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TMS320F2800157-Q1: Crystal oscillator waveform issue

Tonyx.wu
Genius 3075 points
Part Number: TMS320F2800157-Q1

We recently used F2800157QPMR master controller of TI with CSTNE20M0VH3C000R0 ceramic crystal oscillator of Murata. During the crystal oscillator waveform test, we found that the crystal oscillator waveform amplitude of X1 and X2 channels was quite different, and the crystal oscillator amplitude of X1 was close to the amplitude boundary of the data sheet. Please help us analyze this problem.

The schematic and layout are as follows:  CH1：POWER    CH2：X1  CH3： X2

The waveform is as follows:

R501  68R

R501 Change to 0R

R501  Change to 10R

R501  Change to 30R

R501  Change to 51R

R501  Change to 82R

R501  Change to 100R

R501  Change to 120R

Exchange CH2 and CH3 measurement results

Currently, this phenomenon has been found in many PCBAs of two products, and we have done probe cross verification, confirming that it is not caused by probe difference. Please help analyze the cause of the fault, and please contact us in time if you need to provide other test data.

  • 0
    TI__Mastermind 33295 points

    Hi Tony,

    Couple of questions regarding this issue:

     - You indicated that this phenomenon is found in many PCBs of two products.  Does this mean that on some PCBs, the signal levels on X1 and X2 are same?

     - In the scope pictures, I see a different frequency readings but not sure what they are.  I don't see any measurements indicating 20MHz on X1 and X2 pins.  Can you confirm that you can actually observe oscillations at 20MHz?

     - I looked at the ceramic oscillator datasheet available on the web but the parameters I see are only R1 which I do not think is ESR.  It gives the value of CL1/CL2 as 15pF but did not indicate if this is total load capacitance or per pin capacitance.  I also do not see the value of C0 (equivalent shunt capacitance).  These parameters are important to ensure proper oscillation and startup are achieved.  Refer to datasheet 6.13.3.4.7.2 Crystal Equivalent Series Resistance (ESR) Requirements tables and plots for guidance .

     - Even with Rd=0R, amplitudes of X1 and X2 are not the same which is already a concern.  X1 and X2 should have the same amplitude which is close to the 3.3V level.

     - Why was Rd included in the circuit?  Is there a concern that the oscillator circuit drive level is overdriving the external resonator?  Please see section 6.13.3.4.3.4 DL – Drive Level of the datasheet for more guidance.

    Regards,

    Joseph