Multiplexing Input Circuit

(1) Field of the Invention

The present invention relates to an input circuit, more particularly to an input circuit constructed having a plurality of channels provided with input ports and input/outputs ports and where the plurality of channels deal mainly with analog signals.

(2) Description of the Prior Art

A recent trend in many fields is for automatic data processing and, therefore, for introduction of microcomputers and the like. Such data processing involves a great deal of information. The data is input one by one and successively processed. To input the information, an input circuit having a plurality of channels is usually used. Each of the plurality of channels transfers certain kinds of information, designated thereto in advance, to a predetermined internal circuit at specified timings.

Such an input circuit is fabricated as an integrated circuit (IC). The plurality of channels, each including, for example, one or more metal-oxide-semiconductor (MOS) transistors, must be arranged with a very small pitch therebetween. This, contrary to the desired electrical independence of the plurality of channels, results in increased manual interference between, which produces undesired noise such as crosstalk. The undesired noise reduces the accuracy of the information input to the circuit and the accuracy of the data processing.

The above-mentioned problem will be analyzed in greater detail later, but, in conclusion, it is effective to fabricate the input circuit so as to, first, prevent a high voltage signal on one channel from influencing neighboring channels and, second, make the voltage level of each channel stable.

Conventionally, this is done by arranging the plurality of channels with a relatively large pitch therebetween. This, however, is disadvantageous from the viewpoint of achieving high IC density.

It is an object of the present invention to fabricate an input circuit so as to overcome the problems of mutual interference, crosstalk, incorrect input of the information, and reduced data processing accuracy.

The above object is attained by employing three transistors in each channel, each transistor being selectively turned on to fix the voltage of conductors at a predetermined level in each of the channels other than a selected channel.

The present invention will be more apparent from the ensuing description with reference to the accompanying drawings, wherein:

FIG. 1 is a circuit diagram of one example of a prior art input circuit;

FIG. 2 is a circuit diagram of an input circuit according to an embodiment of the present invention;

FIG. 3 depicts waveforms of port selection signals shown in FIGS. 1 and 2; and

FIG. 4 is a partial top plane view of the circuit shown in FIG. 2.

FIG. 1 is a circuit diagram of one example of a prior art input circuit. In FIG. 1, reference numeral 11 represents, as a whole, an input circuit. The input circuit is constructed with a plurality of channels 12-0, 12-1, 12-2, - - - 12-n, each having the same construction. Taking the channel 12-1 as an example, a conductor line 13-1 is arranged therein, one end of which is connected to a port 14-1 and the other end of which is connected to a node N commonly connected to other channels, i.e., 12-0, 12-2, - - - 12-n. A transistor 15-1 is inserted in series at the middle of the conductor line.

When the input circuit 11 is mounted on a semiconductor chip provided with a microcomputer thereon, a branched route is often introduced, brancing, for example, from a portion between the port 14-1 and the transistor 15-1 and reaching a buffer 16-1. In this case, the port 14-1 functions not only as an input port, but also as an input/output port. Incidentally, in this figure, the buffer is illustrated specifically only for a buffer 16-n.

The plurality of channels 12-0 through 12-n are sequentially made active. Thus, the information for the channels is input one after another through the corresponding activated channel. In order to selectively make the plurality of channels active one after another, port selection signals PS.sub.0 through PS.sub.n are applied to gates of the transistors 15-0 through 15-n at predetermined timings. In this case, if, for example, the port 14-2 is inherently used as the input/output port, the transistor 15-2 is maintained in a cut-off state by continuously supplying the signal PS.sub.2 having a "L" (low) level.

The sequentially selected information inputs are supplied one by one to a predetermined internal circuit 17 mounted on the same chip. "Predetermined internal circuit" means a circuit after the input circuit 11. It is not essential in the present invention, however, to clearly define how it is constructed and for what purpose it exists. For example, the predetermined circuit 17 can be a so-called successive approximation type analog/digital (A/D) converter. In such a case, reference numeral 18 corresponds to a comparator; 19 to a successive approximation register; and 20 to a digital/analog (D/A) converter. The reason why such an A/D converter is taken as an example is that the input circuit of the present invention exhibits remarkable merits for analog signals as the aforesaid information inputs received at the ports 14-0, 14-1, and so on. To be more specific, the analog information inputs may indicate unprocessed data, for example, humidity measurement data, temperature measurement data, atmospheric pressure measurement data, or the like. These analog inputs are usually converted into digital signals, then supplied, as corresponding digital data, to the microcomputer for processing. However, the input circuit 11 has a problem, as previously mentioned, with the accuracy of the information inputs. For example, when an analog signal is received at the port with a voltage level of, for example 1 V, the analog signal incorrectly appears at the node N with an error voltage of (1+.alpha.) V. The present inventors found through experiments that the noise of +.alpha.V is produced when one or more of the ports 14-0 through 14-n are used as input/outputs ports. Assuming that, for example, the port 14-2 is used as an input/output port and, at the same time, this input/output port 14-2 receives a digital signal having a magnitude of about 5 V, as in a transistor-transistor logic (TTL) IC, the analog signal of, for example 1 V, inputted from, for example, the port 14-1, varies according to (1+.alpha.) V when reaching the node N. The error voltage of +.alpha.V is for example, about 10 mV. The value of 10 mV itself is considerably small, however, it may result in a 1 bit error when the register 19 of the predetermined internal circuit 17 is formed as an 8-bit register and the allowable maximum voltage for the A/D conversion voltage is rated at 2.56 V.

In the above example, the transistor 15-2 is maintained in a cut-off state, since the related port 14-2 is used for the input/output port. Regardless of the existence of the cut-off transistor 15-2, the noise of +.alpha.V, induced by the digital signal of about 5 V at the port 14-2, is actually produced at the node N.

The reasons for this are not clear, but it is considered that since the transistor 15-2 is usually made of a MOS field effect transistor (FET), a slight voltage variation due to the 5 V digital signal occurs at the node N by way of parasitic capacitors between the source and drain and between the drain and gate thereof. The noise caused by the above-mentioned fact is still maintained, even if the channels are arranged with a large pitch. Also, it is considered that there is interference between the channels. This interference may occur regardless of which port is used as the input/output port. At the conductor lines 13-0 through 13-n, except for the conductor line connected to the input/output port, corresponding analog voltages successively appear at high speed. All of the analog voltage levels are not constant, but vary in accordance with corresponding information inputs. This means that any selected channel is placed under circumstances in which the voltage level is not fixed, but always variable. This fact also causes an unstable voltage level of each information input.

FIG. 2 is a circuit diagram of an input circuit according to an embodiment of the present invention. In FIG. 2, members the same as those of FIG. 1 are represented by the same reference numerals or symbols (same for later figures). Accordingly, is a newly proposed input circuit 11', each of the channels is comprised of three transistors. The channels have the same construction, so only one of the channels, i.e., the channel 12'-1, is referred to in the following explanation. The channel 12'-1 comprises a first transistor 21-1, a second transistor 22-1, and a third transistor 23-1. The first and second transistors 21-1 and 22-1 are inserted in series into the conductor line 13-1. The gates of these transistors are connected with each other via a gate lead 24-1. A third transistor 23-1 is connected between ground GND and a portion of the conductor line 13-1 arranged between the first and second transistors 21-1 and 22-1. Further, a port selection signal PS.sub.1 is applied to the third transistor 23-1 at its gate. The signal PS.sub.1 has an inverted level with respect to the port selection signal PS.sub.1 to be applied to both gates of the first and second transistors 21-1 and 22-1. Therefore, both the first and second transistors 21-1 and 22-1 are in an on or off state with respect to an off or on state of the third transistor 23-1. It should be understood that the port selection signals PS.sub.0 through PS.sub.n, also PS.sub.0 through PS.sub.n, are selected one after another, as in FIG. 1. FIG. 3 depicts waveforms of port selection signals shown in FIGS. 1 and 2. Rows (1), (2), (3), (4), (5), and (6) represent the port selection signals PS.sub.1, PS.sub.2, PS.sub.n, PS.sub.1, PS.sub.2, and PS.sub.n, respectively.

Assuming that the channel 12'-1 is in a selection state now, the corresponding port selection signal PS.sub.1 is an "H" (high) level (the remaining signals PS.sub.0, PS.sub.2 through PS.sub.n are an "L" level). Therefore, the inverted port selection signal PS.sub.1 is the "L" level (the remaining inverted signals PS.sub.0, PS.sub.2 through PS.sub.n are of "H"). Under these conditions, the third transistor 23-1 of the selected channel 12'-1 is turned off by the signal PS.sub.1, and, thereby, the corresponding information input given at the port 14-1 is transferred to the node N by way of the first and second transistors 21-1 and 22-1, both conductive by the signal PS.sub.1. Under the same conditions as above, regarding the other channels 12'-0, 12'-2 through 12'-n, the respective conductors 13-0, 13-n through 13-2, sandwiched by the respective first and second transistors, are forcibly clamped at ground level. This is because, the third transistor of each of the nonselected channels is turned on.

The second transistor (22) for each nonselected channel is needed to prevent the thus forcibly clamped ground level from travelling into the selected channel. The thus forcibly clamped ground level on the conductor lines of the nonselected channels is effective for impeding the transfer of noise to the node N therethrough, the noise induced by signals supplied to the nonselected ports 14-0 and 14-2 through 14-n.

Due to the above-mentioned relationship between the port selection signal (PS) and the inverted port selection signal (PS), taking the channel 12'-1 as an example, the conductor lines 13-0, 13-2, etc. of neighboring channels, such as 12'-0, 12'-2, are fixed at ground level. Further, the gate leads 24-0, 24-1, etc. of the neighboring channels are fixed at the "L" level. Still further, the gate lead 24-1 for channel 12'-1 now in the selection state is fixed at the "H" level. This means that the related channel 12'-1 has a stable or fixed voltage level. This also applies to other selected channels. Consequently, each information input can be transferred to the node N, maintaining the voltage level that is at its port without being affected by any external level variation.

FIG. 4 is a partial top plane view of the circuit shown in FIG. 2, illustrating some sets of the second and third transistors (22, 23) and their neighboring members. The members referenced with the same numerals or characters as those of FIG. 2 are identical to those of FIG. 2. Symbols LPS.sub.0 through LPS.sub.2 and LPS.sub.0 through LPS.sub.2, are gate conductors for transferring the signals PS.sub.0 through PS.sub.2 and PS.sub.0 through PS.sub.2, respectively, shown in FIG. 2. Solid lines referenced by N.sup.+ indicate an enclosure of an N.sup.+ diffusion layer. Hatchings are employed not for representing cross-sectional regions as usual, but for clearly distinguishing polysilicon layers. As understood from FIG. 4, the addition of second and third transistors (22, 23) into the circuit of FIG. 1 does not impede achievement of a high degree of IC integration. That is, the conductors 12'-0, 24-0, 12'-1, and 24-1, etc., are arranged with very small pitches, regardless of the existence of the transistors (22, 23).

As explained above in detail, according to the present invention, there is provided an input circuit, having a plurality of channels which are activated one after another, with the advantage that the information input to each selected channel can be transferred therethrough to the following predetermined internal circuit without being affected by other nonselected channels and, therefore, can be kept at the voltage level that is at its port.