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CC2540 - What to do with USB D+ and D- Lines for low power?

Seth Berggren
Intellectual 910 points
Other Parts Discussed in Thread: CC2540

Hi, I have a CC2540-based device that uses the USB peripheral. The device enters PM2 when it gets disconnected from power and is now running off of 3V coin cell battery. When this happens, all of the ports are set to input with pull-up resistor so that power is conserved.

What should I do with the D+ and D- USB pins? They shouldn't be left floating because then they will cause dynamic power drain. I can't control them through firmware either. Would it mess up the USB communication if I added external pull-down resistors (~10KOHM)?

The CC2540EM does not have USB, so it just ties them off to ground. The CC2540 USB Dongle leaves them floating because the device only operates off of USB power and has no need for low power considerations.

Any ideas?

Thanks,

Seth Berggren

  • 0
    Intellectual 910 points

    To add further to this inquiry...

    How does it affect the overall power consumption of the CC2540 if DVDD_USB is tied to AVDD/DVDD? Would it save energy if DVDD_USB was tied to ground when USB is not plugged in?

    Thanks,

    Seth

  • 0
    Guru 22270 points

    Hello Seth,

    Where and how did you determine that the D- and D+ will cause a dynamic power drain when left to float?

    If you clear the USBCTRL.PLL.EN bit, the PLL clock to the USB will hault.  Clearing the USBCTRL.USB.EN will disable the USB.  I have not seen any electrical specs for the USB on the CC2540 but generally there is a pull down resistor of at least 15K on the D- and D+ lines.  If the 2540EM ties them to ground then they may not have pull downs.

    The USB Dongle doesn't have a 32.768kHz crystal and that is the reason it can only go into PM1.

    If you haven't already tried this and you have both devices, then I would suggest you try this.

    1.  Measure the CC2540EM and USB Dongle in PM1 and that should allow you to determine the effects of having floating or tied to ground lines.

    2.  Measure the current when the PLL is enabled and disabled

    3.  Measure the current when the USB is enabled and reaet

    4.  Measure the current when both PLL and USB are enabled an disabled.

    One last thing.  You can use a Port pin to enable the HS USB instead of tying it to 3.3V.  Set it to high impedance when you go into sleep mode.

    Thanks,

  • 0 in reply to Greenja
    Intellectual 910 points

    Where and how did you determine that the D- and D+ will cause a dynamic power drain when left to float?

    Actually I am not sure what the HW of these pins looks like. I am being cautious because there is not much documentation about the electrical characteristics of D+ and D-

    Thank You for the suggestions, I will have to do some testing to find out more information about these pins.

    -Seth