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F2837x Control Card Reset Circuit

Will D
Prodigy 120 points
Other Parts Discussed in Thread: CONTROLSUITE

Please can someone advise on the reset circuit used on the TI F2837xD controlcard, as per controlsuite.

 

Please can someone explain how it works? And why it is so complicated? From what I can work out:

1) U4 SENSE_OUT will enable the micro when VDD_3V3 > 3.204V, and ENABLE is high. 

2) U3 SENSE_OUT will be enabled as soon as VDD_3V3 > 1.7V, as this is the minimum supply voltage for U3 to work and it meets the requirement for SENSE, ie > 0.5V and the requirement for EN >1.4V. 

My confusion is:

As VDD_3V3 is shorted to U3_SENSE and U3_EN, what is the point of R20, R23 and C5, as they can't pull SENSE/ENABLE high or low, since these pins are short to VDD_3V3. If U3 is monitoring VDD_3V3, when what is U4 monitoring? 

Also, why is one device push-pull and the other open drain, why not make them the same?

Finally is there a reason for R22 being 510K, as opposed to making it the same as R23 - 560K, component reuse?

Many thanks in advance for any help! 

  • 0
    TI__Guru 57255 points

    Will D,

    Part of the reason for the confusion is that there is a bug:
    the connection dot to the left of U3.3 should not be there.  The SENSE node should connect to the 1V2 voltage divider whereas EN should be tied to 3.3V.

    The key thing is that U4 is open-drain because XRSn is open-drain.  U3 could be either open-drain or push-pull.  If you use the open-drain configuration make sure to add a pull up resistor.

    R22 could be the same as R23.  R23 was chosen with 1% tolerance so that the voltage divider could be as accurate as possible.  R22's requirements are less stringent.

    Hopefully this helps.


    Thank you,
    Brett 

  • 0 in reply to Brett Larimore
    Prodigy 120 points

    Brett, 

    Thanks for the information above. Much appreciated.

    Will