Clarification of differences between the variants of the EZ430-Chronos Devkit

I am planning to purchase the EZ430-Chronos Devkit soon, but I wanted to clarify one point from the wiki.  In the FAQs, it states:

"The 868-MHz and 915-MHz watch hardware are physically identical and the radio frequency is software selectable so it's possible to switch frequency if necessary. However, the RF Access Points for the two respective kits are slightly different. The 868-MHz Access Point has a 0Ω resistor next to the antenna and the 915-MHz version does not. The 433-MHz kit is different than the other versions in both software and hardware."

Am I understanding correctly that the 868MHz and 915MHz Access Points are not frequency-switchable like the watches themselves are?  If they are, does switching the frequency require additional hardware?

Thanks,

  - Jeffery MacEachern

  • I have not personally investigated the differences, but from the comment above I believe that the implication is that the two access point versions have the same hardware PCB and component design with the exception of the installation or absence of that one 'resistor'. A zero ohm resistor is commonly used as a jumper to enable or disable a certain portion of a circuit such that the configurations can easily be switched at production time by installing the resistor or not for a particular variant model of the unit. It would not be uncommon to have slightly different RF matching networks or something like that for one frequency versus another similar frequency where the tuning is similar but may be optimized by a slight adjustment like connecting an additional circuit with such a zero ohm resistor. You could investigate the differences yourself by comparing the BOM and the PCB trace gerber files from the hardware documentation / design materials which presumably are available for download. If the PCBs are identical and the BOMs are also except for the installation of that particular resistor, the units should be able to be switched from one configuration to the other by changing that resistor's installation as described. I assume that there might be some software parameter differences too to select the use of one frequency versus the other, but that should be straightforward to reconfigure based on the suggested firmware/settings for the two models of unit. If they wanted the units to be dynamically switched from one frequency to the other in software only, they wouldn't have made the PCB assembly different via that resistor, though, so it will take a little hardware modification to switch frequencies or so it seems. On the other hand if you're willing to compromise the span of frequencies actually used within each band or maybe accept reduction of the sensitivity / power output efficiency you could presumably find a way to reconfigure / retune it to work at a less than optimum efficiency / sensitivity / capability on both bands simultaneously if that is your goal. I suppose if the resistor is in the antenna circuit it is just slightly adjusting the tuning or matching of the antenna. If so it wouldn't be surprising if either unit could receive at least modestly well (if not fully optimally) on both frequencies with no adjustment, though it is possible that you'd lose some transmit efficiency or band optimization adjustment on the other frequency if you were to transmit without making the hardware change. In principle it is seemingly not so different than adjusting a TV antenna to get better reception on one channel versus another even though the same basic antenna and hardware will work for both given the little tuning / alignment adjustment.
  • In reply to C. Hayes:

    Thanks for the comprehensive reply.  I was thinking something along the same lines regarding the "resistor" (albeit not as specifically).  In this case, my motivations are much more simple: Digikey has no stock in the 915MHz model, and plenty of stock in the 868MHz model.  If it was easily switchable, I wouldn't bother waiting to order.  However, I don't really want to mess around with it; furthermore, according to the User's Manual, the USB dongle can only be reprogrammed with a (presumably) TI-specific device that costs as much as the devkit.  I think I'll just wait.

    Thanks again,

      - Jeffery MacEachern

  • In reply to Jeffery MacEachern:

    IMHO there is no reason why the AP should use an antenna circuit that requires a hardware change while the watch uses a different one that doesn't.

    Of course this is possible.
    Yet I think the resistor might be just a jumper that tells the firmware to configure the RF this or that way. While the software in the watch needs to be replaced for a different frequency.

    Only a look into the schematics will tell :)

  • In reply to Jens-Michael Gross:

    I can confirm that the APs from both kits are physically the same save for the suspected 0 ohm resistor.  I would have to assume the software on the AP is different as well, but do not know this for sure.  If reprogrammability of the AP is required then a CCdebugger device is needed and some creative soldering will have to take place to get the new code onto the AP.  The JTAG pins on the AP are brought out to some very small test point surface pads, but they are not in the footprint of any connector.  If development of a custom AP is desired,  the CC1111EMK would be recommended.

  • In reply to Jeffery MacEachern:

    On the Chronos CC1111AP, the 0ohm resistor essentially "extends" the antenna length, essentially switching it from 915 to 868MHz.See the picture below, where the blue lines are the traces, and the red line is the 0ohm resistor that further extends the long trace to the short trace. This 0ohm resistor is present in the 868MHz version of the AP.

     

    However, the CC1111 radio also requires different register configurations to operate at 868MHz // 915MHz, hence different firmwares are needed. If you have installed the Chronos SW package, you should notice 3 different firmware sets for the AP (433, 858, 915) located at C:\Program Files\Texas Instruments\eZ430-Chronos\Recovery\RF Access Point.

    Regards,

     

    Dung

     

    EDIT: 915MHz = no 0Ω resistor
             868MHz = 0Ω resistor populated

     

  • In reply to Dung Dang:

    Dung Dang
    the 0ohm resistor essentially "extends" the antenna, essentially switching it from 868 to 915MHz.[...]This 0ohm resistor is missing in the 868MHz version of the AP.


    Interesting. I always thought that the higher the frequency, the shorter the antenna. One wavelength is 34.6cm for 868MHz and 32.8cm for 915MHz

    But maybe this is extending the (estimated) 1/16 antenna at 868MHz to an 1/12 antenna at 915MHz, making it a little more sensitive.

    If this is the intention, and if it works, it will compensate the range-loss at higher frequencies, making the 915MHz version reach as far as the 868MHz version.

  • In reply to Jens-Michael Gross:

    Jens-Michael,

    sorry, typo, please see edited post. The extended antenna length is for the 868MHz, shorter one is for 915MHz. Nonetheless, the path-loss difference shouldn't be too significant when moving from 868MHz to 915MHz as these two bands are relatively close to each other.

    To answer the the other question: to avoid complication and if you don't want to purchase the additional CC Debugger, you could wait for the 915MHz version to become available.

    More information on the chip antennas (Johanson) of the two versions (from the datasheet, essentially replicated in the Chronos):

    Regards,

    Dung