Abstract:
An interface circuit ( 100 ) and method for interfacing digital signals with a bus, comprising: means (TxduC) for receiving digital signals, for transmission on the bus, relative to a first ground potential; transformer means ( 106 ) for passing edges of the received digital signals; and reconstruction means ( 110 ) for reconstructing signals from the edges of signals passed by the transformer means, so as to produce digital signals, for transmission on the bus, relative to a second ground potential. A similar circuit (CANL,  130, 134 ) interfaces, from the second ground potential to the first ground potential, signals received from the bus. The reconstruction means may use Schmitt triggers, whose bias points may be set by oscillators incorporating further Schmitt triggers ( 120, 142 ) located on the same semiconductor die to reduce temperature variability.

Description:
FIELD OF INVENTION  
         [0001]    This invention relates to interface circuits for digital signals.  
         BACKGROUND OF INVENTION  
         [0002]    In electronic systems such as those in an automobile, electronic control units (ECUs) distributed around a vehicle are typically connected to a communication bus, such as the Controller Area Network (CAN) bus which is well-known in industrial and automotive applications. In use, voltage differences between different (ECUs) on the bus, due for instance to differences in ground potential at the different ECUs, can cause communication errors.  
           [0003]    It has been proposed to overcome this problem by use of opto-isolators to isolate the signals produced by different ECUs. However, such a proposal has a number of drawbacks: firstly, opto-isolators are expensive; secondly, the reliability of opto-isolators may not be sufficiently robust to withstand the exacting mechanical and temperature conditions produced in an automotive environment.  
           [0004]    It is an object of the present invention to provide an interface circuit and method for digital signals, wherein the above-mentioned disadvantages may be overcome or at least alleviated.  
         SUMMARY OF INVENTION  
         [0005]    In accordance with a first aspect of the invention there is provided an interface circuit for digital signals as claimed in claim  1 .  
           [0006]    In accordance with a second aspect of the invention there is provided an interface circuit for digital signals as claimed in claim  7 .  
           [0007]    In accordance with a third aspect of the invention there is provided a method for interfacing digital signals as claimed in claim  9 .  
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0008]    One ECU interface circuit incorporating the invention for isolating signals of units on a CAN bus will now be described, by way of example only, with reference to the accompanying drawings, in which:  
         [0009]    [0009]FIG. 1 shows a schematic circuit diagram of a CAN bus ECU interface circuit.  
     
    
     DETAILED DESCRIPTION  
       [0010]    Referring to FIG. 1, a circuit  100  for interfacing an ECU  200  to a CAN bus  300  includes terminals TxduC, GnduC and RxduC for connection respectively to transmit, ground and receive terminals of the ECU  200 . The interface circuit  100  also has CANH, CANL and CANGnd terminals for connection respectively to high, low and ground connections of the CAN bus  300 .  
         [0011]    The TxduC terminal is connected via a resistance  102  and a capacitance  104  to an end of a winding of a transformer  106 , the other end of the winding being connected to earth. Another winding of the transformer  106  has one end connected to a datum voltage “0”, the other end of the winding being connected via a capacitance  108  to an input of a Schmitt trigger  110 . The output of the Schmitt trigger  110  is connected to the Tx terminal of a conventional CAN Driver  112  which produces signals of appropriate voltage, timing, etc., conforming to the CAN bus standard. The Schmitt trigger  110  also has its output connected via a resistance  114  to the datum voltage “0”, and has its input connected via a resistance  116  to the datum voltage “0”. The input of the Schmitt trigger  110  is also connected via a resistance  118  to the input of a Schmitt trigger  120 . The output of the Schmitt trigger  120  is connected via a resistance  122  and a capacitance  124  to the datum voltage “0”. The input of the Schmitt trigger  120  is also connected to a node intermediate the resistance  122  and the capacitance  124 .  
         [0012]    The GnduC terminal is connected to earth.  
         [0013]    The CAN Driver  112  has its Rx terminal connected (via an optional pulse lengthener  148 ) via a resistance  126  and a capacitance  128  to an end of a winding of a transformer  130 , the other end of the winding being connected to the datum voltage “0”. Another winding of the transformer  130  has one end connected to earth, the other end of the winding being connected via a capacitance  132  to an input of a Schmitt trigger  134 . The output of the Schmitt trigger  134  is connected to the RxduC terminal. The Schmitt trigger  134  also has its output connected via a resistance  136  to earth, and has its input connected via a resistance  138  to earth. The input of the Schmitt trigger  134  is also connected via a resistance  140  to the input of a Schmitt trigger  142 . The output of the Schmitt trigger  142  is connected via a resistance  144  and a capacitance  146  to the earth. The input of the Schmitt trigger  142  is also connected to a node intermediate the resistance  144  and the capacitance  146 .  
         [0014]    The CAN Driver  112  has its CANH and CANL terminals connected respectively to the CANH and CANL terminals of the CAN bus  300 . The CANGnd terminal is connected to the datum voltage “0”.  
         [0015]    It will be appreciated that the interface circuit  100  is made up of two similar, complementary interface circuits  150  (elements  102 - 110  &amp;  114 - 124 ) and  152  (elements  126 - 148 ) which respectively interface pulses for transmission on the bus  300  and pulses received from the bus  300 .  
         [0016]    Pulses (relative to earth potential) received from the TxduC terminal of the ECU  200  are differentiated by the capacitor  102  to produce positive-going and negative-going spikes corresponding to the pulses&#39; positive-going and negative-going transitions are passed by the transformer  106 . The resulting signal (consisting of positive-going and negative-going spikes separated by a DC level) at the output winding of the transformer  106  is applied to the Schmitt trigger  110 , in which a positive-going spike at its input causes its upper trigger level to be crossed (resulting in the output of the Schmitt trigger going low), and a negative-going spike at its input causes its upper lower trigger level to be crossed (resulting in the output of the Schmitt trigger going high). Thus the Schmitt trigger  110  reconstructs (from the pulse edge signal passed by the transformer  106 ) the pulse signal received at the terminal TxduC. However, it will be noted that whereas the pulse signal received at the terminal TxduC is relative to the earth potential (e.g., at the input winding of the transformer  106 ), the reconstructed pulse signal at the output of the Schmitt trigger  110  is relative to the datum voltage “0”. Thus, the interface circuit  150  serves to isolate signals for transmission on the CAN bus  300  between the ground potential (earth) of the ECU  200  and the ground potential (datum voltage “0”) of the CAN bus  300 .  
         [0017]    It will be understood that the complementary interface circuit  152  functions analogously (receiving a pulse signal relative to datum voltage “0” at the Rx terminal of the CAN Driver  112 , passing the edges of this signal through the transformer  130 , and reconstructing from the edge signal a pulse signal relative to earth potential which is equivalent to the pulse signal received at the terminal Rx) to isolate pulse signals received from the CAN bus  300  between its ground potential (datum voltage “0”) and the ground potential (earth) of the ECU  200 .  
         [0018]    The transformers  106  and  130  are small, low-cost transformers which, in use of the interface circuit  100 , act as isolation elements. Only the edges of pulse signals are passed via the transformers, allowing the transformers to be of low inductance (and hence low cost) while achieving very low propagation delay.  
         [0019]    As explained above, signal reconstruction (reconstruction of pulses from the edges passed via the transformers) is performed by the Schmitt triggers  110  and  134  for the circuit&#39;s transmit and receive functions respectively. The optimum bias point for the DC level of signal input to each of the Schmitt triggers  110  and  134  is automatically determined by use of additional Schmitt trigger  120  and  142  respectively, all the Schmitt triggers  110 ,  134 ,  120  and  142  being formed on the same integrated circuit die (not shown). The additional Schmitt triggers  120  and  142  are arranged to function as simple oscillators having a nominal 50% duty cycle. It will be understood that, with variations in temperature, the trigger points of Schmitt triggers vary. However, it will be understood that in the interface circuits  150  and  152  of the circuit  100 , any variation of the trigger point of the Schmitt trigger  110  or  134  due to temperature variation is counteracted by variation of that Schmitt trigger&#39;s bias point due to variation of the duty cycle of the oscillator formed by the additional Schmitt trigger  120  or  142  respectively, because of variation of that additional Schmitt trigger&#39;s bias point caused by the same temperature variation. Thus, the bias points of the Schmitt triggers  110  and  142  used for reconstruction of the pulse signals are dynamically and optimally set.  
         [0020]    It will be appreciated that the interface circuit  100  provides simple and effective isolation of signals on the CAN bus  300  through use of the small, low-cost isolating transformers  106  and  130  and the self-biasing arrangement of Schmitt triggers  110 ,  134 ,  120  and  142 .  
         [0021]    It will be understood that the interface circuit described above could be further enhanced. For example, a pulse lengthener  148  could additionally be used to ensure propagation of pulses of very short duration, as may be required in some CAN applications.  
         [0022]    It will be appreciated that although in the embodiment described above an interface circuit  100  (comprising complementary interface circuits  150  and  152 ) is illustrated as coupling one ECU  200  to the CAN bus  300 , in practice two or more ECU (not shown) may be interfaced by their own respective circuits such as the interface circuit  100  to isolate signals on the CAN bus between the ground potential of the bus and the individual ground potentials of the ECUs.  
         [0023]    It will be further appreciated that in interfacing a particular ECU it may not be necessary for the interface circuit to have the two complementary interface circuits such as  150  and  152  described above. For example, if an ECU is simply to transmit signals onto the bus (without receiving) only an interface circuit such as the interface circuit  150  may be provided, or if an ECU is simply to receive signals from the bus (without transmitting) only an interface circuit such as the interface circuit  152  may be provided.  
         [0024]    It will further be appreciated as a result of the above-discussed properties of good isolation, the interface circuit  100 , although rated for operation at a nominal CAN bus system voltage of 12V, could instead be used with 24V or even 42V systems, the good isolation allowing operation with one wire shorted to battery or ground.