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C6678 PTV15 Pin

Mike H Meyer
Expert 1560 points

All of the documentation available recommends a 45.3 ohm resistor on the PTV15 pin.  I'm working on a design that already utilizes a 47 ohm resistor so it would be beneficial from a sourcing perspective to try to re-use parts.  The eval module has a note showing:

DDR3 Drive Strength:

70 ohm: 6mA

60 ohm: 7mA

50 ohm: 8mA

45 ohm: 9mA

40 ohm: 10mA

36 ohm: 11mA

That's the only reference I can find to adjusting the value beyond the 45.3 ohms documented elsewhere.  Any idea how a 47 ohm resistor would alter the driver?

  • 0
    TI__Mastermind 42545 points

    The external resistor attached to the PTV15 pin is provided to help match the output impedance of the DDR drivers.  The 45.3ohm resistor is recommend because it most closely matches the output driver impedance and was used in all of the characterization of the DDR3 interface.  If you choose to use a value other then the recommended 45.3 ohms you will see different behavior on the DDR3 outputs but we can't quantify the difference because we only characterized with the recommended level and because the IBIS model only supports the discrete values that you've listed above.  While that small a change may not make a difference we always recommend the value used during the testing of the DSP for best results.