• State TI Thinks Resolved
  • Locked Locked
  • Replies 3 replies
  • Answers 1 answer
  • Subscribers 32 subscribers
  • Views 258 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Original question:

TCAN1051-Q1: S pin termination when unused

TCAN1051-Q1: If silent mode loading the CAN Bus.

Tomer Avrahami
Prodigy 40 points
Part Number: TCAN1051-Q1
Other Parts Discussed in Thread: TCAN1044-Q1

Hi.

We are using TCAN1051Q automotive version with silent mode witch disable the transmitter part.

part of the chip characteristic are it's differential input capacitance and resistance, other then stating the CANH + CANL are "Z" according to datasheet and the receiver is still active it's not clear what are this state capacitance and resistance and where the transceiver load the bus.

can you please clearly if in silent mode the transceiver still loads the bus. and if so, what are the differential input capacitance, resistance (CANH, CANL to GND).

Is there any CAN receiver only which does not load the bus as regular transceivers does.

Thanks in advance.

Tomer.

  • 0
    TI__Mastermind 47895 points

    Hi Tomer,

    The capacitive and resistive loads presented by a transceiver are largely the result of receiver circuitry and package parasitics. Because of these inherent parasitics, any receiving or sensing device on a signal line will contribute some load to the system. Regarding TCAN1051-Q1 specifically, I would not expect there to be a significant change in this load when a device is in silent mode where only the driver is disabled (will be very similar to recessive level load - both are high impedance states). This is because switching modes does not change the receiver state nor can it effect package parasitics.  

    I'm not aware of any dedicated CAN receivers and I don't see much advantage to excluding the transmitter portion of such a transceiver. Such solutions could use a typical CAN transceiver with a silent function or simply leave the TX line unused. 

    If bus loading is a concern in your design, you may consider using a transceiver such as TCAN1044-Q1 with lower specified bus load (leaving TX line unused to act as dedicated receiver in place of silent mode), reducing external load sources such as using low-capacitance TVS diodes, or consolidating nodes where possible to reduce bus length or the total number of devices on the bus.

    I hope this information helps. Please let me know if you have any more questions.

    Regards,
    Eric

  • 0 in reply to Eric Schott1
    Prodigy 40 points

    Eric Hi,
    Thanks for your quick reply.

    According to your answer it seems there direct connection between maximum number of ECUs on the bus line, and transceiver loading capacitance in specific data rate. As we know the max ECU's on the bus is 32 according to 11898-2:2016 in 1Mbps.

    Dose the TCAN1044Q with lower capacitance (Cin and Cdiff) will also help to load the max number of ECUs on the bus in same conditions when max rate is 1Mbps? 

    Thanks,

    Tomer.

  • 0 in reply to Tomer Avrahami
    TI__Mastermind 47895 points

    Hi Tomer,

    There are several factors that restrict the maximum data rate and achievable number of nodes on a CAN bus. For simplicity, let's just consider bus capacitance. Bus capacitance will be added to a CAN bus from sources such as cabling, protection circuitry, PCB traces or backplanes, and CAN transceivers. The more capacitance a signal line has, the slower each bit transition will be. This limits the maximum data rate of the system. As more nodes and CAN transceivers are added - thus adding capacitance - to the bus, the more restricted this data rate will be. ISO 11898-2:2016 specifies 32 nodes at 1Mbps. If we generalize, we can assume that if we have less than 32 nodes then we can go faster than 1Mbps, and if we have more than 32 nodes, we will be limited to less than 1Mbps (note that there are many other factors as play here so the numbers here are very system dependent, I would just like to highlight this specific trend). Since capacitance can also be added from other sources, the number of allowable nodes can be increased by decreasing capacitance from elsewhere e.g. reducing cable length. 

    Using transceivers that present smaller capacitive loads will allow more nodes to be added to the bus before the capacitive limitations come into play. Considering this, using TCAN1044-Q1 would allow more nodes per bus than TCAN1051-Q1. As everything on the controller-side of a CAN transceiver presents no additional load to the bus, we only need to consider the number of transceivers present and what load they present. 

    Please keep in mind that bus capacitance is only one factor that limits achievable data rate. Each system will have unique traits to consider when choosing an operational data rate. If your system only has a large number of nodes and is simple in other aspects (well terminated, short/no stubs, short overall bus length, low parasitic protection footprint) then you should have no issue reaching 1Mbps rates with 32 nodes.  

    This response turned out to be a bit more word-y than I hoped, so please let me know if anything is unclear. 

    Regards,
    Eric