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TCAN4550-Q1: Stopped Working: No crystal waveform

Part Number: TCAN4550-Q1

Hello,

We are facing a problem where TCAN was working fine, however after some time it stopped communicating.

During debugging it was found that there is no nominal voltage on CAN Line, further we tried to restart system however it didn't help.

Later on debugging we found that, we are unable to read registers of TCAN IC and there is no crystal output.

Later we just disconnected choke, which was connected on CAN lines of TCAN IC and found that TCAN is again working fine. We got that nominal voltage on CAN lines.

Can anyone please help me to understand what actually happened to crystal and TCAN IC with respect to choke. Also on connecting same choke again it is working fine as expected.

Rergards 

Tirthraj Solanki

  • Hello Tirthraj,

    It sounds like the crystal oscillator circuit needs to be optimized to ensure stable performance.  The crystal (clock) drives the digital core of the device and provides the time quanta needed by the CAN FD controller to generate and receive CAN messages.  If this clock is disrupted, then both the SPI and CAN communication will have errors because the digital core does not have a valid clock.

    The TCAN4550-Q1 supports both a crystal and a single-ended clock.  A comparator is used on the OSC2 pin to check for a "grounded" pin by monitoring for a voltage on this pin below approximately 100mV.  If this low voltage is detected, then the device will disable the crystal oscillator amplifier from sourcing current out the OSC1 pin to the crystal and expect a single-ended clock to be supplied as an input on the OSC1 pin. 

    When the crystal circuit is not optimized with the right loading, too much current can flow through the crystal increasing the power consumption or Drive Level of the crystal.  A higher Drive Level will create larger mechanical vibrations in the device which will produce a larger peak-to-peak voltage in the oscillation waveform seen by the OSC2 pin.  If the amplitude becomes to large, the lowest peak voltage can cross this single-ended comparator detection threshold and cause the device to switch to single-ended mode and stop providing current to the crystal.  This will cause the oscillations amplitude to naturally decay from parasitic losses and at some point the OSC2 pin voltage will rise above the threshold and the device will switch back to crystal mode. 

    Adding a series dampening resistor between the OSC1 pin and the crystal will reduce the amount current flowing through the crystal and reduce the drive level.  If that is not available as an option, increasing the value of the load capacitors will lower the total resistive load and lower the drive level as well.  Either or both of these options will stabilize the clock.

    Additional information can found in our TCAN455x Clock Optimization and Design Guidelines application note and in previous forum posts such as this one.

    I'm not sure I understand your choke issue, but I suspect it is coincidence and stems from the clock issue.  I suspect that after you optimize and stabilize the clock, you won't have a CAN issue.

    Regards,

    Jonathan

  • I will once go through this and will give an update after some testing.

  • Hi Tirthraj,

    OK.  Let me know if you have any additional questions.

    Regards,

    Jonathan

  • Hi Jonathan

    As i told you we are not facing this issue very frequent, so this is still under testing. Will give you an update after some testing.

    Regards 

    Tirthraj

  • Hi Tirthraj,

    Thanks for the update.

    Regards,

    Jonathan