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SN65HVDA1040A-Q1 CANH level

Other Parts Discussed in Thread: SN65HVDA1040A-Q1

I would like to confirm CANH level for SN65HVDA1040A-Q1 in STB state.

In operating CAN bus, CANH level was 2.5V, but in STB state, its level was 2.3V.

Is this correct operation?

If it is correct operation, what is the cause of the voltage drop in STB state?

The condition is below;

Nodes: 5, 4 nodes: reccesive (operate: this IC only)

Best regards,

Atsushi Yamauchi

  • Yamauchi-san,

    In normal operation, the device biases its recessive level to Vcc/2 (2.5 V). In standby mode, the CAN driver is shut off and the bus pins present a high resistance to ground. When you measure 2.3 V in this state, is any other CAN device present on the bus? If so, you may be observing the recessive-level biasing from the other device.

    You can read more about the standby behavior of the device in Section 8.3.1.1 of the current datasheet.

    Regards,
    Max
  • Dear Max-san,

    Thank you for your reply.

    I almost understood the operation. 

    I would like to confirm the voltage drop.

    The cause of the voltage drop (about 0.2V) is the degradation of the impedance at bus pin by four other CAN

    device (recessive mode). Is this correct?

    Best regards,

    Atsushi Yamauchi

  • Yamauchi-san,

    There are a couple of different possibilities. One is that 2.3 V is just the recessive level of the other transceivers on the bus, so that is what is measured when HVDA1040A is inactive. Another possibility is that the other transceivers have a slightly higher recessive level (like 2.5 V), but that the loading caused by the inactive HVDA1040A is causing the voltage to be pulled a little lower. (This could happen since the recessive level is weakly biased by a CAN transceiver, so even high-valued pull-up or pull-down resistances can affect the voltage.)

    Max
  • Dear Max-san,

    Thank you for your reply.
    The customer checked it. The cause was the loading pull down resister.

    I have questions.

    1. Is the high resistance for pull down (STB) "Rin"?
    If it is not correct, how much is the value of the high resistance ?
    2. Does Io(stb) impact for this voltage (about 2.3V)?

    Best regards,
    Atsushi Yamauchi
  • Yamauchi-san,

    Yes, the receiver input current is the reason that the recessive level is lower than normal when the device is in standby mode. CAN drivers only weakly drive the recessive output level (i.e., the output impedance is very high), so even small currents can pull the voltage down. The spec item you mentioned, Io(stb), refers specifically to the SPLIT pin leakage current, though. The bigger impact comes from the CANH and CANL pins, which are biased differently in standby mode versus normal mode. You can see Figures 16 and 17 in the datasheet to better understand this.

    For your first question, I am having trouble understanding what you mean. Could you please try to clarify? Sorry for my confusion.

    Regards,
    Max
  • Dear Max-san,

    Thank you for your reply.

    I'm sorry for the question No.1.

    I changed the question.

    Is "Rin" specified in the datasheet the high resistance for weak pull down in STB mode?

    Or is it other value? 

    I would like to confirm whether "Rin" impacts to the voltage drop or not.

    Best regards,

    Atsushi Yamauchi 

  • Yamauchi-san,

    Thanks for the clarification. Yes, the "Rin" resistance is the pull-down value in STB mode.

    Regards,
    Max