Abstract:
The invention relates to a receiver for a differential data bus with two resistive branches ( 1, 2, 3; 4, 5, 6 ), with a differential amplifier with two transistors ( 9, 10 ), with a resistor ( 13 ), and with a control logic ( 16 ) that controls a switch ( 15 ) with which a current from a current source ( 14 ) is switchable to either side of the resistor ( 13 ), which resistor couples the two transistors ( 9, 10 ), and with two operational amplifiers ( 17, 18 ) which are coupled to the two transistors ( 9,10 ) of the differential amplifier with opposite poles, in which receiver the control logic detects from the output signals of the two operational amplifiers ( 17,18 ) whether a “0” or a “1” is expected on the bus and which receiver sets the switch ( 25 ) accordingly so that a comparison with the received bus signal is made.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a receiver for receiving data from a differential data bus with two lines which can detect a positive and a negative level on the bus lines. 
     Such receivers usually have two resistive input branches, which are used to weaken the input signals from the data bus. According to solutions in the state of the art, two voltage sources in combination with two comparators are used for detecting the two levels. The exact detection levels are mainly defined by the voltage sources. If the voltage sources do not deliver exactly the same voltage, witch can easily occur in practice, the positive and negative detection levels are not equal, which should be avoided. 
     It is an object of the invention to provide a receiver for a differential data bus which ensures symmetrical detection levels of positive and negative signals. 
     This object is achieved by the receiver having the features according to claim  1 : 
     Receiver for a differential data bus with two resistive branches, with a differential amplifier with two transistors, with a resistor, and with a control logic that controls a switch with which a current from a current source is switchable to either side of the resistor, which resistor couples the two transistors, and with two operational amplifiers which are coupled to the two transistors of the differential amplifier with opposite poles, in which receiver the control logic detects from the output signals of the two operational amplifiers whether a “0” or a “1” is expected on the bus and which receiver sets the switch accordingly so that a comparison with the received bus signal is made. 
     The receiver according to the invention uses only one voltage source instead of two in order to avoid level mismatches. This one voltage source is realized with one current source and one resistor. By switching the resistor between two branches of a differential amplifier this voltage source can be used for detecting a positive level on one line and a negative level on the other line, or vice versa. This ensures an absolutely symmetrical detection of levels of the two polarities, which has the consequence of a very low jitter. 
     The control logic puts the switch with which the current is switched on either side of the differential amplifier in accordance with the falling edge last received. 
     According to the advantageous measures of claim  2 , two transistors can be used as the differential amplifier, thus providing a simple circuitry. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be further described with reference to the drawings, in which: 
         FIG. 1  is a schematic block diagram of an embodiment of a receiver according to the invention with only one voltage source used for the detection of positive as well as negative levels, and 
         FIG. 2  is a timing diagram of the receiver according to  FIG. 1  with several bits received from the data bus. 
     
    
    
     DETAILED DESCRIPTION 
     The schematic block diagram of  FIG. 1  shows a receiver for a differential data bus with two lines bm and bp. As this bus works with differential signals, the signals always have opposite polarities, if the bus is not disturbed. The bus may be, for example, one according to the FlexRay standard. 
       FIG. 1  shows that the input stage of the receiver is provided with two branches with resistive dividers, which are used to accommodate high common input voltages. 
     The first divider comprises three resistors  4 ,  5  and  6  in series. Resistor  4  is coupled to the line bp of the bus. The second divider comprises three resistors  1 ,  2  and  3 , also in series, of which resistor  1  is coupled to the other line bm of the data bus. 
     The connection between resistor  1  and resistor  2  is coupled to the input of an inverter  7 , whose output is coupled to a connection between resistors  3  and  6 . In the same way a second inverter  8  is coupled to the connection between resistors  4  and  5  and the connection between the last transistors  3  and  6  of the dividers. 
     The connection between the resistors  5  and  6  is coupled to the base of a first bipolar npn-transistor  9 , while the connection between the transistors  2  and  3  is coupled to the base of a second bipolar npn-transistor  10 . 
     The collectors of the transistors  9  and  10  are coupled via current sources  11  and  12  to a power source V+ with positive voltage. 
     The emitters of the two transistors  9  and  10  are coupled via a resistor  13  which, together with a current source  14 , forms a voltage source which is used for detecting positive and negative levels, as will be explained below. The two transistors  9  and  10  and the resistor  13  form a differential amplifier. 
     The current source  14  can be switched to either side of the resistor  13  by a switch  15 , which is controlled by a control logic  16 . 
     The data outputs are realized by a second comparator  17  and a first comparator  18 , which deliver the output signals RXD 1  and RXD 0 . 
     The negative input of the second comparator  17  and the positive input of the first comparator  18  are coupled to the collector of the second transistor  10 , while the positive input of comparator  17  and the negative input of comparator  18  are coupled to the collector of the first transistor  9 . 
     As already mentioned, instead of using two separate voltage sources, one voltage source is formed with one resistor  13  and one current source  14 . The switch  15  serves to determine whether a positive or a negative differential voltage has to be detected. 
     The control logic  16  detects only the falling edges of the output signals RXD 0  and RXD 1 . A falling edge of RXD 0  causes the control logic  16  to set the switch  15  to a second position, in which the current source  14  is coupled to the emitter of transistor  10 , whereas a falling edge of RXD 1  causes the control logic  16  to set the switch  15  to a first position, in which the current source  14  is coupled to the emitter of transistor  9 . 
     With reference to the timing diagram in  FIG. 2 , it will now be explained how the receiver according to  FIG. 1  works when data bits appear on the bus. The timing diagram shows the voltages of several signals in the receiver. 
     The first two signals in  FIG. 2  are the bus signals bm and bp on the bus lines. As this is a differential bus, for example a bus according to the FlexRay-standard, the signals bm and bp have opposite polarities. 
     The next two signals in the diagram are the signals V 1  and V 2 . These are bus signals bm and bp which have been weakened by input dividers formed by the resistors  1  to  6 . The signals V 1  and V 2  are applied to the bases of the transistors  9  and  10 , respectively. 
     The next two signals show the voltages at the collectors of the transistors  9  and  10 . 
     The last two signals are the output signals RXD 1  and RXD 0  of the comparators  17  and  18  and of the receiver. 
     In the timing diagram of  FIG. 2 , RXD 0  first is negative, so that a falling edge (not shown) must have appeared in this signal last. That is why the control logic had set the switch  15  in the second position. Now a falling edge appears in bm and a rising edge in signal bp. Consequently, the same edges appear in the weakened versions V 1  and V 2  of these signals. As V 2  is coupled to the base of the transistor  9 , this transistor switches and the potential at its collector falls, as can be seen in the timing diagram. At the same time V 1  shows a falling edge, so that transistor  10  closes and the potential at its collector shows a rising edge. This has the consequence that the level of the output signal RXD 1  changes from high to low, whereas the output signal RXD 0  goes from low to high. 
     The fact that the signal RXD 1  shows a falling edge causes the control logic to set the switch  15  to the first position, in which the current source  14  is coupled to the emitter of the second transistor  9 , as now a falling edge in the signal bm is expected. 
     The timing diagram shows that in fact at the next transition the signal bp changes from high to low and that signal bm from low to high. This time, the transistors  9  and  10  are switched to the opposite positions, so that the potential at the collector of transistor  9  goes up and that at the collector of transistor  10  goes down. Consequently, the signal RXD 1  shows a rising edge and RXD 0  shows a falling edge this time, so that the control logic  16  switches the switch  15  to its second position, in which the current source is coupled to the emitter of transistor  10 . 
     Now a falling edge in the signal bp is expected next and the process is repeated as described above. 
     The switching process of the two transistors will be explained in detail below: 
     The outputs of the bipolar transistors  9  and  10  switch at the point where the currents through the emitters are equal. So, at this moment the emitter currents are I/2, wherein I is the tail current of the differential pair formed by the current source  14 . At this moment the current through the resistor  13  is I/2. It can thus be calculated that: V 1 −Vbe−½I*R=V 2 −Vbe. Now the collector of transistor  10  goes down and the collector of the transistor  9  goes up. Output RXD 0  goes from 1 to 0 and output RXD 1  goes from 0 to 1. The falling edge of RXD 0  causes the control logic to switch the switch  15  to the other side of the resistor (The control logic only reacts to negative edges of RXD 0  and RXD 1 , positive edges are of no influence). After this, the differential pair switches its outputs again when the point is reached where: V 2 −Vbe−½*I*R=V 1 −Vbe. RXD 1  goes from 1 to 0, RXD 0  goes from 0 to 1, and the control logic switches the tail current back to the other side of the resistor. 
     In the example of  FIG. 2 , the bus lines were already high or low at the beginning. However, this may not be the case when starting up a data bus. This means no differential voltage on the bus for a specified time. When the idle state is detected, the switch  15  is set to the first (default) position. In this position the receiver is ready to detect a “0”, as, for example according to the FlexRay-standard, a “0” is always the first bit after idle. A “0” means that RXD 0  will have a falling edge and on this edge the switch position will be set to the second position so that the receiver is ready to detect a “1”. Now the receiver and the switching position are in the normal routine as described above. 
     If for some reason the first bit after idle is not a “0” but a “1”, it would seem that the first bit will be missed by the system. This, however, is not the case for the following reason: the falling edges of RXD 0  and RXD 1  are translated into an RXD signal. So a negative edge on RXD 0  makes RXD “0” and a negative edge on RXD 1  makes RXD “1”. Most protocols require that RXD is high when the bus is idle, so if the first bit is “1” no negative edge will arise on RXD 1 , RXD will stay high, and the comparator will still wait for a “0” (which should come after the previous “1”). So, the system will still work in this case.