Abstract:
A buffer of reduced size includes a logic gate to raise the potential level of input digital data having a first logic level to a potential equal to a low power supply potential, and to produce intermediate data if a validation signal is active. The buffer also includes a tristate inverter to produce output data, at an output, that are logically inverse to the intermediate data if the validation signal is active and having its output at high impedance if otherwise. Such a buffer is particularly useful as an output buffer for contact cards using a power supply potential different from a potential powering a reader with which the card communicates.

Description:
FIELD OF THE INVENTION 
     The invention relates to a buffer, for example for a contact card. The invention is particularly related to a contact card using a power supply potential VDD 1  different from a power supply potential VDD 2  used by a reader with which the card communicates. 
     BACKGROUND OF THE INVENTION 
     A contact card C 1  (FIG. 1) generally comprises an integrated circuit CI grouping together all the electronic circuits capable of performing all the functions of the card. The circuit CI is connected to at least one contact pad or terminal PAD. When the card is used, the contact terminal/terminals of the card come into contact with one of the corresponding terminals of the card reader into which the card is inserted. A pad or terminal PAD can be used as input terminal for the circuit CI to receive data coming from the reader C 2  or as output terminal so that the circuit CI can transmit data to the reader. A same terminal PAD can also be used as input terminal for certain functions of the circuit CI and as output for other functions of the circuit CI. 
     When the card power supply potential VDD 1  is different from the reader power supply potential VDD 2 , then an input and/or output buffer has to be used between the terminal PAD and the circuit CI of the card to adapt the amplitude of the signals. An input buffer BE thus has the function of converting signals received from the reader, which range from 0 to VDD 2 , into signals ranging from 0 to VDD 1  that can be exploited by the circuit CI. Similarly, an output buffer BS has the function of converting signals to be transmitted to the reader and ranging from 0 to VDD 1  into signals ranging from 0 to VDD 2  that are exploitable by the card reader. 
     When a terminal PAD is used as input and output, the input and output buffers should not disturb each other at the common point at the terminal PAD. To this end, in particular, the output buffer BS should have a high impedance output when it is not used. In one example, we consider a card powered by an internal power supply potential VDD 1  powering the card. This potential VDD 1  is lower than the potential VDD 2  powering the reader. The internal power supply potential VDD 1  powering the card is produced, by a known regulation circuit, from the potential VDD 2  received at an input of the card. An input buffer for a card of this kind is made according to known approaches using, in particular, inverters supplied with the potential VDD 1 . An output buffer for a card of this kind is shown in FIG.  2 . It includes two potential step-up circuits  10 ,  20  and one tristate inverter  30 . 
     The potential step-up circuit  10  receives a logic control signal VAL that takes either of two values, 0 or VDD 1 , and it produces a logic signal HVAL of a higher level, taking two values: 
     
       
         HVAL=0 when VAL=0 
       
     
     
       
         HVAL=VDD 2  when VAL=VDD 1 . 
       
     
     In the example of FIG. 2, the potential step-up circuit  10  comprises two P type transistors T 1 , T 3 , two N type transistors T 2 , T 4 , and one simple inverter I 1  powered by VDD 1 . The transistors T 1 , T 2  are series-connected. The potential VDD 2  is applied to the source and the well of T 1  and the source of T 2  is connected to a ground of the circuit. The common drain of the transistors T 1 , T 2  is connected to the gate of T 3 . The transistors T 3 , T 4  are also series-connected. The potential VDD 2  is applied to the source and the well of T 3 , and the source of T 4  is connected to the ground of the circuit. The common drain of the transistors T 3 , T 4  is connected to the gate of T 1  and forms the output of the potential step-up circuit  10  at which the signal HVAL is produced. Finally the gate of T 2  is connected to the gate of T 4  via the inverter I 1 . The gate of T 2  forms the input of the step-up circuit to which the signal VAL is applied. 
     The step-up circuit  20  receives a logic signal DATA, taking either of two values, 0 or VDD 1 , and it produces a logic signal INT that is the inverse of the signal DATA but has a higher level. The signal INT thus takes two values: 
     
       
         INT=VDD 2  when DATA=0 
       
     
     
       
         INT=0 when DATA=VDD 1 . 
       
     
     The step-up circuit  20  is made in the same way as the step-up circuit  10 ; a simple inverter I 2  powered by the potential VDD 2  has simply been added to the output of the step-up circuit  20 . The inverter  30  receives the validation signal HVAL and the data signal INT, which take either of two values, 0 or VDD 2 . The inverter  30  has an output terminal OUT connected to the terminal PAD of the card. The inverter  30  works as follows: it produces a logic signal HDATA that is the inverse of INT, e.g. the same logic value as DATA, at the output OUT when the signal HVAL is active (in the example equal to 0); its output OUT is at high impedance when the signal HVAL is inactive. 
     The inverter  30  has two P type transistors T 5 , T 6 , two N type transistors T 7 , T 8  and one simple inverter I 3 . The transistors T 5 , T 6 , T 7 , T 8  are series-connected between a ground of the circuit and a power supply terminal to which the potential VDD 2  is applied. The potential VDD 2  is applied to the wells of the transistors T 5 , T 6  and to the source of the transistor T 5  whose drain is connected to the source of T 6 . The source of T 8  is connected to the ground of the circuit and its drain is connected to the source of T 7 . The drains of the transistors T 6 , T 7  are connected together to the output OUT of the inverter  30 . The gates of the transistors T 5 , T 8  are connected together and receive the signal INT. Finally, the gate of T 6  is connected to the gate of T 7  via the inverter I 3  powered by VDD 2 . The gate of T 6  receives the control signal HVAL. 
     The inverter  30  works as follows. When HVAL=VDD 2 , the transistors T 6 , T 7  are off and the output OUT is at high impedance, whatever the value of INT and whatever the state of the transistors T 5 , T 8 . Conversely, when HVAL=0, the transistors T 6 , T 7  are on. Depending on the value of the signal INT, the transistor T 5  or the transistor T 8  is on and the logic signal HDATA, which is the inverse of the signal INT, is produced at the output OUT. The signal HDATA is finally identical to the signal DATA from a logic point of view, but is at a higher potential level. It must be noted that the signals HVAL and INT must necessarily reach the value VDD 2  to turn off the operation of the inverter  30 , and more specifically to turn off the transistor T 6 , whence the necessity of using the potential step-up circuits  10 ,  20 . 
     The buffer of FIG. 2 has the drawback of using a large number of transistors. Indeed, given that a simple inverter (like I 1 , I 2  or I 3 ) is made from a P type transistor and an N type transistor that are series-connected, it is necessary to use a total of 20 transistors to make the buffer. The fact that the number of transistors is large naturally entails a large-sized circuit, and also substantial power consumption. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to make a buffer with a reduced number of transistors, to make a buffer that is smaller-sized and consumes less power than the prior art buffers having the same function. 
     This and other objects are attained by a buffer according to the invention that includes a logic gate to raise the potential level of input digital data having a first logic level (“ 1 ”) to a potential equal to a low power supply potential (VDD 1 ), and to produce intermediate data if a validation signal is active. The intermediate data has a first logic level (“1”) whose potential is equal to a high power supply potential (VDD 2 ), and the intermediate data is logically inverse to the input data. The buffer also includes a tristate inverter to produce output data, at an output, that are logically inverse to the intermediate data if the validation signal is active and having its output OUT at high impedance if this is not the case. 
     A buffer according to the invention has the same function as a prior art equivalent buffer: at its output, it produces data which, from a logic point of view, is identical to the data that it receives at its input but has a different potential level. However, the buffer according to the invention uses only one potential step-up logic gate and therefore uses a far smaller number of transistors than a prior art buffer, and shall be seen more clearly here below in examples. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention and the advantages that flow from it shall appear more clearly from the following description of exemplary embodiments of a buffer according to the invention. The description must be read with reference to the appended drawings, of which: 
     FIG. 1, which has already been described, is a schematic drawing of a typical contact card; 
     FIG. 2, already described, is an schematic diagram of a prior art buffer; and 
     FIG. 3 is an schematic diagram of a buffer according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A buffer according to the invention may be used, for example, in a card such as that of FIG. 1, as an output buffer BS between the circuit CI and a terminal PAD of the card. The terminal PAD is used in the example as an input terminal during normal operation of the card and as an output terminal during a phase of testing of the card. The integrated circuit CI of the card uses a low power supply potential VDD 1 , for example equal to 3 V. The buffer is used to transmit data to a circuit (for example a card reader) using a high-power supply potential VDD 2 , equal for example to 5 V. 
     The buffer receives digital data DATA in the form of a logic signal taking either of two logic values, 0 or VDD 1 , and a validation logic signal VAL also taking either of two values, 0 or VDD 1 . The buffer raises the potential of the data DATA and then, when the signal VAL is active, it produces a logic signal HDATA containing the data DATA at an output OUT. The signal HDATA varies between 0 and VDD 2  which is greater than VDD 1 . When the signal VAL is inactive, the output OUT of the buffer is at high impedance. Thus, the buffer does not disturb its environment when it is inactive (here in the normal mode of operation of the card). 
     The buffer comprises a potential step-up logic gate  50  and a tristate inverter  60 . The logic gate  50  is a NAND type logic gate, comprising four N type transistors T 11 , T 12 , T 16 , T 17  and four P type transistors T 13 , T 14 , T 15 , T 18 . The drain of the transistor T 11  is connected to the gate of the transistor T 15  and the source of T 11  is connected to the drain of the transistor T 12  whose source is ground-connected. The potential VDD 2  is applied to the source and the well of the transistor T 15  and to the source and the well of the transistor T 18 . The drain of T 15  is connected to the gate of T 18 . The potential VDD 1  is applied to the source of the transistors T 13 , T 14 . The drain of T 13  and the drain of T 14  are connected together to the gate of the transistor T 16  and to the drain of the transistor T 17 . The source of T 16  and the source of T 17  are ground-connected. The drain of T 16  and the gate of T 17  are connected together to the drain of T 15  and to the gate of T 18 . Finally, the gate of T 11  and the gate of T 13  are connected together to an input terminal of the logic gate  50  to which the signal VAL is applied. The gate of T 12  and the gate of T 14  are connected together to another input terminal of the logic gate  50  to which the signal DATA is applied. The drain of T 11  and the drain of T 18  are connected together to an output terminal of the logic gate  50  at which a signal /HDATA is produced. 
     The logic gate  50  works as follows. When the signal VAL is inactive at 0, T 11  is off and T 13  is on; the potential VDD 1  appears at the drain of T 13  and at the gate of T 16 , thus turning the transistor T 16  on. The zero potential then appears at the drain of the transistor T 16  and at the gate of T 18 . T 18  is thus turned on and the signal /HDATA at its drain is equal to VDD 2 . 
     When the signal VAL is active, and equal to VDD 1  corresponding to a logic “1”, the transistor T 11  is on and the transistor T 13  is off. When the signal DATA is equal to 0, T 12  is off and T 14  is on; the potential VDD 1  appears at the drain of T 14  and at the gate of T 16 ; the transistor T 16  is on, the zero potential appears at the drain of T 16  and therefore at the gate of T 18 ; the transistor T 18  is on and the signal /HDATA at the drain of T 18  is equal to VDD 2 . Conversely, when the signal DATA is equal to VDD 1 , T 12  is on and T 14  is off; the zero potential appears at the drain of T 12  and then at the drain of T 11  and at the gate of T 15 ; T 15  is on and the potential VDD 2  appears at the drain of T 15  and at the gate of T 18  which is off; the signal /HDATA is equal to 0. 
     In short, the logic gate  50  receives the signals VAL and DATA ranging from 0 to VDD 1  and it produces a signal /HDATA which is: 
     
       
         
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 /HDATA = VDD2 when VAL is inactive at 0, 
               
               
                   
                 /HDATA = 0 if DATA = VDD1 and VAL is active at VDD1, 
               
               
                   
                 /HDATA = VDD2 if DATA = 0 and VAL is active at VDD1. 
               
               
                   
                   
               
             
          
         
       
     
     The tristate inverter  60  comprises two P type transistors T 21 , T 22  and two N type transistors T 23 , T 24  connected in series. The potential VDD 2  is applied to the source of the transistor T 21  whose drain is connected to the source of T 22 . The potential VDD 2  is also applied to the wells of the transistors T 21 , T 22 . The ground of the circuit is connected to the source of the transistor T 24  whose drain is connected to the source of T 23 . The drains of the transistors T 22 , T 23  are connected together to an output terminal OUT of the inverter. The gate of T 22  is ground-connected and the validation signal VAL is applied to the gate of the transistor T 23 . Finally, the signal /HDATA containing the intermediate data is applied to the gates of the transistors T 21 , T 24 . 
     The inverter  60  works as follows. When the signal VAL is inactive and equal to 0, the signal /HDATA is equal to VDD 2 ; the transistors T 21  and T 23  of the inverter are therefore off and OUT is at high impedance. When the signal VAL is active and equal to VDD 1  (corresponding to a logic “1”), the transistors T 22 , T 23  are on. In this case, the inverter produces the signal HDATA at its output OUT. This signal is the inverse of the signal /HDATA. The value of /HDATA varies according to whether it is the transistor T 21  or the transistor T 24  that is on. 
     It will be noted that, according to the diagram of FIG. 3, the buffer according to the invention comprises 12 transistors only, giving a gain of about 40% as compared with a known buffer, in terms of silicon surface area and power consumed. 
     If the capacitive charge at the output OUT of the inverter  60  is great, then substantial current has to be given to the transistors T 21 , T 24  so that the inverter  60  can switch over without difficulty. For this purpose, it is possible to add two simple inverters (shown in dashes in FIG. 3) powered by the potential VDD 2  and series-connected between the gate  50  and the inverter  60 . The buffer then becomes slightly bigger (16 transistors), but nevertheless remains smaller than the prior art buffers.