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TCAN4550: OSC voltage is lower than datesheet

Part Number: TCAN4550

Dears:

Customer use external 40M Hz OSC, the same circuit with TCAN4550EVM, they found OSC1 voltage is lover than 0.85*Vio=0.85*3.3V=2.805V, It's just around 0.85V showing in below waveform, they also found OSC1 voltage is the same with EVM. Could you please help check what happened ? Thanks.

  • Hi Haiwen,

    The TCAN4550-Q1 supports both a Crystal and a Single-Ended Clock options.  The 0.85*VIO is the specification for the single-ended clock voltage, not the crystal.

    When the device is used in single-ended clock mode (OSC2 pin is grounded), the OSC1 pin is used as an "Input" for the single-ended clock signal that must have a high voltage level of at least 0.85V * VIO, or 2.805V as you have calculated.

    When the device is used in crystal mode, the OSC1 pin is used as an "output" pin to source current to the crystal to start ans sustain oscillations.  The OSC2 pin is then used as the "input" for the crystal clock signal.  The crystal amplitude should have a peak-to-peak amplitude of 1V or less which is what you are measuring.

    I would also recommend looking at the TCAN455x Clock Optimization and Design Guidelines application note (Link) for more information about the configuring this clock circuit.

    Regards,

    Jonathan

  • Jonathan:

    Thanks for your sharing. now customer found another abnormal waveform showing in below

    There is abnormal drop in the beginning with 120 ohm loading. Could you please help check if it's ok ? Thanks. 

  • Hi Haiwen,

    This is a normal observation and there are two possible factors. 

    The first is from common mode ringing due to a slight impedance mismatch between the CAN bus and the termination.  This is normal and you can see it affects the CANH and CANL signals the same.  By this I mean that the voltage drop is in the same direction which does not affect the "Differential" voltage between the CANH and CANL signals which is all that matters in CAN. 

    The second factor is a voltage drop from a sudden change in the current needed to change from an Idle or recessive state to an active state.  This may indicate some additional decoupling capacitance may be needed.

    This is a common observation and can change with the physical properties of the CAN bus, data rate, bus loading (active vs. idle time), etc.  The important factor is that the differential voltage between CANH and CANL is still OK.

    Regards,

    Jonathan