Patent Application
20210167989
The controller area network (CAN) is a bus standard designed to allow microcontrollers and devices to communicate with one another in applications without a host computer. The CAN bus protocol is a message-based protocol, particularly suitable for multiplexed electrical wiring within automobiles but has usefulness in other applications,
In one example, a transceiver includes a driver stage and a transient-triggered ring suppression circuit. The driver stage has a first transistor coupled between a supply voltage terminal and a first bus terminal and a second transistor coupled between a ground and a second bus terminal. The transient-triggered ring suppression circuit is coupled to the first and second transistors. The transient-triggered ring suppression circuit is configured to be enabled upon transition of the transceiver from a dominant state to a recessive state. Further, while the transceiver is in the recessive state, the transient-triggered ring suppression circuit is configured to attenuate ringing on at least one of the first or second bus terminals.
For a detailed description of various examples, reference will now be made to the accompanying drawings in which:
As CAN bus speeds have increased, ringing on the bus due to improper electrical termination has also increased. As a CAN bus transceiver transitions from a “dominant” state to a “recessive” state, reflections from improperly terminated stubs may cause ringing on the transceiver. If the magnitude of the ringing is high enough, a transceiver will misinterpret the ring as a dominant bit. As such, ringing can cause bit errors. The examples described herein include a CAN bus transceiver that includes a transient-triggered ring suppression circuit which is enabled upon transition of the transceiver to the recessive state. Any ringing on the bus is attenuated through the transient-triggered ring suppression circuit thereby resulting in a smaller amplitude and shorter duration ringing signal thereby resulting in fewer bit errors. The transient-triggered ring suppression circuit described herein may have applicability to other bus protocols besides CAN.
The driver stage 140 includes transistors M1-M5 and a driver 142. M1, M3, and M6 are p-type metal oxide semiconductor field effect transistors (PMOS) and M2, M4, and M5 are n-type metal oxide semiconductor field effect transistors (NMOS). As PMOS or NMOS devices, each such transistor includes a control input (gate) and current terminals (source and drain). Other types of transistors can be used as well, such as bipolar junction transistors, which also have control inputs (base) and current terminals (collector and emitter).
M1-M3 are connected in series between the supply voltage terminal (VCC) and CANH, with the source of M1 coupled to VCC, the drain of M1 connected to the source of M2 at node N1, the drain of M2 connected to the source of M3 at node N2, and CANH taken from the drain of M3. Similarly, M4-M6 are connected in series between ground and CANL, with the source of M4 coupled to ground, the drain of M4 connected to the source of M5 at node N3, the drain of M5 connected to the source of M6 at node N4, and CANL taken from the drain of M6. A termination resistor Rterm (e.g., 120 ohms) is connected between CANH and CANL, but the transceiver can be terminated in other ways as well (e.g., with a series-connected 60-ohm resistors between CANH and CANL and capacitor connected between the node between the resistors and ground).
The gates of PMOS transistors M3 and M6 are connected to ground and thus M3 and M4 remain on continuously. The sources of M3 and M6 remain fixed at the transistor's threshold voltage above ground (e.g., 0.7 V). M3 and M6 operate to block large negative voltages from the respective bus terminal CANH or CANL from damaging the transceiver. The gates of M2 and M5 are connected to VCC and block large positive voltages from the respective bus terminal CANH or CANL from damaging the transceiver.
The driver 142 receives the transmit signal TxD on its input and drives complementary outputs 143 and 144 which are connected to the gates of M1 and M4, respectively. CANH and CANL are either driven to the dominant state with CANH voltage being higher than the CANL voltage, or not driven and pulled by passive resistors to the recessive state with the CANH voltage being below or equal to the CANL voltage. A “0” data bit encodes the dominant state, while a “1” data bit encodes the recessive state. For the dominant state, TxD is set equal to 0 and for the recessive state, TxD is set equal to 1. With TxD being 0 (dominant state), output 143 of driver 142 is 0 (low) and output 144 is 1 (high). With output 143 being a 0 and output 144 being a 0, PMOS transistor M1 and NMOS transistor M4 are both turned on, thereby pulling CANH up towards VCC and CANL down toward ground. In accordance with the CAN bus protocol, in the dominant state the CAN bus differential voltage is nominally 2V. In the recessive state, TxD is a 1 and thus driver 142 output 143 is a 1 and output 144 is a 0 and both M1 and M4 are turned off. With M1 and M4 being off, the voltages on CANH and CANL passively become approximately equal to VCM through resistors Rterm and Rid. In the example provided above, VCM is equal to VCC/2. In an application in which VCC is 5V, VCM is 2.5V and, in the recessive state, CANH and CANL are both approximately equal to 2.5V (approximately zero differential voltage).
The recessive nulling circuit includes NMOS transistors M7-M12. The gates of M7 and M8 are connected together and to pulse generator 110. The drain of M7 is connected to the drain of M1 and source of M2 (node N1). M9 is connected between the drain of M8 and the drain of M2 and source of M3 (node N2). M9 is biased on and is operative to block large positive voltages on N2 from damaging the transceiver. The sources of M7 and M8 are connected together and to the sources of M10 and M11. The gates of M10 and M11 are connected together and to pulse generator 110. The drain of M10 is connected to the drain of M4 and to the source of M5 (node N3). M12 is connected between the drain of M11 and the drain of M5 and source of M6 (node N4). M12 is biased on and is operative to block large positive voltages on N4 from damaging the transceiver.
The pulse generator 110 generates pulses 114 and 116 on outputs 111 and 112, respectively, responsive to 0-to-1 transition of TxD. The width of the pulses can be fixed or programmable. In one example, the width is 200 nanoseconds. During the pulses, M7, M8, M10, and M11 are on. The recessive nulling circuit 130 functions to force each of nodes N1-N4 to be equal to VCM for a short period of time (e.g., 200 ns) upon transition into the recessive node to help force the voltages on CANH and CANL to be equal to each other and to VCM. Once the pulses 114 and 116 end, CAN and CANL remain at VCM.
The transient-triggered ring suppression circuit 120 helps to suppress ringing on the CAN bus upon the transition from the dominant state into the recessive state. The transient-triggered ring suppression circuit 120 includes switches SW1 and SW2, resistors R1 and R2, capacitors C1 and C2, and NMOS transistors M21 and M22. SW1 is coupled between VCC and the gate of M21. In one example, SW1 may be a PMOS transistor. R1 also is coupled between VCC and the gate of M21. C1 is coupled between the source and gate of M21. SW2 is coupled between ground and the gate of M22. In one example, SW may be an NMOS transistor. R2 also is coupled between ground and the gate of M22. C2 is coupled between the gate and drain of M21.
During the dominant state (TxD is 0), M1 is on. With M1 on, node N1 is pulled up to VCC and thus source of M21 is VCC. The source of M22 is coupled to VCM. During the dominant state, control signals 121 and 122 cause switches SW1 and SW2 to be closed. In this example, control signals 121 and 122 are generated by the pulse generator 110. If SW1 is implemented as a PMOS transistor, control signal 121 may be asserted by the pulse generator 110 to track the logic state of TXD (i.e., when TXD is high, control signal 121 is forced high, and vice versa). If SW2 is implemented as an NMOS transistor, control signal 122 may be asserted by the pulse generator 110 to track the logical inverse of the logic state of TXD (i.e., when TXD is high, control signal 122 is forced low, and vice versa). In one example, the pulse generator 110 includes a buffer to generate control signals 121 and 122 (the buffer having a positive and negative outputs). With SW1 closed, the gate of M21 is pulled up to VCC. As such, the gate-to-source voltage across M21 is insufficient to turn on M21 and thus M21 is off. With SVV2 closed, the gate of M22 is pulled maintaining M22 in an off state.
Upon entry into the recessive state, control signals 121 and 122 change logic state and cause switches SW1 and SW2 to be open to thereby enable the transient-triggered ring suppression circuit. R1 pulls the gate of M21 high thereby maintaining M21 in an off state. However, any ringing signal on CANH propagates through M3 and M2 to capacitor C1. C1 becomes charged due to the ringing signal and if the magnitude of the ringing signal is large enough, C1 will charge to a sufficiently large voltage (at least a threshold voltage above VCM) to turn on M21. The resistor R1, which is connected between VCC and the gate of M21, discharges the gate of M21 thereby eventually turning of M21. As such, the ringing signal on CANH is dissipated through that portion of the transient-triggered ring suppression circuit 120 coupled to CANH (i.e., R1, C1, and M21).
In the recessive state and on the CANL side of the bus, R2 pulls the gate of M22 low thereby maintaining M22 in an off state. Any ringing signal on CANL propagates through M6 and M5 to capacitor C2. C2 becomes charged due to the ringing signal and if the magnitude of the ringing signal is large enough, C2 will charge to a sufficiently large voltage (at least a threshold voltage above M22's source which is connected to VCM) to turn on M22. The resistor R2, which is connected between ground and the gate of M22, discharges the gate of M22 thereby eventually turning of M22. As such, the ringing signal on CANL is dissipated through that portion of the transient-triggered ring suppression circuit 120 coupled to CANL (i.e., R2, C2, and M22).
The term “couple” is used throughout the specification. The term may cover connections, communications, or signal paths that enable a functional relationship consistent with the description of the present disclosure. For example, if device A generates a signal to control device B to perform an action, in a first example device A is coupled to device B, or in a second example device A is coupled to device B through intervening component C if intervening component C does not substantially alter the functional relationship between device A and device B such that device B is controlled by device A via the control signal generated by device A.
Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims.
This application claims priority to U.S. Provisional Application 62/942,763, filed Dec. 3, 2019, titled “Circuit Technique to Absorb RF Energy and Improve Immunity in CAN Transceivers,” which is hereby incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 62942763 | Dec 2019 | US |
