Abstract:
Disclosed is a decoder circuit designed to reduce the number of transistor elements and to increase the speed of operation. The decoder includes a plurality of output selectors for respectively drawing out different decoded output signal lines in group having the common input components. In each output selector, a plurality of serial-connected transistors is provided to execute AND operations for the common input components, and a plurality of parallel-connected transistors to execute OR operation for the remaining and their remaining inverted input components. The final one of the serial-connected transistors is coupled to each of parallel-connected transistors, and thereby the entire AND operation results for the common inputs and remaining inputs, or for the common inputs and remaining inverted inputs is obtained at the predetermined one of the decoded output signal lines.

Description:
This is a continuation of application Ser. No. 07/382,272 filed on Jul. 20, 1989 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a decoder that outputs a signal to the output port corresponding the combination of input signals applied, in particular, to a decoder using a level shifter that improves economy by reducing the area occupied in the chip and by increasing the operational speed. 
     2. Description of the Prior Art 
     In general, the decoder, which is used as a typical function circuit for the combination of gates within various digital systems such as 1-chip microprocessors, custom integrated circuit (IC&#39;s), and controllers for electrical devices, performs the role of finding the memory address by decoding the data from various combinations of signals. Therefore, the decoder that is used within a digital system as a function circuit for control purposes consists of a combination of several complementary metal oxide semiconductor NAND or NOR gates, which occupy a large area of semiconductor chip. As a result, the unit price and yields can be affected, since the decoder occupies a large portion of the digital system and, accordingly, the circuit operates with limited speed of operation. 
     For example, as shown in FIG. 1, let&#39;s consider a 3-input decoder composed of CMOS NOR gates. In the circuit, based upon each combination of the 3 input signals A,B, and C, 8 NOR gates are requisite to find a specific bit line using the decoder, because we must select a specific piece of data out of 8 outputs 01 thorugh 08. In this case, three p-channel transistors should be connected in parallel with the serially connected three n-channel transistors per NOR gate; hence six transistors are required in total. In the line run, a total of 48 transistors are used in a 3-input decoder. In the case of a 4-input decoder, not only is a combination of 96 transistors and their corresponding area required, but there is also the disadvantage of having limited operational speed, since the speed of setting an output for the 3-input decoder corresponds to the delay time of the three p-channel transistors. 
     SUMMARY OF THE INVENTION 
     The purpose of this invention is to provide a decoder using a level shifter that improves the overall economy of the circuit by reducing the number of transistors constituting the decoder, and by increasing the speed of operation. 
     According to the features of the present invention, a decoder circuit for generating the decoded signal is provided to one specific output line in response to the input signals of a specific combination, said decoder circuit including a plurality of output selectors for respectively drawing out different decoded output signal lines in a group having the common input components. 
     Each of the output selectors is provided with a plurality of serial-connected transistors for executing logic AND operations for the common input components received by the respective gates thereof, and a plurality of parallel-connected transistors for executing logic OR operations for the remaining input components and the remaining inverted input components received by the respective gates thereof. 
     The coupled sources (or drains) of the parallel-connected transistors are connected to the drain (or source) of the final one of the serial-connected transistors while the different decoded output signal lines are drawn from the respective drains (or sources) of the parallel-connected transistors. Therefore, logic AND operation results may be selectively output to one of the decoded output signal lines for the common inputs and remaining inputs, or for the common inputs and remaining inverted inputs, based upon the conditions of these remaining inputs. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objects and other advantages of the present invention will become more apparent in the following detailed description of the preferred embodiment of the present invention, with reference to the attached drawings, in which: 
     FIG. 1 shows a circuit diagram illustrating the conventional structure of a 3-input decoder; 
     FIG. 2 shows a circuit diagram illustrating the structure of the level shifter circuit used in the decoder according to the invention; and 
     FIG. 3 shows a circuit diagram illustrating the structure of the 3-input decoder of the invention, using the level shifter. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A preferred embodiment of the invention will be explained in detail by referring to FIG. 2 and FIG. 3. 
     FIG. 3 is a circuit diagram depicting an example of the operation of the invention, the 3-input decoder. The structure is as follows; 
     Assume that A,B and C are the three inputs and A,B, and C are the inverted signals of the 3 inputs. 01, 02, 03, 04, 05, 06, 07, and 08 represent outputs of the decoder. In this invention, only the n-channel transistors are used for the decoder, since the drift velocity of an electron is three times faster than that of a hole and, hence, the structure of the circuit can be simplified and the operational speed can be increased. 
     
                       TABLE 1______________________________________A      B            C     Decoder Output______________________________________0      0            0     010      0            1     020      1            0     030      1            1     041      0            0     051      0            1     061      1            0     071      1            1     08______________________________________ 
    
     The decoder, which outputs logic &#34;1&#34; to one of eight outputs according to the condition of the three inputs, follows the logic table shown in Table 1. Here, if the logic &#34;1&#34; signal appears at output 01, the circuit must be constructed to satisfy the logic expression 01=A·B·C. In the same manner, for outputs 02-08, the circuits satisfying the logic expressions 02=A·B·C, 03=A·B·C, 04=A·B·C, 05=A·B·C, 06=A·B·C, 07=A·B·C, and 08=A·B·C must be accordingly constructed. 
     In order to construct the circuits for generating the outputs 01-08, the invention utilizes the following characteristic scheme to simplify the complexity of the circuit. 
     First, for the outputs (01-08) having these logical expressions the outputs having the same input components are so grouped as to be drawn from one output selector unit. For example, the outputs 01 and 02 are configured to be drawn from the same output selector unit 1, since they have the same input component A·B· The following output pairs are drawn from the output selector units 2, 3, and 4 since 03 and 04 have the common input A·B, 05 and 06 have A·B, and 07 and 08 have A·B, respectively. Moreover, in this invention, in order to minimize the number of metal oxide semiconductor transistors that form the output selector construction, the common inputs included in these different outputs are arranged to use the same transistors at the corresponding output selector unit. Therefore, the common inputs in this example are individually applied to the gates of the serially connected transistors that execute the logical AND operation i.e. to perform preliminary logic products within the corresponding output selector unit. That is to say, for the different outputs 01 and 02 that have a common input A·B, the common inputs A and B are applied to the gates of the serially connected transistors Q11 and Q12 in the output selector 1. In the same way, the common inputs A B for outputs 03 and 04 are applied to the gates of the transistors Q21 and Q22 in output selector 2, the common input A and B for output 05 and 06 are applied to the gates of the transistors Q31 and Q32 in output selector 3, and the common input A and B for output 07 and 08 are applied to the gates of transistors Q41 and Q42 in output selector 4. 
     In addition, for the input conditions to the output pairs (01, 02), (03, 04), (05, 06), and (07, 08), the remaining inputs, and the inverted inputs C and C are respecively applied to each gate of the parallel-connected transistors (Q13, Q14), (Q23, Q24), (Q33, Q34), and (Q43, Q44), which execute the logical OR operations, i.e., perform secondary logic products within the corresponding output selector units 1, 2, 3, and 4 to generate secondary logic products. Moreover, the drains of the final ones Q12, Q22, Q32, and Q42 among the serially connected transistors within the respective output selector units 1, 2, 3, and 4 are joined with the sources of the parallel-connected transistors (Q13, Q14), (Q23, Q24), (Q33, Q34), and (43, Q44); and different decoding outputs 01,02,03,04,05,06,07, and 08 are drawn from the drains of the parallel-connected transistor pairs (Q13,Q14), (Q23,Q24), (Q33,Q34), and (Q43,Q44). With this configuration, logical AND operation results i.e., to perform an output logic operation to generate output logic results selectively appear at each output selector unit for the common inputs and remaining inputs, or for the common inputs and remaining inverted inputs, based upon the conditions of these remaining inputs. 
     In order to attain an accurate logic voltage for logical &#34;0&#34; (0 Volt) (RESET signal) at the output ports of those output selector units which are not selected by the input conditions, the pull-down transistors (QV1, QV2), which act as load transistors, are respectively connected between the drains of the parallel-connected transistors (Q13,Q14), (Q23,Q24), (Q33,Q34), and (Q43,Q44), within the output selector unit, and the ground terminal. Moreover, each gate of these pull-down transistors is applied to the output port VO of the level shifter, which generates the voltage that can turn these transistors ON. 
     The level shifter is used to maintain a low impedance condition by shifting the direct current voltage in parallel between the input and the output ports, and has the structure shown in FIG. 2. Its operation will be discussed below. In order to set the output port VO under the low impedance condition, assume that the DC voltage Vcc=5 V, the threshold voltage VT=0.8 V, and the aspect ratios of the transistors M1, M2, M3, and M4 are (W/L) 1  =(12/5), (W/L) 2  =(5/7), (W/L) 3  =(16/10), and (W/L) 4  =(8/4), respectively. The aspect ratios can be adjusted according to the output conditions desired. Suppose that both the transistors M3 and M4 are saturated, then the relation between the terminal voltage VA and the output voltage VO is 
     
         β(W/L).sub.1 (VA-VO-VT).sup.2 =β(W/L).sub.4 (VO-VT).sup.2 
    
     Insert the appropriate values in the equation and solve the equation, which becomes: ##EQU1## 
     Therefore, supposing that the transistors M1, M2, and M3 are all saturated, then from the equation 
     
         β(W/L).sub.1 (Vcc-VA-VT).sup.2 =β(W/L).sub.2 (VA-VT).sup.2 +β(W/L).sub.3 (VA-VO-VT).sup.2 
    
     the following results are obtained: 
     VA=2.8 V 
     VO=1.4 V 
     If the output port VO of the level shifter is always assigned to &#34;SET&#34; (HIGH) condition, the pull-down transistors (QV1) and (QV2) connected to the output port VO always remain &#34;ON&#34;, and those outputs which are not selected by any input combinations will be set to the &#34;LOW&#34; condition, with an accurate output level (=0 Volt). 
     In addition, in order to increase the speed of operation, only the n-channel transistors are used in this invention. Therefore the operational speed is two times faster than that of conventional circuitry, because the requisite time for specifying the state of one output line corresponds to the delay time of the three n-channel transistors. 
     The following is the description of the procedures on how the decoder adopting the level shifter according to the invention can decode arbitrary input signals and select a particular output. For example, assume that the 3-bit signal 010 is entered. Then, the signal line A, A, B, B, C, and C will become &#34;LOW&#34;, &#34;HIGH&#34;, &#34;HIGH&#34;, &#34;LOW&#34;, &#34;LOW&#34;, and &#34;HIGH&#34;, respectively. The transistors Q14, Q24, Q34, and Q44 along the &#34;LOW&#34;-state signal line C will be turned off, and, therefore, the pull-down transistor QV2, being always turned on due to &#34;HIGH&#34; signal from the output port VO of the level shifter, regardless of other transistors, so that the output lines 02, 04, 06, and 08 can be always in the &#34;LOW&#34; state. Transistors Q31 and Q41 are turned off by the &#34;LOW&#34; state signal line A and the output lines 05 and 07 are kept in the &#34;LOW&#34; state by the pull-down transistor QV1 being always turned on. The remaining problem is to select one of the output lines 01 or 03. Since the transistor Q12, being turned off by the signal line B of the &#34;LOW&#34; state, enables the output line 01 to be kept in &#34;LOW&#34; state, only the output line 03 can be finally selected. That is, since the transistors Q21, Q22, and Q23 of output selector unit 2 are turned on by the &#34;HIGH&#34; state input lines A, B, and C, the voltage of power supply Vdd sets the output line 03 to the &#34;HIGH&#34; state and, as a result, only one output line 03 out of the output lines 01-08 can be selected. The operations of other input combinations are performed in the same way, to enable selection of a specific output line. 
     As discussed above, the number of transistors including the conventional and the newly-invented circuits, and the operating speed for a 3-input decoder, are compared in Table 2. 
     
                       TABLE 2______________________________________     Type       Conventional 3-input                      Newly-InventedPerformance decoder        3-input decoder______________________________________No. of Tr&#39;s 48             28Operational 2×       1×Speed______________________________________ 
    
     The decoder according to the present invention can be used not only for 3-input and 8-input types, but also for 4-input and 16-output types: and can double the integrity from that is provided by the conventional decoder, and can increase the speed of operation to twice of conventional decoders.