Abstract:
A bus interface having a first circuit based on a first pair of transistors of opposite types having a control electrode and a common electrode for providing a first output potential. A second circuit has a second pair of transistors of opposite types and having a common electrode for providing a second potential switching in opposite direction from the former. This device has a first capacitive coupling means for feeding a portion of the signal existing at said first potential back into said control electrode of said second transistor pair and second capacitive coupling means for feeding a portion of the signal existing at said second potential back into said control electrodes of said first transistor pair. Thus variations between the rise and decay times of the transistors of each pair can be compensated for.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to bus control circuits, and more particularly to a bus control circuit including a switching time servo system. 
     2. Description of the Related Art 
     Communication systems continuously develop and operation speeds increase constantly. Bus control circuits—also called bus drivers or bus buffers—operate at increasingly higher speeds. 
     A typical example is provided by the serial interface defined in the CCITT standard, Universal Serial Bus, which is intended to control operation of the serial interface between data processing systems, and in particular between computer devices. In one of the latest versions of this standard—i.e., standard known as USB 2.0—bus control circuits are designed to operate at three different speeds namely, low speed, full speed and high speed. Full speed is considered to be 12 Mbits per seconds and this standard imposes particularly severe constraints in particular regarding switching times of both output circuits (or “buffers”) driving the serial cable. 
     FIG. 1 shows a conventional architecture for such a control circuit comprising a twin control circuit having a low impedance—typically 6 ohms—for example switching to 6 MHz for 50 picoFarads capacitive loads. 
     A first control circuit or driver  100  is based on a pair of NMOS-PMOS transistors comprised of a PMOS transistor  10  and a NMOS transistor  20 , respectively, the drains of these transistors are connected to a common D+ electrode. Each one of transistors  10  and  20  has a coupling capacitor— 11  and  21  respectively—which makes it possible to set to an absolute value the rise and decay times of terminal D+ output potential fed by a capacitor C LP    15 . A respective current generator, respectively  12  and  22 , controls the gates of transistors  10  and  20 . 
     Similarly, the control circuit comprises a second circuit  200  which is also based on a pair of NMOS-PMOS transistors comprised of transistors  30  and  40 , controlled by power sources  32  and  42  respectively, and associated with a coupling capacitor,  31  and  41  respectively. The common junction of PMOS transistor  30  and NMOS transistor  40  provides a potential D−, to which a (presumably capacitive) load C LP    35  is connected, and potential D− is supposed to switch exactly contrary to the common potential D+ of transistors  10  and  20 . 
     The assembly of circuits  100  and  200  makes up a twin control structure for a bus. It is however apparent that characteristics of the NMOS and PMOS transistors are difficult to pair to generate virtually identical rise and decay times for potentials D+ and D. Pairing NMOS and PMOS transistors so that they show substantially similar internal characteristics, as is currently done, for example, with resistors or capacitors is one of the great difficulties encountered in manufacturing processes. 
     BRIEF SUMMARY OF THE INVENTION 
     An embodiment of this invention provides a solution to the problem of designing a simple structure for a control circuit that allows for compensation of differences between active components, and particularly NMOS and PMOS components, and to appreciably pair the switching times of transistors, in particular their rise and decay times. 
     An embodiment of the present invention provides a control or interfacing structure for a serial cable having virtually identical rise and decay time values. 
     An embodiment of the invention is directed to a bus interface circuit that comprises: 
     a first circuit comprising a first pair of transistors of opposite types having a control electrode and a common electrode for providing a first output potential (D+); 
     a second circuit comprising a second pair of transistors of opposite types and having a common electrode for providing a second potential (D−) switching in opposite direction from the former; 
     first capacitive coupling means for feeding a portion of the signal existing at said first potential (D+) back into said control electrodes of said second pair of transistors and, 
     second capacitive coupling means for feeding a portion of the signal existing at said second potential (D−) back into said control electrodes of said first pair of transistors, in order to compensate for the internal characteristics of the transistors and to standardize rise and decay times. 
     Preferably, coupling will be carried out by cross coupling capacitors and it will then be possible, by simply pairing the capacitors, which is easy to carry out, to compensate for variations of the internal characteristics of the NMOS and PMOS transistors and to obtain identical rise and decay times. 
     In a preferred embodiment, the first pair of transistors comprises a PMOS-type first transistor, having a source electrode connected to a positive potential (V+) and having a gate that receives a control signal. An NMOS-type second transistor has a source electrode connected to a negative potential (V−) and also has a gate receiving a control signal. Both drain electrodes of the first and second transistors are connected together so as to provide a potential to the output electrode (D+). A first coupling capacitor is respectively connected between the gate and drain of the first transistor and, in the same way, a second coupling capacitor is inserted between the gate and the drain of the second transistor. Cross capacitive coupling is then realized by means of first and second cross coupling capacitors, having one electrode connected to the output (D−) potential of the second control circuit and a second electrode respectively connected to the gate of the first transistor and the gate of the second transistor. 
     The second circuit comprises a PMOS-type third transistor having a gate and having a source electrode connected to a positive potential (V+). The third transistor is serially assembled to a NMOS-type fourth transistor having a source electrode connected to a negative potential (V−) and having a common drain electrode with the third transistor, this common electrode being connected to the output potential (D−). A NMOS-type fourth transistor has a source electrode connected to a negative potential (V−) and its gate receives a control signal. A third output coupling capacitor is connected between the gate and the drain of the third transistor and, in the same way, a fourth output coupling capacitor is connected between the gate and the drain of the fourth transistor. 
     Cross capacitive coupling is then realized by means of third and fourth cross coupling capacitors, each having a first electrode connected to the output potential (D+) of the first control circuit and a second electrode that is connected to the gate of the third and the gate of the fourth transistor respectively. 
     In a preferred embodiment, values of the four cross coupling capacitors C 1 , C 2 , C 3  and C 4  will be set according to the following formulas: 
     
       
         
           C 
           1 
           =K×C 
           2 
         
       
     
     
       
         C 3 =C 1   
       
     
     
       
         C 4 =C 2   
       
     
     where K corresponds to the ratio of the bias current values in sources  12  and  42  respectively. 
     Alternatively, a dual structure can be realized in which the first, second, third and fourth transistors are NMOS, PMOS, NMOS and PMOS transistors respectively. 
     Preferably, the control circuit will be associated with a device for avoiding simultaneous conduction of the transistors of the first and second pairs in order to avoid power overdrain. 
     In a preferred embodiment, cross coupling capacitors between the first and second circuits will advantageously be realized by means of manufacturing techniques based on capacity arrays 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 illustrates the structure of a driver or control circuit known in the art. 
     FIG. 2 illustrates a control circuit as improved by the teachings of this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A preferred embodiment will now be described which will be used to build an interfacing circuit for a USB-type serial port. It is however obvious that the invention can be used to realize any other high-speed interface circuit. Furthermore, an embodiment of the invention utilizes NMOS transistors having a source electrode connected to ground and PMOS transistors receiving a positive supply voltage, will be described. People qualified in the art will be able to adapt the circuit so as to form a dual structure in which NMOS transistors are connected to the ground and PMOS transistors are supplied with a negative voltage. 
     When examining in detail the structure shown in FIG. 2, it can be seen that the first circuit is composed of a first PMOS transistor  10  that receives a positive supply voltage V+ via its source electrode and has its drain electrode connected to the output electrode D+ of the bus control circuit. A NMOS transistor  20  of opposite polarity has a drain electrode connected to the output electrode D+ and a source electrode connected to a negative reference potential V−. The gates of transistors  10  and  20  are controlled by a power source, respectively a first source  12  and a second source  22  supplied with voltage V−. A first coupling capacitor  11  is connected between the gate and drain electrodes of PMOS transistor  10 . In the same manner, a second coupling capacitor is inserted between the gate of NMOS transistor  20  and its drain electrode. In the diagram of FIG. 2, it is assumed that the output electrode D+ is connected to a capacitive load  15  (C LP ), which is a relatively close representation of a typical load for a serial interface. It should be understood, however, that this representation is given for illustration purposes only and that the invention is not in any way limited thereto. 
     The second circuit is composed of a third transistor  30 , of the same type as transistor  10 , and that has a source electrode connected to the positive potential V+ and a drain electrode connected to the output electrode D−, that is loaded with a load  35 , also supposed to be of a capacitive nature. A fourth transistor  40 —of the same type as NMOS second transistor  20 —has a drain electrode that is connected to the output electrode D− and a source electrode that is connected to the negative potential V−. Gates of transistors  30  and  40  are respectively controlled by a third power source  32  and a fourth power source  42  (V+) that are to receive the switching signal. As for the transistor pair  30  and  40 , a third coupling capacitor  31  is inserted between the gate and drain electrodes of PMOS transistor  30 , and a fourth coupling capacitor  41  between the gate and drain electrodes of NMOS transistor  40 . 
     In order to compensate for internal characteristic differences of the transistor pairs  10 - 20  and  30 - 40 , a new cross capacitive coupling is then advantageously realized between control circuits  100  and  200 , especially by means of a set of four capacitors  13 ,  23 ,  33  and  43 . The capacitors  13 ,  23  comprise a first coupling circuit  300 , and capacitors  33  and  43  comprise a second coupling circuit  400 . In the semiconductor manufacturing process, the first and second coupling circuits  300 ,  400  have process parameters that are more controllable than the process parameters for the transistor pairs  10 - 20  and  30 - 40 . 
     More precisely, a first capacitor  13 —having a value C 1 —is inserted between the gate of first PMOS transistor  10  and the opposite output D− of the bus control circuit. Similarly, a second capacitor  23 —having a value C 2 —is inserted between the gate of second NMOS transistor  20  and the output D− of the other control circuit. Thus a first cross-capacitive coupling is realized between the gate inputs of transistors  10  and  20  and the output of the second bus control circuit formed by transistor pair  30 - 40 . 
     In a symmetrical way, a second cross-capacitive coupling is realized between the inputs of the gates of transistors  30  and  40  and the output of the first circuit D+. For this purpose, a third capacitor  33 , with a value C 3 , is inserted between the gate of transistor  30  and the output D+, and a fourth capacitor  43 , with a value C 4 , is inserted between the gate of transistor  40  and the output D+. Supply sources  12 ,  22 ,  32  and  42 , that drove transistors  10 ,  20 ,  30  and  40  respectively, remain unchanged. 
     As can be seen in FIG. 2, output potentials D+ and D− switch in opposite direction. Cross coupling capacitors  13 ,  23 ,  33  and  43  make it possible to feed back part of the information existing at the output of any of the control circuits—for example D+ at the output of the transistor pair  10 - 20 —to modify the behavior of the transistor pair of the other circuit ( 30  and  40  in this case). If ever, because of internal characteristics of active components, one of the circuit switches more quickly than the other—for example the pair of transistors  10 - 20 —the realized cross coupling substantially increases the switching speed of the opposite pair, thus reducing any variation between the switching times of the transistors. 
     With this cross-capacitive coupling, an effective compensation of the internal characteristics of the active components  10 ,  20 ,  30  and  40  that are so difficult to pair, can thus be achieved. Such compensation is carried out by pairing the passive components comprised of the four capacitors  13 ,  23 ,  33  and  43 , which is more easy to realize regarding manufacturing processes. 
     More precisely, the values of capacitors C 1 , C 2 , C 3  and C 4  will be set according to the following formulas: 
     
       
         
           C 
           1 
           =K×C 
           2 
         
       
     
     
       
         C 3 =C 1   
       
     
     
       
         C 4 =C 2   
       
     
     Where K=I 12 /I 42  with I 12  and I 42  correspond respectively to the bias currents in power sources  12  and  42 , respectively. People qualified in the art will be able to then adjust the value of capacitor C 1  (other values being then derived from the preceding formulas) so as to set values of the rise and decay times to the desired values. This adjustment of the values of the capacitors will provide substantially equal slew rates on the output signals D− and D+. 
     As can be seen, the invention makes it possible to substantially compensate for the differences existing between internal characteristics of transistors  10 ,  20 ,  30  and  40 , by means of a simple adjustment of capacitor values. It will be noted that all known techniques for pairing the values of these capacitors  13 ,  23 ,  33  and  43  could advantageously be employed. Thus, people qualified in the art will be able to advantageously arrange capacitors  13 ,  23 ,  33  and  43  in capacitor arrays on the semiconductor element so as to avoid component mismatch effects, for example, resulting from diffusion gradients on the semiconductor element or physical layout differences. 
     Similarly, it is observed that control circuits driving the transistor gates can advantageously include known devices that are usually used in this type of structure. Thus, a power conservation circuit ensuring that two transistors do not conduct exactly at the same time can clearly be adapted thereto, as is conventionally done in such architectures. 
     As mentioned previously, the types of the transistors  10 ,  20 ,  30  and  40  can be changed. An interfacing circuit in which transistors  10  and  30  are NMOS-type transistors while transistors  20  and  40  are PMOS-type transistors can then be realized. Especially, the first transistor pair is realized with a NMOS-type first transistor, the source electrode of which is connected to a negative potential (V−) and the gate of which receives a control signal. The first pair further comprises a second transistor (a PMOS-type transistor) having a source electrode connected to a positive potential (V+). Both drain electrodes are connected together in order to provide the output electrode (D−) potential. A first (resp. a second) coupling capacitor is connected between the gate and the drain of the first (resp. second) transistor, respectively. Cross capacitive coupling is then realized by means of a first (second) capacitor that has one electrode connected to the output potential (D+) of the second control circuit and has a second electrode connected to the gate of the first (resp. second) transistor, respectively. 
     Similarly, the second circuit comprises an NMOS-type third transistor having a gate and having a source electrode connected to a negative potential (V−). The third transistor is serially mounted with a PMOS-type fourth transistor having a source electrode connected to a positive potential (V+) and having a common drain electrode with the third transistor, this common electrode being connected to the output potential (D+). A third output coupling capacitor is connected between the gate and drain of the third transistor and, in the same way, a fourth output coupling capacitor is connected between the gate and drain of the fourth transistor. Cross capacitive coupling is then realized by means of a third (resp. fourth) capacitor having an electrode connected to the output potential (D−) of the first control circuit and a second electrode that is connected to the gate of the third (resp. fourth) transistor, respectively. 
     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 and the equivalents thereof.