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[OMAPL138] Resistance Specification for Internal Pull-Up and Pull-down Resistors

Michael Questo
Expert 6000 points
Other Parts Discussed in Thread: OMAPL138

OMAPL138 Champs,

I've looked through both the datasheet and the TRM but have been unable to find the internal reistance values of the Pull up and Pull down resistors that are  determined by the PUPD_ENA and PUPD_SEL registers.  These registers have default states which are listed in 11.5.22 Pullup/Pulldown Enable Register and 11.5.23 Pullup/Pulldown Select Register in the TRM

There is also information on how oppose and compliment the resistors.

Opposing - A 1-kohm resistor can be used to oppose the IPU/IPD while meeting the above criteria.
Complimenting - A 20-kohm resistor can be used to compliment the IPU/IPD on the boot and configuration pins while meeting the above criteria.
 
However, I can't find the actual value of the resistance?  
 
Thanks,
  • 0
    TI__Guru* 82795 points

    Hi Michael

    You will find the following posts useful

    http://e2e.ti.com/support/dsp/omap_applications_processors/f/42/t/113557.aspx

    https://e2e.ti.com/support/dsp/omap_applications_processors/int_omapl1x_dsps/f/116/p/13220/149022.aspx  (internal post)

    Regards

    Mukul

  • 0
    Expert 1070 points

    Hi Michael,

    This post might be useful also:

    http://e2e.ti.com/support/dsp/omap_applications_processors/f/42/p/124259/445855.aspx

    Regards,

       Denis 

     

  • 0 in reply to fdm
    TI__Expert 6000 points

    Thanks to everyone who responded, your links were useful.  Upon reviewing them i've come to the conclusion that the equivalent resistance of the IPU/IPD is aproximately 12.2kohms max.  Please correct me if i'm wrong.

    Thanks!

  • 0 in reply to Michael Questo
    Expert 1070 points

    My rough estimations for the equivalent resistance are (10...50)kOhm for 3.3V and (5...28)kOhm for 1.8V

      Denis 

  • 0 in reply to fdm
    TI__Expert 6000 points
    Bringing all the discussion together, here are the calculations.
     
     
    Section "5.3 Recommended Operating Conditions" in Datasheet
    Recommended I/O Operating Range (DVDD)
    3.3V I/O - 3.15V to 3.45V
    1.8V I/O - 1.71V to 1.89V
     
    Section "5.5 Electrical Characteristics Over Recommended Ranges of Supply Voltage and Operating JunctionTemperature"
    LVCMOS Pull-Up Current
    Min 70 uA
    Max 310 uA
    LVCMOS Pull-Down Current
    Min -75 uA
    Max -270 uA
     
    Resistance Calulations: Pull Down
    Resistance Pull Down 3.3V I/O:
    Max : 3.15V / 75 uA = 42k
    Min : 3.45V / 270 uA = 12.8k
    Typ : 3.30V / 172.5 uA = 19.1k

    Resistance Pull Down 1.8V I/O:
    Max : 1.71V / 75 uA = 22.8k
    Min : 1.89V / 270 uA = 7k
    Typ : 1.80V / 172.5 uA = 10.4k

    Resistance Calulations: Pull Up
    Resistance Pull Up 3.3V I/O:
    Max : 3.15V / 70 uA = 45k
    Min : 3.45V / 310 uA = 11.1k
    Typ : 3.30V / 190 uA = 17.4k

    Resistance Pull Up 1.8V I/O:
    Max : 1.71V / 70 uA = 24.4k
    Min : 1.89V / 310 uA = 6.1k
    Typ : 1.80V / 190 uA = 9.5k

    Similar Calculations for the OMAPL137:
  • 0 in reply to Michael Questo
    Expert 1070 points

    Hi Mike,

    Taking into account that these IPU/IPD structures are implemented as current source/sink rather than constant resistance (see that post) I suppose that UMAX/IMIN and UMIN/IMAX pairs should formally be used instead.

    From the practical point of view, it is more useful to calculate maximum values of external RPU/RPD to oppose IPD/IPU (as it suggested e.g. in OMAP-L138 Errata Advisory 2.3.23) rather than internal resistances.

    In addition we should account for tolerance of DVDD and resistors and also add some noise margin.

      -Denis