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USB Dongle and Keyfob reference designs

Daniel A.
Intellectual 360 points
Other Parts Discussed in Thread: CC2540

Hi,

I'm trying to understand the main differences from the USB dongle and keyfob designs.

The USB dongle looks much simpler than the keyfob, starting by the fact that the dongle doesn't have the 32KHz xtal and the antenna circuit is much simpler. Would this circuit be suitable for a peripheral device battery powered? What are the main differences between the two approaches?

Many thanks

Daniel

  • 0
    TI__Genius 12545 points

    The 32kHz crystal is required if the sleep mode in the device is to be used. In the USB dongle, the CC2540 does not go to sleep as it is connected to USB power and not battery power.

    The antenna is the same used in the USB dongle and the key fob. The keyfob uses a discrete component balun that is more easily tunable, while the USB dongle uses a chip balun which does the job of the discrete balun in very little area. The difference, the cost. The chip balun is a cost addition, while the passives not so much. Also, if you want to save on time, board area and cost is not an issue, go for the chip balun.

    The CC2540 can work in a battery operated application. The RF balun or antenna can be your choice.

  • 0 in reply to Chatto
    Intellectual 360 points

    Many thanks Chatto.

    I've got my hands on the MiniDk dev kit.

    However now I'm playing around with the PAN1720 by Panasonic which,  apparently, doesn't include the 32KHz xtal and I'm still able to get it into sleep mode in a battery operated application. How is it explained? I had to change the configuration to let the project know that there was no external 32KHz xtal installed but, apart from that, everything works smoothly. Does this mean that the xtal is not necessary for the sleep timer and it would use an internal (less precise?) one?

    Regards,

    Daniel

  • 0 in reply to Daniel A.
    Expert 1290 points

    have a look at http://www.panasonic.com/industrial/includes/pdf/PAN1720_Datasheet_v0.1.pdf

    page 4 and page 8 clearly state the existence of two crystals: 26Mhz and 32kHz...

  • 0 in reply to Andre S
    Intellectual 360 points

    Indeed, you're right...

    I was totally wrong. What actually happened is that I picked up the USB related thing from the USB dongle sources and I had to specify that I had a 32KHz oscillator, otherwise the connection was dropped off automatically after the supervisor timeout. Sorry about that.

    Thanks!

    Daniel