that the output node N is also settled to be “0” finally. Thereby, the output signal Z is determined to be “1”.

For example, as illustrated inFIG. 3A, when the selection signal S1changes from “0” to “1” in a state where the input signal Q0is “0” and the input signal Q1is “1”, the N-channel MOS transistor MN2comes to be in the ON state and the P-channel MOS transistor MP2comes to be in the OFF state. As a result that the N-channel MOS transistor MN2comes to be in the ON state, the output node N comes to be “0” via the N-channel MOS transistors MN1, MN2, so that the output signal Z comes to be “1”. As a result that the output signal Z comes to be “1”, the inversion signal XZ comes to be “0”, the N-channel MOS transistor MN4comes to be in the OFF state, and the P-channel MOS transistor MP4comes to be in an ON state, but no influence is given since both the N-channel MOS transistor MN3and the P-channel MOS transistor MP3are in the OFF states due to the input signals Q0and Q1.

If it is assumed that the input signal Q0is “X” (indefinite), as illustrated inFIG. 3B, while the output node N is pulled down to “0” via the N-channel MOS transistors MN1, MN2, there can exist a path connected to the output node N via the N-channel MOS transistors MN3, MN4. In such a case, as a result that the N-channel MOS transistors MN3, MN4operate similarly to a cascode current source of the N-channel MOS transistor as described above, on-resistances exhibited by the N-channel MOS transistors MN1, MN2are substantially smaller in relation to resistances exhibited by the N-channel MOS transistors MN3, MN4, so that the output node N comes to be almost “0” level. Thereafter, the N-channel MOS transistor MN4comes to be in the OFF state and the path constituted with the N-channel MOS transistors MN3, MN4are interrupted, so that the output, node N comes to be “0”.

When the input signal Q1is “0”, the P-channel MOS transistor MP3comes to be in an ON state and the N-channel MOS transistor MN1comes to be in an OFF state. As a result that the N-channel MOS transistor MN1comes to be in the OFF state, a path from the output node N to the ground line is disconnected. In a case where the output node N is “0”, the inversion signal XZ is also “0”, and thus the P-channel MOS transistor MP4is in the ON state, so that a path connecting a power supply and the output node N, the path being constituted with the P-channel MOS transistors MP3, MP4, is conducted. Thereby, the output node N transits to “1”, and the output signal Z is determined to be “0” in response thereto. In a case where the output node N is “1”, the output signal Z has originally been determined to be “0”, so that transition does not occur. As a result that the output signal Z is determined to be “0”, the inversion signal XZ comes to be “1”, and the P-channel MOS transistor MP4comes to be in an OFF state and the N-channel MOS transistor MN4comes to be in the ON state. A path into which the N-channel MOS transistor MN4is inserted is the path connecting the power supply voltage (selection signal S1, in practice) and the output node N as described above, the output node N being “1” regardless of whether that path is in an ON state or in an OFF state, so that transition of the state does not occur and no influence is given.

For example, as illustrated inFIG. 4, when the selection signal S1changes from “0” to “1” in a state where the input signal Q0is “1” and the input signal Q1is “0”, the N-channel MOS transistor MN2comes to be in the ON state and the P-channel MOS transistor MP2comes to be in the OFF state. Then, the output node N comes to be “1” via the P-channel MOS transistors MP3, MP4, and the output signal Z comes to be “0”. As a result that the output signal Z comes to be “0”, the inversion signal XZ comes to be “1”, the N-channel MOS transistor MN4comes to be in the ON state, and the P-channel MOS transistor MP4comes to be in the OFF state, but the signal of the output node N does not change since the output node N is connected to the power supply voltage (selection signal S1, in practice) via the N-channel MOS transistors MN3, MN4.

When the selection signal S1is “0”, the P-channel MOS transistor MP2comes to be in an ON state, and the N-channel MOS transistor MN2comes to be in an OFF state, so that a signal path of the selector circuit illustrate inFIG. 1becomes as illustrated inFIG. 2B. The P-channel MOS transistor MP3to which the input signal Q1is inputted exists in the path that connects the ground voltage (selection signal S1, in practice) and the output node N, and the P-channel. MOS transistor MP4to which the inversion signal XZ is inputted is inserted in series into that path.

When the input signal Q0is “0”, the N-channel MOS transistor MN3comes to be in the OFF state and the P-channel MOS transistor MP1comes to be in an ON state. In a case where the P-channel MOS transistor MP1is in the ON state, even if the P-channel MOS transistors MP3, MP4are in the ON states, the P-channel MOS transistors MP3, MP4operate in a saturation region (operates similarly to a current source), whereby the potential of the output node N comes to be a potential almost equal to a power supply voltage level. As a result that the potential of the output node N comes to be almost equal to the power supply voltage level, the output signal Z comes to be “0” and the inversion signal XZ comes to be “1”. The P-channel MOS transistor MP4comes to be in the OFF state finally and the current in this path is interrupted so that the output node N is also settled to be “1” finally. Thereby, the output signal Z is determined to be “0”.

When the input signal Q0is “1”, the N-channel MOS transistor MN3comes to be in the ON state and the P-channel MOS transistor MP1comes to be in an OFF state. As a result that the P-channel MOS transistor MP1comes to be in the OFF state, the path from the output node N to the power supply line is disconnected. In a case where the output node N is “1”, the inversion signal XZ is also “1”, and thus the N-channel MOS transistor MN4is in the ON state, so that the path connecting the ground voltage and the output node N, the path being constituted with the N-channel MOS transistors MN3, MN4, is conducted. Thereby, the output node N transits to “0” and the output signal Z is determined to be “1” in response thereto. In a case where the output node N is “0”, the output signal Z has originally been determined to be “1”, so that transition does not occur. As a result that the output signal Z is determined to be “1”, the inversion signal XZ comes to be “0”, and the N-channel MOS transistor MN4comes to be in the OFF state and the P-channel MOS transistor MP4comes to be in the ON state. The path into which the P-channel MOS transistor MP4is inserted is the path connecting the ground voltage (selection signal S1, in practice) and the output node N as described above, the output node N being “0” regardless of whether that path is in an ON state or in an OFF state, so that transition of the state does not occur and no influence is given.

As described above, in a case where the selection signal S1is “1”, the selector circuit illustrated inFIG. 1outputs the output signal Z of “1” when the input signal Q1is “1 and outputs the output signal Z of “0” when the input signal Q1is “0”, regardless of the state of the input signal Q0. In a case where the selection signal S1is “0”, the selector circuit outputs the output signal Z of “1” when the input signal Q0is “1” and outputs the output signal Z of “0” when the input signal Q0is “0”, regardless of the state of the input signal Q1. Therefore, the selector circuit illustrated inFIG. 1can perform a selector function without using an inversion signal of the selection signal S1, so that a selector circuit which operates at a high speed can be realized. For example, since the selector circuit in the present embodiment can realize the selector function without using the inversion signal of the selection signal S1, it is possible to realize an operation at a higher speed in a decision feedback equalizer (DFE) or the like in which generation of a selection signal becomes a bottleneck of an operation speed of a circuit.

The reason why the selector function can be realized without using the inversion signal of the selection signal S1is a circuit configuration which utilizes a fact that a behavior of an ON state and an OFF state in a case where a signal is given to a gate terminal is inverted in a P-channel MOS transistor and an N-channel MOS transistor. A path in which an input signal Q0mainly works is constituted with P-channel MOS transistors and a path in which an input signal Q1mainly works is constituted with N-channel MOS transistors, whereby exclusive selection is realized by the same selection signal S1. With these paths only, there is a case where an output node N comes to be open depending on a combination of states of the selection signal S1and the input signals Q0, Q1, and thus, a path connecting the selection signal S1and the output node N is created and further a shoot-through current path is interrupted by using the signal XZ obtained by inverting the output signal Z so as that a steady shoot-through current does not flow. Since interruption of the shoot-through current is independent of a signal processing and is not required to be performed at a high speed, usage of a signal obtained by inverting an output does not cause a problem in operation.

Note that since the output node N is of inversion logic of the output signal Z, the output node N can be used as an inversion output of the selector circuit as illustrated inFIG. 5. The output node N is determined faster than the output signal Z by one stage of an inverter, and in a case where a configuration using an inversion output of a selector circuit in a circuit using the selector circuit is adopted, it becomes possible to operate at a faster speed.

In the selector circuit illustrated inFIG. 1, the P-channel MOS transistor MP4and the N-channel MOS transistor MN4to gates of which the inversion signal XZ of the output signal Z is inputted are provided, and the shoot-through current path is interrupted by using the inversion signal XZ so that the stable shoot-through current may not flow. However, in a case where a shoot-through current may flow, it is possible that the P-channel MOS transistor MP4and the N-channel MOS transistor MN4to the gates of which the inversion signal XZ is inputted are omitted (source/drain short circuit) as illustrated inFIG. 6.

FIG. 6is a diagram illustrating another configuration example of the selector circuit in the present embodiment. InFIG. 6, the same reference symbol is given to a component the same as a component illustrated inFIG. 1. The selector circuit illustrated inFIG. 6includes P-channel MOS transistors MP1to MP3, N-channel MOS transistors MN1to MN3, and an inverter INV1.

The first P-channel MOS transistor MP1to a gate of which a first input signal Q0is inputted and the second P-channel MOS transistor MP2to a gate of which a selection signal S1is inputted are connected in series between a power supply line and an output node N. The first N-channel MOS transistor MN1to a gate of which a second input signal Q1is inputted and the second N-channel MOS transistor MN2to a gate of which the selection signal S1is inputted are connected in series between a ground line and the output node N.

The third P-channel MOS transistor MP3to a gate of which the second input signal Q1is inputted is connected between the output node N and the gate of the second P-channel MOS transistor MP2. The third N-channel MOS transistor MN3to a gate of which the first input signal Q0is inputted is connected between the output node N and the gate of the second N-channel MOS transistor MN2. The inverter INV1logically inverts a signal of the output node N and outputs as an output signal Z.

An operation of the selector circuit illustrated inFIG. 6will be described. When the selection signal S1is “1”, the P-channel MOS transistor MP2comes to be in an OFF state and the N-channel MOS transistor MN2comes to be in an ON state, so that a signal path of the selector circuit illustrated inFIG. 6becomes as illustrated in FIG.7A.

When the input signal Q1is “1”, the P-channel MOS transistor MP3comes to be in an OFF state and the N-channel MOS transistor MN1comes to be in an ON state. In a case where the N-channel MOS transistor MN1is in the ON state, even if the N-channel MOS transistor MN3is in ON states, the N-channel MOS transistor MN3operates in a saturation region (operating similarly to a current source), whereby a potential of the output node N comes to be a potential almost equal to a ground level. As a result that the potential of the output node N becomes the potential almost equal to the ground level, the output signal Z becomes “1”.

For example, as illustrated inFIG. 8A, when the selection signal S1changes from “0” to “1” in a state where the input signal Q0is “0” and the input signal Q1is “1”, the N-channel MOS transistor MN2comes to be in the ON state and the P-channel MOS transistor MP2comes to be in the OFF state. As a result that the N-channel MOS transistor MN2comes to be in the ON state, the output node N comes to be “0” via the N-channel MOS transistors MN1, MN2, so that the output signal Z comes to be “1”.

If it is assumed that the input signal Q0is “X” (indefinite), as illustrated inFIG. 8B, while the output node N is pulled down to “0” via the N-channel MOS transistors MN1, MN2, there can exist a path connected to the output node N via the N-channel MOS transistor MN3. In such a case, as a result that the N-channel MOS transistor MN3operates similarly to a current source as described above, on-resistances exhibited by the N-channel MOS transistors MN1, MN2are substantially smaller in relation to a resistance exhibited by the N-channel MOS transistor MN3, so that the output node N comes to be almost “0” level.

When the input signal Q1is “0”, the P-channel MOS transistor MP3comes to be in an ON state and the N-channel MOS transistor MN1comes to be in an OFF state. As a result that the N-channel MOS transistor MN1comes to be in the OFF state, a path from the output node N to the ground line is disconnected. When the output node N is “0”, as a result that the P-channel MOS transistor MP3comes to be in the ON state, a path connecting a power supply and the output node N is conducted. Thereby, the output node N transits to “1”, and the output signal Z is determined to be “0” in response thereto. When the output node N is “1”, the output signal Z has originally been determined to be “0”, so that transition does not occur.

For example, as illustrated inFIG. 9, when the selection signal S1changes from “0” to “1” in a state where the input signal Q0is “1” and the input signal Q1is “0”, the N-channel MOS transistor MN2comes to be in the ON state and the P-channel MOS transistor MP2comes to be in the OFF state. Then, the output node N comes to be “1” via the P-channel MOS transistor MP3and the output signal Z comes to be “0”.

When the selection signal S1is “0”, the P-channel MOS transistor MP2comes to be in an ON state and the N-channel MOS transistor MN2comes to be in an OFF state, and thus the signal path of the selector circuit illustrate inFIG. 6becomes as illustrated inFIG. 7B.

When the input signal Q0is “0”, the N-channel MOS transistor MN3comes to be in an OFF state and the P-channel MOS transistor MP1comes to be in an ON state. In a case where the P-channel MOS transistor MP1is in the ON state, even if the P-channel MOS transistor MP3is in the ON states, the P-channel MOS transistor MP3operates in a saturation region (operates similarly to a current source), whereby the potential of the output node N comes to be a potential almost equal to a power supply voltage level. As a result that the potential of the output node N comes to be almost equal to the power supply voltage level, the output signal Z comes to be “0”.

When the input signal Q0is “1”, the N-channel MOS transistor MN3comes to be in an ON state and the P-channel MOS transistor MP1comes to be in an OFF state. As a result that the P-channel MOS transistor MP1comes to be in the OFF state, the path from the output node N to the power supply line is disconnected. In a case where the output node N is “1”, the N-channel MOS transistor MN3comes to be in the ON state, whereby the path connecting the ground voltage and the output node N is conducted. Thereby, the output node N transits to “0” and the output signal Z is determined to be “1” in response thereto. In a case where the output node N is “0”, the output signal Z has originally been determined to be “1”, so that transition does not occur. As described above, also in the selector circuit illustrated inFIG. 6, a selector function can be performed without using an inversion signal of the selection signal S1, so that a selector circuit which operates at a high speed can be realized.

The selector circuit in the present embodiment is not limited to the two-input selector circuit but is applicable also to a selector circuit which has three or more inputs.FIG. 10Ais a diagram illustrating an example in which a four-input selector circuit is constituted with the selector circuit illustrated inFIG. 1in the present embodiment.FIG. 10Bis a diagram illustrating a truth table of the selector circuit illustrated inFIG. 10A. The four-input selector circuit illustrated inFIG. 10Aincludes three selector circuits SEL1A, SEL1B, SEL2which are constituted similarly to the two-input selector circuit illustrated inFIG. 1.

The first selector circuit SEL1A includes P-channel MOS transistors MP1A to MP4A, N-channel MOS transistors MN1A to MN4A, and inverters INV1A, INV2A which are connected similarly to the selector circuit illustrated inFIG. 1. To the first selector circuit SEL1A, a first input signal Q0and a second input signal Q1are inputted as input signals and a zeroth bit of a selection signal S is inputted as a selection signal. The first selector circuit SEL1A outputs an inversion signal of the first input signal Q0when the zeroth bit of the selection signal S is “0”, and outputs an inversion signal of the second input signal Q1when the zeroth bit of the selection signal S is “1”.

The second selector circuit SEL1B includes P-channel MOS transistors MP1B to MP4B, N-channel MOS transistors MN1B to MN4B, and inverters INV1B, INV2B which are connected similarly to the selector circuit illustrated inFIG. 1. To the second selector circuit SEL1B, a third input signal Q2and a fourth input signal Q3are inputted as input signals and the zeroth bit of the selection signal S is inputted as a selection signal. The second selector circuit SEL1B outputs an inversion signal of the third input signal Q2when the zeroth bit of the selection signal S is “0”, and outputs an inversion signal of the fourth input signal Q3when the zeroth bit of the selection signal S is “1”.

The third selector circuit SEL2includes P-channel MOS transistors MP1C to MP4C, N-channel MOS transistors MN1C to MN4C, and inverters INV1C, INV2C which are connected similarly to the selector circuit illustrated inFIG. 1. To the third selector circuit SEL2, an output of the first selector circuit SEL1A and an output of the second selector circuit SEL1B are inputted as input signals and a first bit of the selection signal S is inputted as a selection signal. The third selector circuit SEL2outputs an inversion signal of the output of the first selector circuit SEL1A when the first bit of the selection signal S is “0”, and outputs an inversion signal of the output of the second selector circuit SEL1B when the first bit of the selection signal S is “1”.

By combining the three selector circuits illustrated inFIG. 1as described above, it is possible to realize the four-input selector circuit which selects four input signals Q0to Q3for the two bits selection signal S<1:0>. Since the output signal Z is outputted through two stages of the selector circuits illustrated inFIG. 1, the output signal Z of positive logic can be outputted at a high speed by using all outputs of nodes NA, NB, NC being inversion outputs of the selector circuit.

FIG. 11is a diagram illustrating a configuration example of a decision feedback equalizer to which the selector circuit in the present embodiment is applied. The decision feedback equalizer illustrated inFIG. 11is used for a receiver or the like of a serializer/de-serializer (SerDes), for example.

A comparison circuit12A performs binary decision to an input serial signal IN which is inputted via a buffer11by using a first reference voltage V0, and outputs a decision result. A comparison circuit12B performs binary decision to the input serial signal IN which is inputted via the buffer11by using a second reference voltage V1, and outputs a decision result. The first reference voltage V0is a reference voltage corresponding to a case where previous data is “0”. The second reference voltage V1is a reference voltage corresponding to a case where the previous data is “1”. The second reference voltage V1is higher than the first reference voltage V0.

A flip-flop13A latches the decision result outputted from the comparison circuit12A in synchronization with a clock signal CK which performs sampling of a serial signal, and outputs the latched decision result to a selector circuit14as a first input signal Q0. A flip-flop13B latches the decision result outputted from the comparison circuit12B in synchronization with the clock signal CK, and outputs the latched decision result to the selector circuit14as a second input signal Q1.

The selector circuit14is the two-input selector circuit in the present embodiment described above. The selector circuit14selectively outputs the first input signal Q0or the second input signal Q1in correspondence with a selection signal S1. In the present embodiment, the selector circuit14selects the first input signal Q0and outputs as an output signal Z when the selection signal S1is “0”, and selects the second input signal Q1and outputs as the output signal Z when the selection signal S1is “1”. A flip-flop15latches the output signal Z of the selector circuit14in synchronization with the clock signal CK, and outputs the latched signal as an output signal OUT and outputs the latched signal to the selector circuit14as the selection signal S1related to the next data.

In the decision feedback equalizer, for the input serial signal IN whose high-frequency component is weakened by passing through a transmission path or the like, codes are decided to different reference voltages which facilitate decision of codes of input signals in the next sampling respectively in correspondence with decision results of the codes in previous sampling in order to supplement the lost high-frequency component. Here, decision results to the reference voltages V0, V1corresponding to “0” and “1” being decision results in the previous sampling are prepared in advance, and the selector circuit14selects the proper input based on the codes (data) in the previous sampling. In the decision feedback equalizer, the selector circuit is required of a high-speed operation in order to quickly reflect a previous sampling result on selection of the next sampling result, and the high-speed operation is possible according to the selector circuit in the present embodiment, which realizes the selector function without using the inversion signal of the selection signal.

FIG. 12is a diagram illustrating a configuration example of a semiconductor integrated circuit which includes the decision feedback equalizer illustrated inFIG. 11. The semiconductor integrated circuit21in the present embodiment includes a reception circuit22which has a function of a de-serializer that converts an input serial signal into a parallel signal and an internal circuit29such as a logic circuit which performs a processing operation after receiving the parallel signal (data) from the reception circuit22.

The reception circuit22includes a front end circuit23, a clock data recovery circuit27, and a clock generator28. The front end circuit23includes a differential buffer24, a decision feedback equalizer25, and a demultiplexer26. The differential buffer24receives differential input serial signals RXIN, RXINX transmitted via a transmission path or the like. The decision feedback equalizer25is the decision feedback equalizer illustrated inFIG. 11, for example, and decides a code (data) of the input serial signal. The demultiplexer26performs serial-parallel conversion to an output of the decision feedback equalizer25and outputs as a parallel signal RXOUT and outputs a reception data clock RXCLKO.

The clock data recovery circuit27properly controls a phase of a clock signal which the clock generator28outputs based on a received signal. The decision feedback equalizer25performs sampling of the input serial signal at a proper timing by using the clock signal which the clock generator28outputs. The parallel signal RXOUT outputted from the reception circuit22is taken into the internal circuit29by a flip-flop30which operates by the reception data clock RXCLKO, and is subjected to a processing or the like.

The aforementioned embodiments merely illustrate concrete examples of implementing the present invention and are not intended to limit the interpretation of the technical scope of the present invention. In other words, the present invention can be implemented in various manners without departing from the technical spirits or main features thereof.

A disclosed selector circuit selectively outputs a first input signal or a second input signal in correspondence with a selection signal, and a high-speed operation can be realized without using an inversion signal of the selection signal.