BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention refers to a pull-up circuit for input/output terminals of electronic appliances.  
           [0003]    2. Description of the Related Art  
           [0004]    In the prior art, pull-up resistances are used in connections between digital appliances, for example between computers and printers. In fact in communications between computers and printers the lines that carry the data are terminated by pull-up resistances with a set value. Said resistances are arranged between each data line and the power supply and each data line is connected to a pin or terminal of an interface, i.e., a data input/output (I/O) pin.  
           [0005]    If we consider any pull-up resistance Rp arranged between an I/O pin and a terminal of the cable on which a supply voltage Vcc is present, as shown in FIG. 1, it is possible that when the voltage on the I/O pin is higher than the supply voltage Vcc or vice versa there is an undesired flow of current between the I/O pin and the power line. Said current flow consumes energy, which consumption must be as low as possible in many applications such as portable appliances of small dimensions.  
         BRIEF SUMMARY OF THE INVENTION  
         [0006]    An embodiment of the present invention is a pull-up circuit for input/output terminals of electronic appliances that enables the disadvantage described above to be lessened.  
           [0007]    The pull-up circuit is arranged between an input/output terminal and a supply-voltage terminal, and includes a first transistor and a resistance connected serially and coupled between said input/output terminal and said supply-voltage terminal, circuitry suitable for driving said transistor so as to switch it on or off depending on whether the values of the voltage of the input/output terminal belong or do not belong to a set range of voltage values within the supply-voltage value. As a result, pull-up circuit minimizes the absorption of current on the input/output terminals and on the supply-voltage terminals. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)  
       [0008]    The characteristics and advantages of the present invention will appear evident from the following detailed description of its embodiments thereof, illustrated as non-limiting examples in the enclosed drawings, in which:  
         [0009]    [0009]FIG. 1 is a diagrammatic view of a pull-up resistance used in a circuit according to the prior art;  
         [0010]    [0010]FIG. 2 is a diagrammatic view of the circuit according to an embodiment of the present invention;  
         [0011]    [0011]FIG. 3 is a more detailed diagrammatic view of the circuit in FIG. 2;  
         [0012]    [0012]FIG. 4 is a diagram of the variation in currents on the terminal I/O on the supply-voltage terminal depending on the voltage VI/O on the terminal I/O using the circuit shown in FIG. 3;  
         [0013]    [0013]FIG. 5 is a diagram of the variation in currents on the terminal I/O and on the supply-voltage terminal depending on the voltage VI/O on the terminal I/O using a pull-up resistance as in FIG. 1. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]    With reference to FIG. 2 a pull-up circuit according to the present invention is diagrammatically shown. The circuit comprises a p-channel MOS transistor M 1  serially connected to a resistance Rs between the supply Vcc and input/output terminals I/O, preferably an input/output terminal for digital data of an electronic appliance not shown in the Figure. More precisely, the source terminal of the transistor M 1  is connected to the supply-voltage terminal Vcc, the drain terminal is connected to a terminal of the resistance Rs, the other terminal of which is connected to the terminal I/O. The dimensions of the transistor M 1  should be such that the sum of its switch-on resistance Ron and of the resistance Rs falls within the specifications required for the value of the pull-up resistance.  
         [0015]    The gate of the MOS transistor M 1  is driven by circuitry comprising a driving device  1  coupled to the terminals Vcc and I/O; device  1  commands the switch-on or the switch-off of the transistor M 1 , depending on the values assumed by the voltage VI/O on the input/output terminal I/O and on the power supply Vcc.  
         [0016]    The circuitry also comprises another device  2  coupled to the terminals Vcc and I/O that supplies the bulk of the MOS transistor M 1  and is coupled to the device  1 ; the device  2  enables the bulks of the MOS transistors of the circuit to be biased to the highest potential between a voltage value slightly below Vcc and a voltage value slightly below or the same as the voltage on the terminal I/O.  
         [0017]    [0017]FIG. 3 describes in greater detail the circuit of FIG. 2. Device  1  comprises a first p-channel MOS transistor M 10  and a second n-channel MOS transistor M 11 , with drain terminals connected together, the gate terminals being connected to the supply-voltage terminal Vcc whilst the source terminal of the transistor M 11  is grounded and the source terminal of the transistor M 10  is connected to its bulk terminal. The device  1  also comprises third and fourth pchannel MOS transistors, M 12  and M 13 , the source terminals of both of which are connected to the supply-voltage terminal Vcc; the transistor M 12  has its gate terminal coupled to the input/output terminal I/O and has its drain terminal connected to the drain terminals of the transistors M 10  and M 11  and to an output terminal Out 1  of the device  1  that is connected to the gate terminal of the transistor M 1 ; the transistor M 13 &#39;s gate terminal is grounded and its drain terminal is connected to the terminal I/O. The transistors M 10 -M 13  have their bulk terminals coupled to an output terminal Out 2  of the device  2 .  
         [0018]    The transistor M 13  is always switched-on and its switch-on resistance is much greater than the sum of the resistances Rs and of the switch-on resistance Ron 1  of the transistor M 1 .  
         [0019]    When the voltage on the input/output terminal I/O is high (forced by the electronic appliance to which the terminal I/O is connected) so as to keep the transistor M 12  switched off, the voltage on the gate terminal of the transistor M 1  is at a low value because the transistor M 11  is switched on; in this way the transistor M 1  is switched on and for this reason there is a current between the supply Vcc and terminals I/O via the resistances Rs and Ron 1 .  
         [0020]    When the voltage on the input/output terminal I/O is low (forced by the electronic appliance to which the terminal I/O is connected) so as to switch on the transistor M 12 , the voltage on the drain terminal of the transistor M 12  and therefore on the gate terminal of the transistor M 1  increases and thereby causes the transistor M 1  to switch off and prevents the flow of current from the terminal Vcc towards the terminal I/O. This occurs for as long as the voltage on the terminal I/O is lower than the difference between the supply voltage Vcc and the voltage V 12  provided by the threshold voltage Vth 12  and a voltage Vx 1  that indicates the amount by which the source-gate voltage Vgs of the transistor M 12  must exceed the threshold voltage Vthl  2  in order to switch off the transistor M 1   
           VI/O&lt;Vcc −( Vth 12+ Vx 1). 
         [0021]    The voltage Vx 1  needs to be added to the threshold voltage Vth 12  to offset the effect of the transistor M 11  on the drain terminal of the transistor M 12 ; the transistor M 11  is always switched on unless the voltage Vcc is zero or near to zero and the value of the voltage Vx 1  depends on the dimensions of transistors M 11  and M 12 .  
         [0022]    The transistor M 13  ensures that when the terminal I/O is disconnected from the electrical appliance to which it is connected and the voltage on the terminal I/O is at a low or high value, the value of the voltage on the terminal I/O reaches the supply voltage Vcc so as to enable the switch-off of the transistor M 12  and consequently enable the transistor M 1  to be switched on.  
         [0023]    The device  2  enables biasing of the bulk terminals of the p-channel MOS transistors M 1 , M 10 , M 12 , M 13  to the highest potential between a voltage value slightly less than the supply voltage Vcc and a voltage value slightly less or the same as the voltage on the terminal I/O. Said device comprises a p-channel MOS transistor M 21  with a gate terminal connected to the supply voltage Vcc, the drain terminal being connected to the drain terminal I/O and the source and bulk terminals being connected to the output terminal Out 2  of the device  2 . The device  2  also comprises two p-channel MOS transistors M 22  and M 23 , both being connected as a diode and sharing a gate terminal and being connected to output terminal Out 2 , whereas the source terminal of the transistor M 22  is connected to the supply-voltage terminal Vcc and the source terminal of the transistor M 23  is connected to the terminal I/O. If the supply voltage Vcc is higher than the voltage on the terminal I/O the voltage on the gate terminals of the transistors M 22  and M 23  and therefore on the terminal Out 2  will be the same as Vcc−Vth 22  wherein Vth 22  is the threshold voltage of the transistor M 22 . If the voltage VI/O is higher than the supply voltage Vcc, considering the threshold voltages of the transistors M 23  and M 21 , the voltage on the terminal Out 2  will vary from the value VI/O−Vth 23  (where Vth 23  is the threshold voltage of the transistor M 23 ) to a value that is the same as the voltage on the terminal I/O due to switch-on of the transistor M 21 .  
         [0024]    If the voltage on the terminal I/O is high and voltage Vcc=0, the transistor M 12  is switched off, the transistor M 10  is switched on and the transistor M 1  is switched off. In this case, as the voltage of the bulk of the MOS transistors is the same as the voltage on the terminal I/O, a current will run through the transistor M 21  and the transistors M 10  and M 11 .  
         [0025]    VI/O&gt;Vcc+Vth 10 +Vx 2  may occur, where the voltage Vx 2  is the voltage to be added to the voltage Vth 10 , the threshold voltage of the transistor M 10 , to switch off the transistor M 1 ; the voltage Vx 2  depends on the dimensions of the transistors M 10  and M 11 . In this case, as in the previous case, the transistor M 12  is switched off, the transistor M 10  is switched on and the transistor M 1  is switched off; as the voltage of the bulk of the MOS transistors is the same as the voltage on the terminal I/O, a current will flow through the transistor M 21  and the transistors M 10  and M 11 .  
         [0026]    The circuit in FIG. 3 therefore acts as a pull-up resistance when the voltage on the terminal I/O is high with voltage values in the range between Vcc−(Vth 12 +Vx 1 ) and Vcc+Vth 10 +Vx 2 . For all the voltage values not included in said range the current absorbed by the terminal I/O is zero or very low. For example, said range of voltage values (Vcc−Vth 12 −Vx 1 , Vcc+Vth 10 +Vx 2 ) comprises voltage values that are no more than two volts above or below the value of the supply voltage Vcc; in fact, for example, the possible voltage values of Vx 1 , Vx 2 , Vth 10  and Vth 12  are Vth 10 =Vth 12 =0.9V, Vx 1 =0.4V and Vx 2 =0.5V.  
         [0027]    The graphs in FIGS. 4 and 5 show variations in the currents lin/out on the terminal I/O and Icc on the supply-voltage terminal depending on the voltage VI/O for the circuit in FIG. 3 (FIG. 4) and for a prior-art circuit consisting of a pull-up resistance as in FIG. 1 (FIG. 5). The value of the supply voltage is Vcc=3V and the voltage VI/O varies between 0 and 7V. We can see that the currents lin/out and Icc for the circuit in FIG. 3 have values near zero except for a range of VI/O values around the VI/O=Vcc value, wherein the values of the currents lin/out and Icc for the circuits in FIGS. 3 and 1 coincide.  
         [0028]    All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and nonpatent publications referred to in this specification and/or listed in the Application Data Sheet, are incorporated herein by reference, in their entirety.  
         [0029]    From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.
A pull-up circuit for input/output terminals of electronic appliances is disclosed. The circuit is arranged between an input/output terminal and a supply voltage terminal and includes a first transistor and a resistance serially connected and coupled between the input/output terminal and the supply-voltage terminal and circuitry suitable for driving the transistor so as to switch it on or off depending on whether the values achieved by the voltage of the input/output terminal belong or do not belong to a set range of voltage values within the supply-voltage value.