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TCAN1042-Q1: CAN bus impedance measurement

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Replies: 3

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Part Number: TCAN1042-Q1

Hi sir,

We prefer to measure CAN bus impedance as followed Table 3. in TCAN1042 datasheet.

Especially when VCC<UVLO, VIO>UVLO, the STB be high or low.

Can you provide us measure method? It is important to our customer. Thanks.

B.R.

Hedy

  • Hi sir,

    Updated test result as below, the condition [2b] and [3] should be high impedance, but the data shows 25Kohm, it doesn't make sense.

    The result is using multi-meter.

    Any configuration or test method should be adjusted? 

    Condition [1] [1] [2a] [2b] [3] [4]
    CANs VCC=5.25V,
    VIO=3.3V,
    STB="0"
    VCC=5.25V,
    VIO=3.3V,
    STB="1"
    VCC=0V,
    VIO=3.3V,
    STB="1"
    VCC=0V,
    VIO=3.3V,
    STB="0"
    VCC=3.3V,
    VIO=0V,
    STB="0"
    VCC=0V,
    VIO=0V,
    STB="0"
    HS1_BUS_P 24.43 Kohm 24.72 Kohm 24.80 Kohm 24.78 Kohm 24.75 Kohm
    HS1_BUS_N 24.50 Kohm 24.69 Kohm 24.62 Kohm 24.74 Kohm 24.75 Kohm
    P_N 46.55 Kohm 48.57 Kohm 46.90 Kohm 48.71 Kohm 48.55 Kohm 48.57 Kohm
    V_BUS_P 2.60 V 0.00 V 0.00 V 0.00 V 0.00 V 0.00 V
    V_BUS_N 2.62 V 0.00 V 0.00 V 0.00 V 0.00 V 0.00 V
    CAN BUS Per TXD Per TXD Recessive High Impedance High Impedance High Impedance

    B.R.,

    Hedy

  • In reply to Hedy Hung1:

    Hedy,

    We are looking into this and will have a response by tomorrow US time. Thank you for your patience.

    Regards,

    Eric Hackett

    Texas Instruments | INT-TRX| Applications Engineer

  • In reply to Hedy Hung1:

    Hi Hedy,

    I understand where the confusion is coming from here as the table in the datasheet makes it appear that there are only two passive bus outputs states - recessive, or high impedance. In reality, there are several different states in which the bus can be considered "high impedance" that may have different measured impedances. 

    Let's consider all passive bus states (so not including a dominant state). In the recessive state in Normal or Standby Mode, the CAN driver is enabled and weakly biasing the CAN bus to a recessive level. In this state, the weak bias provides a relatively low input impedance, but is still weak enough to be overcome by another dominant-driving transceiver. This is denoted in Table 3 as "Recessive".

    In any Protected Mode (or any time the device is powered really), one of two receivers will be active on the bus - the main receiver, or the low-power wake-up monitoring receiver. These receivers add some load to the bus in order to measure the bus state and thus present a high-impedance load as a leakage path (the ~25k-ohm measurement). The specification "receiver input resistance (CANH or CANL)" refers to the device in this state. 

    Lastly, when the device is completely unpowered (Vcc = Vio = 0V), both receivers can be deactivated. In this state, neither of the transceivers load the bus and the internal recessive biasing is also disabled. This allows he CAN lines to be at their highest input impedance. This state provides very little load to the CAN bus and will have minimal impact on the network. 

    The last two states mentioned - protected and unpowered - are lumped together in the table for simplified reading. Because both states do not actively bias the bus to a recessive or dominant state, they are both considered high impedance. 

    Let me know if this makes sense and if you have any more questions.

    Regards,
    Eric Schott

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