Abstract:
A pull-up switching device for controlling connection and non-connection of an input terminal IN and a first supply VDD and a pull-down switching device for controlling connection and non-connection of the input terminal IN and a second supply VSS are provided. The pull-up switching device and the pull-down switching device are operated exclusively on and off in time division to hold and output the state of the input terminal during each operating state from the two output terminals.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The entire disclosure of Japanese Patent Application No. 2008-154833 including specifications, claims, drawings, and abstract is incorporated herein by references. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a ternary value input circuit for digitally realizing ternary value inputs. 
         [0004]    2. Description of the Related Art 
         [0005]    Ternary value input circuits are known for communicating input terminal states of high level, low level, and open as a level of an output terminal level. 
         [0006]    For example, a ternary value input circuit disclosed in  FIG. 11  is composed of a voltage divider circuit formed from resistors  10  and  12  connected to an input terminal IN and two inverter circuits  14  and  16  having different threshold voltages. 
         [0007]    The first inverter circuit  14  is formed from a P-channel field-effect transistor  14 a and an N-channel field-effect transistor  14   b  and the second inverter circuit  16  is formed from a P-channel field-effect transistor  16   a  and an N-channel field-effect transistor  16   b . The inverter circuit  14  is connected between the input terminal IN and an output terminal OUT 1 . The inverter circuit  16  is connected between the input terminal IN and an output terminal OUT 2 . 
         [0008]    A ternary value input circuit of the related art analogically detects the state of the input terminal IN in accordance with the transistor threshold voltages and varies the output states of the output terminals OUT 1  and OUT 2 . This resulted in a problem of increased device area due to setting a desired threshold voltage at each transistor. 
         [0009]    Furthermore, since it is necessary to complementarily connect each transistor, a problem also results where the operation of the ternary value input circuit becomes unstable when the threshold voltage fluctuates. In addition, there is also a problem of a complex design since it is necessary to take into consideration the fluctuation of the threshold voltage of each transistor. 
       SUMMARY OF THE INVENTION 
       [0010]    One aspect of the present invention is a ternary value input circuit for converting and outputting a combination of states of two output terminals, comprising a pull-up switching device for controlling connection and non-connection of the input terminal and a first power supply, and a pull-down switching device for controlling connection and non-connection of the input terminal and a second power supply. The pull-up switching device and the pull-down switching device are operated exclusively on and off in time division to hold and output the state of the input terminal at each operating state from the two output terminals. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    Preferred embodiments of the present invention will be described in detail based on the following drawings, wherein: 
           [0012]      FIG. 1  shows a configuration of the ternary value input circuit in an embodiment of the present invention; 
           [0013]      FIG. 2  is a timing chart showing the operation of the ternary value input circuit in an embodiment of the present invention; 
           [0014]      FIG. 3  is a timing chart showing the operation of the ternary value input circuit in an embodiment of the present invention; 
           [0015]      FIG. 4  is a timing chart showing current of an input terminal of ternary value input circuit in an embodiment of the present invention; 
           [0016]      FIG. 5  shows a configuration of a signal generator circuit in a modified example 1 of the present invention; 
           [0017]      FIG. 6  is a timing chart showing the operation of the signal generator circuit in modified example 1 of the present invention; 
           [0018]      FIG. 7  is a timing chart showing the operation of the ternary value input circuit in modified example 1 of the present invention; 
           [0019]      FIG. 8  shows a configuration of the signal generator circuit in a modified example 2 of the present invention; 
           [0020]      FIG. 9  is a timing chart showing the operation of the signal generator circuit in modified example 2 of the present invention; 
           [0021]      FIG. 10  is a timing chart showing the operation of the ternary value input circuit in modified example 2 of the present invention; and 
           [0022]      FIG. 11  shows a configuration of the ternary value input circuit in the related art. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0023]    As shown in  FIG. 1 , a ternary value input circuit  100  in an embodiment of the present invention includes a pull-up transistor  20 , a pull-down transistor  22 , a Schmitt trigger buffer  24 , a pull signal generator circuit  26 , a pull-up output circuit  28 , and a pull-down output circuit  30 . 
         [0024]    The pull-up transistor  20  can be configured from a P-channel field-effect transistor. The drain of the pull-up transistor  20  connects to a supply voltage VDD and the source connects to the input terminal IN. The gate of the pull-up transistor  20  connects to an output terminal of a NOT gate  32 . The pull-down transistor  22  can be configured from an N-channel field-effect transistor. The drain of the pull-down transistor  22  connects to the input terminal IN and the source is grounded. 
         [0025]    The Schmitt trigger buffer  24  converts an input value and outputs a pulse. More specifically, if the input voltage is lower than an upper threshold voltage, the output becomes a low level (L), and if the input voltage reaches the upper threshold voltage, the output changes to a high level (H). Even though the input voltage drops below the upper threshold voltage in this state, the high level (H) is maintained provided the input voltage is at a state higher than the lower threshold voltage. If the input voltage is lower than the lower threshold voltage, the output becomes a low level (L). At this time, the difference between the upper and lower limits of the threshold voltage is referred to as a hysteresis voltage and the larger the value the higher the noise resistance. 
         [0026]    The pull signal generator circuit  26  includes a NOT gate  26   a , a buffer  26   b , an OR gate  26   c , an AND gate  26   d , and a NOT gate  26   e . To the input terminal of the NOT gate  26   a  is input a pull signal PULL and the output terminal is connected to input terminals of the buffer  26   b , OR gate  26   c , and AND gate  26   d . The output terminal of the buffer  26   b  is connected to the input terminals of the OR gate  26   c  and the AND gate  26   d . The output terminal of the OR gate  26   c  is connected to the input terminal of the NOT gate  26   e . The output of the NOT gate  26   e  is output as a pull-up signal PUON to the pull-up output circuit  28  and the NOT gate  32 . The output of the AND gate  26   d  is output as a pull-down signal PDON to the pull-down output circuit  30  and the gate of the pull-down transistor  22 . 
         [0027]    The pull-up output circuit  28  includes an AND gate  28   a , a NOT gate  28   b , an AND gate  28   c , an AND gate  28   d , an OR gate  28   e , and a flip-flop  28   f . To the AND gate  28   a  are input an enable signal ENABLE and a pull-up signal PUON. The output terminal of the AND gate  28   a  is connected to the input terminal of the NOT gate  28   b  and an input terminal of the AND gate  28   d . The output terminal of the NOT gate  28   b  is connected to an input terminal of the AND gate  28   c . To an input terminal of the AND gate  28   c  is connected the output terminal of the flip-flop  28   f . To an input terminal of the AND gate  28   d  is connected the output of the Schmitt trigger buffer  24 . The output terminals of the AND gate  28   c  and the AND gate  28   d  are connected to the input terminals of the OR gate  28   e  and the output terminal of the OR gate  28   e  is connected to the input of the flip-flop  28   f . To the clock terminal of the flip-flop  28   f  is input a system clock signal CLK. The output of the flip-flop  28   f  is a pull-up output signal. The pull-up output circuit  28  latches the state of the input terminal IN when the enable signal ENABLE and the pull-up signal PUON are high levels. 
         [0028]    The pull-down output circuit  30  includes an AND gate  30   a , a NOT gate  30   b , an AND gate  30   c , an AND gate  30   d , an OR gate  30   e , and a flip-flop  30   f . To the AND gate  30   a  are input the enable signal ENABLE and the pull-down signal PDON. The output terminal of the AND gate  30   a  is connected to the input terminal of the NOT gate  30   b  and an input terminal of the AND gate  30   d . The output terminal of the NOT gate  30   b  is connected to an input terminal of the AND gate  30   c . An input terminal of the AND gate  30   c  is connected to the output terminal of the flip-flop  30   f . An input terminal of the AND gate  30   d  is connected to the output terminal of the Schmitt trigger buffer  24 . The output terminals of the AND gate  30   c  and the AND gate  30   d  are connected to the inputs of the OR gate  30   e  and the output terminal of the OR gate  30   e  is connected to the input terminal of the flip-flop  30   f . To the clock terminal of the flip-flop  30   f  is input the system clock signal CLK. The output of the flip-flop  30   f  is the pull-down output signal. The pull-down output circuit  30  latches the state of the input terminal IN when the enable signal ENABLE and the pull-down signal PDON are high levels. 
         [0029]    An operation of the ternary value input circuit  100  configured in this manner will be described with reference to the timing chart of  FIG. 2 . Hereinafter, the operation will be described when high level and low level signals are input by the input terminal IN and when the input terminal IN is open. The high level of the input terminal IN is set higher than the upper threshold voltage of the Schmitt trigger buffer  24  and the low level of the input terminal IN is set lower than the lower threshold voltage of the Schmitt trigger buffer  24 . Furthermore, the supply voltage VDD applied to the drain of the pull-up transistor  20  is a voltage greater than or equal to the high level for the logic circuits and the supply voltage VSS applied to the pull-down transistor  22  is a voltage less than or equal to the low level for the logic circuits. 
         [0030]    The system clock signal CLK is a pulse signal having a predetermined frequency. Here, the enable signal is a pulse signal having four times the period of the system clock signal and the pull signal is a pulse signal having double the period of the system clock signal. 
         [0031]    When the pull signal input by the pull signal generator circuit  26  becomes a high level, the output PDON of the AND gate  26   d  becomes a low level at a slightly delayed timing, and the output PUON of the NOT gate  26   e  becomes a high level at a slightly further delayed timing. Furthermore, when the pull signal input by the pull signal generator circuit  26  becomes a low level, the output PUON of the NOT gate  26   e  becomes a low level at a slightly delayed timing, and the output PDON of the AND gate  26   d  becomes a high level at a slightly further delayed timing. When the output PUON becomes a high level, the pull-up transistor  20  turns on, and when the output PDON becomes a high level, the pull-down transistor  22  turns on. 
         [0032]    The pull signal generator circuit  26  varies the outputs PUON and PDON so that the timing of the changes do not coincide. This prevents the pull-up transistor  20  and the pull-down transistor  22  from switching simultaneously and causing the input of the Schmitt trigger buffer  24  to become unstable. 
         [0033]    When the input terminal IN is a high level, the pull-up transistor  20  turns on and the pull-down transistor  22  turns on so that in any event the input of the Schmitt trigger buffer  24  is a high level and a high level is output from the Schmitt trigger buffer  24 . 
         [0034]    The pull-up output circuit  28  holds the output QUP at a high level at the rising edge of the system clock signal CLK when the output of the Schmitt trigger buffer  24 , the enable signal, and the output PUON are high levels. On the other hand, the pull-down output circuit  30  holds the output QDN at a high level at the rising edge of the system clock CLK when the output of the Schmitt trigger buffer  24 , the enable signal, and the output PDON are high levels. 
         [0035]    In this manner, when the input terminal IN is a high level, both outputs QUP and QDN become high levels. 
         [0036]    Next, when the input terminal IN is a low level, the pull-up transistor  20  turns on and the pull-down transistor  22  turns on so that in any event the input of the Schmitt trigger buffer  24  is a low level and a low level is output from the Schmitt trigger buffer  24 . 
         [0037]    The pull-up output circuit  28  holds the output QUP at a low level at a rising edge of the system clock signal CLK when the output of the Schmitt trigger buffer  24  is a low level and the enable signal and the output PUON are high levels. On the other hand, the pull-down output circuit  30  holds the output QDN at a low level at a rising edge of the system clock CLK when the output of the Schmitt trigger buffer  24  is a low level and the enable signal and the output PDON are high levels. 
         [0038]    In this manner, when the input terminal IN is a low level, both outputs QUP and QDN become low levels. 
         [0039]    Next, when the input terminal IN is open and when the pull-up transistor  20  is on, the input of the Schmitt trigger buffer  24  becomes a high level and a high level is output from the Schmitt trigger buffer  24 . When the pull-down transistor  22  is on, the input of the Schmitt trigger buffer  24  becomes a low level and a low level is output from the Schmitt trigger buffer  24 . 
         [0040]    The pull-up output circuit  28  holds the output QUP at a high level at a rising edge of the system clock CLK when the output of the Schmitt trigger buffer  24  is a high level and the enable signal and the output PUON are high levels. On the other hand, the pull-down output circuit  30  holds the output QDN at a low level at a rising edge of the system clock CLK when the output of the Schmitt trigger buffer  24  is a low level and the enable signal and the output PDON are high levels. 
         [0041]    In this manner, when the input terminal IN is open, the output QUP becomes a high level and the output QDN becomes a low level. 
         [0042]    Thus, the ternary value input circuit  100  in the embodiment varies the combination of the output QUP and the output QDN in accordance with the three states of the input terminal IN of high level, low level, and open. Namely, (1) the input terminal IN can be indicated as a high level when the output QUP and the output QDN are both high levels, (2) the input terminal IN can be indicated as a low level when the output QUP and the output QDN are both low levels, and (3) the input terminal IN can be indicated as open when the output QUP is a high level and the output QDN is a low level. 
         [0043]    With the enable signal ENABLE always at a high level in the ternary value input circuit  100 , the output QUP and the output QDN according to the three states of the input terminal IN can be obtained as shown in  FIG. 3 . 
         [0044]    Furthermore, transistors  34   a  and  34   b  for ESD protection may be provided for absorbing sudden changes in the signal input from the input terminal IN. 
       MODIFIED EXAMPLE 1 
       [0045]    In the configuration of the ternary value input circuit  100  in the above-mentioned embodiment, frequent switching of pull-up and pull-down causes current to flow via the pull-up transistor  20  or the pull-down transistor  22  as shown by the current at the input terminal in  FIG. 4  and increases the power consumption of the circuit. 
         [0046]    Consequently, it is preferable to generate the enable signal ENABLE and the pull signal PULL with a signal generator circuit  200  shown in  FIG. 5 . 
         [0047]    The signal generator circuit  200  includes a counter  36 , a first selector  38 , a second selector  40 , and flip-flops  42  and  44 . The counter  36  receives the system clock CLK, cyclically counts from 0 to a predetermined value, and outputs the counter value. In the embodiment, the counter  36  counts up from 0 to 63, returns the counter value from 63 to 0, and repeats the count up. The first selector  38  receives the counter value output from the counter  36  and outputs a high level (H) when the counter value is a first value or the first value plus 1 and outputs a low level (L) when the counter value is any other value. For example, a high level (H) is output when the counter value is 62 or 63 and a low level (L) is output when the counter value is any other value. The second selector  40  receives the counter value output from the counter  36  and outputs a low level (L) when the counter value is the first value plus 1 and outputs a high level (H) when the counter value is any other value. For example, in the embodiment, a low level (L) is output when the counter value is 63 and a high level (H) is output when the counter value is any other value. The flip-flop  42  receives the output from the first selector  38  and latches and outputs the output state from the first selector  38  at the timing of the input system clock CLK. The flip-flop  44  receives the output from the second selector  40  and latches and outputs the output state from the second selector  40  at the timing of the input system clock CLK. 
         [0048]      FIG. 6  shows a timing chart of the enable signal and the pull signal generated by the signal generator circuit  200 . As shown in  FIG. 6 , the first selector  38  becomes a high level when the counter value is 62 or 63 and the first selector  38  becomes a low level when the counter value returns to 0. In conjunction a pulse signal is output where the output of the flip-flop  42  becomes a high level when the counter value reaches 62 and the clock signal changes from a low level to a high level and returns to a low level when the counter value changes to 0 and the clock signal changes from a low level to a high level. Furthermore, when the counter value is 63, the second selector  40  becomes a low level and when the counter value returns to 0, the second selector  40  becomes a low level. In conjunction a pulse signal is output where the output of the flip-flop  44  becomes a low level when the counter value reaches 63 and the clock signal changes from a low level to a high level and returns to a high level when the counter value changes to 0 and the clock signal changes from a low level to a high level. 
         [0049]    By having the ternary value input circuit  100  input the ENABLE signal and the PULL signal generated by the signal generator circuit  200 , the outputs shown in the timing chart of  FIG. 7  can be obtained. Namely, (1) outputs QUP and QDN are both high levels when the input terminal IN is a high level, (2) outputs QUP and QDN are both low levels when the input terminal IN is a low level, and (3) the output QUP is a high level and the output QDN is a low level when the input terminal IN is open. 
         [0050]    Furthermore, in the case where the input terminal IN is a high level, current flows to the input terminal IN only when the pull-down transistor  22  is on so that the current flowing at the input terminal IN is less than the case shown in  FIG. 4 . 
       MODIFIED EXAMPLE 2 
       [0051]    It is preferable to generate the enable signal ENABLE and the pull signal PULL with a signal generator circuit  300  shown in  FIG. 8 . 
         [0052]    The signal generator circuit  300  includes the counter  36 , the first selector  38 , the flip-flops  42  and  44 , and a third selector  46 . Since the signal generator circuit  300  is identical to the signal generator circuit  200  except for the third selector  46 , mainly the operation of the third selector  46  will be described hereinafter. 
         [0053]    The third selector  46  receives the counter value output from the counter  36 , the output signal QUP of the pull-up output circuit  28 , and a feedback signal of the output PULL of the flip-flop  42 , and outputs the output signal QUP when the value of the counter is the first value plus 2, outputs a low level when the value of the counter is the first value plus 1, and outputs the feedback signal otherwise. 
         [0054]    For example, when the first value is 62, a low level (L) is output when the counter value is 63, which is the first value plus 1. Furthermore, when the counter value is 0, which is the first value plus 2 (cyclically counted from 63 to 0), a high level (H) is output if the output signal QUP is a high level (H) and a low level (L) is output if the output signal QUP is a low level (L). When the counter value is other than 0 or 63, the feedback signal is output. 
         [0055]      FIG. 9  shows a timing chart of the enable signal and the pull signal generated by the signal generator circuit  300 . As shown in  FIG. 9 , the first selector  38  becomes a high level when the counter value is 62 or 63 and the first selector  38  becomes a low level when the counter value returns to 0. In conjunction a pulse signal is output where the output of the flip-flop  42  becomes a high level when the counter value becomes 62 and returns to a low level when the counter value becomes 0. 
         [0056]    Furthermore, in the case where the output signal QUP is a high level, the third selector  46  becomes a low level when the counter value is 63 and the third selector  46  becomes a high level when the counter value returns to 0. In conjunction the output of the flip-flop  44  becomes a low level when the counter value becomes 63 and the clock signal changes from a low level to a high level, and latches to a high level when the counter value becomes 0 and the clock signal changes from a low level to a high level. Thereafter, until the counter value again becomes 63 and the clock signal changes from a low level to a high level, the third selector  46  continues to output the PULL signal, which has been latched to a high level by the flip-flop  44 , and as a result the flip-flop  44  also continues to be latched to a high level. 
         [0057]    On the other hand, in the case where the output signal QUP is a low level, the third selector  46  becomes a low level when the counter value is 63 and the third selector  46  maintains the low level also when the counter value returns to 0. In conjunction the output of the flip-flop  44  is latched to a low level when the counter value is 0 or 63 and the clock signal changes from a low level to a high level. Thereafter, until the counter value again becomes 63 and the clock signal changes from a low level to a high level, the third selector  46  continues to output the PULL signal, which has been latched to a low level by the flip-flop  44 , and as a result the flip-flop  44  also continues be latched to a low level. 
         [0058]    By having the ternary value input circuit  100  input the ENABLE signal and the PULL signal generated by the signal generator circuit  300 , the outputs shown in the timing chart of  FIG. 10  can be obtained. Namely, (1) outputs QUP and QDN are both high levels when the input terminal IN is a high level, (2) outputs QUP and QDN are both low levels when the input terminal IN is a low level, and (3) the output QUP becomes a high level and the output QDN becomes a low level when the input terminal IN is open. 
         [0059]    Furthermore, in the case where the input terminal IN is a high level, current flows to the input terminal IN only when the pull-down transistor  22  is on so that the current flowing at the input terminal IN is less than the case shown in  FIG. 4 . Moreover, in the case where the input terminal IN is a low level, current flows to the input terminal IN only when the pull-up transistor  20  is on so that the current flowing at the input terminal IN is less than the case shown in  FIG. 4 . 
         [0060]    While there has been described what are at present considered to be preferred embodiments of the invention, it will be understood that various modifications may be made thereto, and it is intended that the appended claims cover all such modifications as fall within the true spirit and scope of the invention.