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debugging reset function on f28m36

N.AB
Intellectual 390 points

Hi,

we have done a custom electronic design around f28m36p63 uC.

We have debugged all the functionnalities of the uC, the only think not working yet is the reset of the uC from external button.

our reset circuit is joined.

both ars and xrs are commony actuated from the reset signal.

we have verified that the button push drops down the voltave on the accesible test point od the reset signal. this signal is normally liked directly to xrs an ars pins.

Unfortunately as the uC have a BGA package it is not possible to put a probe on the pin it self.

We wonder what possible steps would allow us to find the problem in order to solve it?

Best Regards

  • 0
    Intellectual 390 points
    Hello,
    I would give some extra informations:
    on the input of the R94 which is linked to pin7 of the ADM708 we measure 3.3V when the reset button is off and 0V when the buton is on.
    on the output of the R94 resistor which is directly connected to XRS and ARS of the uC, we measure 3.3V when the reset button is off and 1.4V when the buton is puched down.
    Actually, we expected to have 0V when the buton is puched down.
    Is this observation the expected behavior of the F28M36 uC?
    Is there any work around to get the reset work correctly?
    Thank you
  • 0 in reply to N.AB
    TI__Guru 62975 points

    Your observation makes sense.

    Based on the 1.4V measured voltage and 4.7k series resistor, we can estimate that 1.4V / 4.7k = 298uA of current is flowing through R94.

    According to the datasheet, the combined pull-up currents for XRS + ARS can range from (50uA + 100uA = 150uA) to (230uA + 400uA = 630uA). The estimated 298uA of current is within the expected range of 150uA to 630uA.

    The value of R94 should be changed so that under worst case conditions, VIL is satisfied at the inputs of XRS and ARS when ADM708 pin 7 is outputting VOL.

    -Tommy

  • 0 in reply to tlee
    Intellectual 390 points

    Thank you for the reply.

    I changed the R94 by a 3k3 one. o XRS we emasure 1V when the reset buton is down.

    The reset works well in this case.

  • 0 in reply to N.AB
    TI__Guru 62975 points

    You may want to adjust R94 some more so that you can accommodate the worst case conditions when XRS + ARS are sourcing 630uA and ADM708 pin 7 is outputting a VOL(max) voltage that is greater than 0V.