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Keyfob power management

Andy Warne
Intellectual 615 points
Other Parts Discussed in Thread: CC2540, CC2530

I have completed a design of a keyfob using the reference design CC2540DK-MINI Keyfob. 

Although it is working I need to look at power management in this application. There seems to be very little documentation of how this is implemented in the reference design, other than POWER_SAVING is enabled and osal_pwrmgr_device( PWRMGR_BATTERY );

When measuring the current, it is around 0.4 mA until the button is pressed to start advertising. Thats fine. But from that point onwards it never drops below 1.9 mA.

It seems the CPU is going into a PM state (which one?) but I am assuming the radio is never turning off. Is it possible to turn it off? If so, what would be needed to turn it back on when the button is pressed?

The datasheet specifies power management in the microamp range, but I cant see how to achieve this.

  • 0
    Guru 22270 points

    Hello Andy,

    For some of your power consumption issues, I suggest you take a look at the following two post.  They may clear up some issues.

    http://e2e.ti.com/support/low_power_rf/f/538/t/224115.aspx

    http://e2e.ti.com/support/low_power_rf/f/155/p/122947/441487.aspx#441487

    There is also the Design Note DN097 that covers current consumption for the CC254x.

    The power mode that is selected depends on if the number of EVENTS that are pending within a certain time period.  It is outlined in one of the guides, I just can't remember which one.  For setting which power management state, you can refer to the Power Management API found in the OSAL Application Interface swra194.pdf.

    I am not sure about you question of the radio never being turned off.  If the radio is not in Tx mode or Rx mode, I would consider this to be off.  The radio also requires the 32Mhz OSC to be running, so the PM mode that shuts off the crystal should turn the radio off as well.  Access to the radio on the CC2540 is limited. In chapter 24 of the CC254x User Guide swru191D.pdf, you can view the registers to see some the states the radio is in.

    Thanks,

  • 0 in reply to Greenja
    Intellectual 615 points

    Thanks for this. One factor in this is I was measuring power consumption with the debugger still connected. This does not give valid readings!

    Now, everything is as expected with one exception. When the keyfob is first powered on, but not advertising, the consumption is around 0.3 uA which is great. After advertising is started and when connected it of course goes up. But when the button is pressed again I have added code which disconnects and stops advertising. The power does not drop back to 0.3uA. In fact it goes to 8 uA.

    Still investigating this.

  • 0 in reply to Andy Warne
    Intellectual 290 points

    All,

    Can anyone tell me why all the power pins require their own capacitor off of VCC?  They are all in parallel and the net effect sums all the capacitors. Right? Seems like one capacitor equal to the sum of them should be attached to VCC and then all power pins wired together.

    Thanks,

    Tony 

  • 0 in reply to Tony Megna
    Guru 22270 points

    Hello Tony,

    Your question does not seem related to this thread.  It should have been asked in a new post.

    The VCC pins are separated into DVDD, AVDD and the DVDD_USB for the CC2530.  The use of individdual caps on each pin allows for better decoupling.  Also, in the case of the analog inputs, I believe most, but not all of them are tied together.

    You can search the Internet for the benefits of using small decoupling caps and PCB layouts for more detailed explanations.

    Thanks,