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TPS3808G01DBVEVM: RESET WAVE FORM REVIEW

Mohammed Azlum
Intellectual 835 points
Part Number: TPS3808G01DBVEVM

HI TI Community, 

I'm using TPS3808G01DBVR for generating reset for my board. I just checked the reset waveform from Supervisory Circuit. I'm getting a short pulse before the reset release. 

Please find the schematic and waveform attached below. May I know the reason ?

  • 0
    TI__Genius 12630 points

    Hi Mohammed,

    Thank you for using our device. I have not seen this kind of behavior before with this device. I also noticed that you have /MR pin connected to the VCC_3V3 supply. This pin has an internal 90kohm pull-up resistor inside that pulls up to Vcc so I don't think R208 is necessary? I would like to get some more information to try to figure out what is going on.

    1. Does this problem occur every time? Is this occurring only at start up or each reset during normal operation?

    2. Have you tried with a new device? Are you using the EVM for your testing?

    3. Is anything connected to the /RESET pin not shown in the schematic above?

    4. Could you repeat the test and include the voltage waveform at VCC_1V03 and VCC_3V3? I just want to make sure nothing strange is happening on these pins.

    Thank you and I look forward to your reply.

    -Michael

  • 0 in reply to Michael_DeSando
    Intellectual 835 points

    Hello Micheal,

    Please find below inline reply.

    1. This problem occurs only during the startup time. Manuel reset this problem never observed.

    2. This issue existing all the boards and tried with new devices. Almost 5 pcs of boards tested and issue is existing in all 5pcs of boards.

    3. RESET connected to the reset pin of the processor. I didn't connect any other chips/load.

    4. 1.03V, 1.8V and 3.3V with respect to RESET probed and attached below.

     

  • 0 in reply to Mohammed Azlum
    TI__Genius 12630 points

    Hi Mohammed,

    I believe what is happening is that the RESET pin is undefined as long as VDD is below 0.8V.

    So this means that before VDD reaches 0.8V, if RESET is connected to a separate supply through a pull-up resistor that is already high at 1.8V, the RESET may be pulled-up briefly until VDD reaches the 0.8V which then causes the RESET to become active so it drops back low since VDD is below VIT. If you use the same supply for VDD and RESET, does the pulse on RESET drop to 0.8V or disappear completely?

    -Michael

  • 0 in reply to Michael_DeSando
    Intellectual 835 points

    Hello Michael,

    Yes, you are correct. Problem solved after connecting VDD to 1.8V. 

    Reason :- 1.8V is present before the VDD 3.3. Hence reset signal initially gone to high. Once the VDD (3.3V) present, the reset is coming to low and wait for sensing voltage. Once the sense voltage is present, reset release to high. 

    Regards,

    Azlum