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AM5718: Strange PORz behavior

Fredda
Intellectual 625 points

Part Number: AM5718
Other Parts Discussed in Thread: SN74LVC1G08

Hi,

I've got a custom AM5718 board, which is based on the AM571x Industrial Development Kit (IDK). The AM57XX_PORZn net connects to PORZ on the MPU and is expected to go to 3.3 V when the PMIC releases the PMIC_RESET_OUTn reset. However, when measuring the AM57XX_PORZn voltage I get a reading of approximately 1.5 V.

The PMIC_RESET_OUTn also measures 1.5 V, but when desoldering the SN74LVC1G08 AND gate, PMIC_RESET_OUTn measures 3.3 V as it should. I have desoldered all components on the AM57XX_PORZn net so that nothing drives it and it still measures 1.5 V. Note that AM57XX_PORZn is connected to PORZ, RTC_PORZ and RTC_ISO on the MPU. It seems like the MPU in some strange way drives the AM57XX_PORZn net to 1.5 V. Very confusing. Any ideas?

Regards

/F

  • 0
    TI__Guru**** 393215 points
    What is the VDDSHV3 voltage on your board? The PORz I/O cell is powered from this rail. If VDDSHV3 is 1.8V the PORz signal should not exceed 1.8V.
  • 0 in reply to Biser Gatchev-XID
    Intellectual 625 points

    VDDSHV3 is 3.3 V.

    The PORZ input as well as the RTC_PORZ and RTC_ISO inputs use IHHV1833 buffer (table 5-14 in SPRS957F) and according to table 4-2 they have pullups and puldowns. How strong are these pullups and puldowns? Are they causing the 1.5 V I'm seeing?

    I have also tried to connect PORZ directly to PMIC_RESET_OUTn (bypassing the SN74LVC1G08 AND gate in the schematic). PMIC_RESET_OUTn has a 10k pullup to 3.3 V and thus should drive PORZ to 3.3 V, but I'm still measuring 1.5 V. Is the pull to weak to drive the inputs to 3.3 V?

    In any case, the input high-level threshold of PORZ is at 1.2 V according to table 5-14 and thus the reset should be deasserted at 1.5 V. Right?

  • 0 in reply to Fredda
    TI__Guru**** 393215 points
    How about VDDSHV5 voltage? RTC_PORZ and RTC_ISO are powered from this rail. I don't suspect this is caused by internal resistors, they are too weak to pull the signal this much.
  • 0 in reply to Biser Gatchev-XID
    Intellectual 625 points

    Oops. I see now that I have an oscillating PMIC_RESET_OUT_N. The 1.5 V measurement made with just a voltmeter was just the mean value of a square wave. The PMIC_RESET_OUT_N is high 3.12 ms and low 3.6 ms. So, I need to see what is causing the PMIC_RESET_OUT_N to oscillate. I'll get back...

  • 0 in reply to Fredda
    Intellectual 625 points
    By some reason we had a 10k pull-up on RSTOUTN from the MPU. After removing the resistor the board works fine.