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SN65HVD232Q-Q1: SN65HVD232Q-Q1 Implicit Level Issue

jeno.zhang
Genius 3395 points

Part Number: SN65HVD232Q-Q1

Hello,

The customer uses the SN65HVD232Q-Q1 product without an external 120 ohm resistor, and the hidden voltage level of CAN-H and CAN-L is only 1.3V, which is much lower than the 2.3V in the manual. The impedance test is 38K ohms. Excuse me, is this a normal phenomenon? Or what could be the cause?

Thanks.

  • 0
    TI__Guru 73582 points

    Hi Jeno,

    The measured 38 kohms between CANH and CANL should be consistent with a bus that does not have external termination resistors. Furthermore, note that the specified 2.3 V is a typical value and 1.3 V may be expected dependent on the VCC levels (if < 3 V for example) or other parameters based on the nodes or components connected to the bus.

    Hence, I would recommend also double checking the transceiver operating mode (RS pin) to ensure normal mode / high-speed mode, supply voltage levels / stability, presence of other connected nodes and disconnect to reverify, thanks.

    Best Regards,

    Michael.

  • 0 in reply to Michael Ikwuyum
    Genius 3395 points

    Hello Michael,

    Before the CAN network, there were 8 nodes, but only 2 nodes were powered on. The impedance of CANH and CANL to ground was 18K ohms, with an implicit level of 0.88V. After removing 4 nodes, CAN communication returned to normal, with an impedance of 38K ohms, which is the impedance of CANH and CANL to ground, not between CANH and CANL. The implicit level is 1.3V. The explicit level test is normal. At present, the node cannot be disconnected. Will not powering on have an impact on the hidden voltage level? Does not connecting a 120 ohm matching resistor affect the stealth level?


    The following is a schematic diagram, in which the diode has been replaced with a 0 ohm resistor.

    Thanks.

  • 0 in reply to jeno.zhang
    TI__Guru 73582 points

    Hi Jeno,

    This seem to strongly suggest that the recessive bus bias is being loaded down by the unpowered transceiver nodes, rather than a problem with the SN65HVD232-Q1 itself. I.e., the resistance more than doubled, meaning the removed nodes were contributing significant loading and the recessive voltage increased which further suggests the loading is reduced.

    The 120 ohm may affect the bus voltage slightly, but it is probably not the main cause (termination should influence signal integrity, reflections, etc). I would recommend to power all the nodes to verify if the voltage returns closer to ~2 V to narrow down any unpowered-node leakage. May further measure the resistance to compare all nodes powered OFF. 2 nodes powered ON and all nodes powered ON.

    Hence, I suspect that the most probable root cause is due to the four removed nodes that were unpowered and created additional loading on CANH / CANL, thereby reducing the recessive bias voltage. However, dominant level being normal should indicate the CAN driver stage is healthy, thanks.

    Best Regards,

    Michael.

  • 0 in reply to Michael Ikwuyum
    Genius 3395 points

    Hello Michael,

    1. Could you please help check if the waveform of the CAN bus is correct? I think there is no problem, but in idle state, if the voltage of CANH and CANL to GND exceeds about 1V, theoretically it should also be an invisible level of 2.3V?
    2. Will nodes that are not powered on affect the CAN bus and the hidden voltage? The data manual states that it has no impact.

    Thanks.

  • 0 in reply to jeno.zhang
    TI__Guru 73582 points

    Hi Jeno,

    1. Please note that 2.3 V is typical and VOD seems to be above the data sheet's min of 1.5 V. Hence, looks correct to me.

    2. Unpowered nodes may accumulate and shift the recessive common-mode voltage. However, may not affect the differential signaling as observed but will recommend proper termination at all times, regardless, thanks.

    Best Regards,

    Michael.