I spent some time reading inHM-TRP Series 100mW Transceiver data sheet as well as SiK source code I wanted to know that if the 433MHz and 913 MHz is something fixed in the chip, that is why modules are sold either 433 or 913 or it is something you can change EASILY :)

Luckly I found that frequency is something not fixed, however in the source code of SiK, it is stored in board_frequency as input preset parameter when building the code, and it is passed to the firmware part in main.c where it is used for initialization.

Well I decided to make minor change,  I added a new variable -the 16th variable- called Main_Freq 

it can be given the following values

FREQ_433 = 0x43,

FREQ_470 = 0x47,    

FREQ_868 = 0x86,

FREQ_915 = 0x91,

These values are defined in board_info.h file 

Typical steps to change frequency are:

  1. +++ to enter AT commands mode.
  2. ATS16=67 to change to 433 MHz
  3. AT&W to save settings
  4. ATZ to reboot (Mandatory)

Another interesting thing is that the chip originally support frequencies as low as 240 MHz, however the firmware does support the above four frequencies only.

Now if you need to change frequency using only Firmware without the need to update the Bootloader you can use my changes...I hope you find this useful. 

Please note that you should use proper antenna for each frequency


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  • Hello to All. I have question for You Guys. I need to change freq from 915 to 433 in ma oryginal SIK  can i do it without special dongles ? I have never do that all i have  is my pixhawk with which i see i can upload firmware to my radios and maybe some usb rs232 converters. Can i do it with that on some windows machine ??

  • Almost worked for me!! Have a connection problem no one seems to be able to solve!!

  • Anybody got any updates to this post .. i wanted to take a RFM23B  433 to 466?

    • Based on replies the antenna circuitry will affect the range, but I guess for two adjacent frequencies it might work ok.

      you need to try and remember all firmware could be reversed back. 

  • beside all the discussion about circuitry and frequencies - I flashed my (use in Europe is a bad Idea) 915Mhz Kits - my mistake in ordering - to a legal 868 Version.

    next week I'll do some range test.

    @ MHefny you made my day!

  • Okay so after reading this thread,

    is my understanding correct, the cards can do either 433 or 915 BUT the filters are different...

    is that because they are physically different?


    because the analogue output is simply giving different voltage/current to the filter?!

    • as Marcin Krawczyk  said ... the chip is the same ... the output LC is different, so u still can change frequency using chip , but will not be able to practically use it for any useful range.

      • Ahh ok :) i really enjoy fiddling with these things! 

    • Because they are physically different.

      433 or 868 or 915MHz

  • Developer

    I'm afraid this doesn't work in any reasonable way (and yes, I did try it a long time ago).

    Each board has a different analog output stage, with different filtering. So if you run a 915 radio at 433 then you can communicate at very short distances, but it will be like using a 433 radio with a attenuator attached to it. The attenuation will be large enough that the range is cut to just a few meters.

    I have used this trick to test 433 mode when I didn't have a 433 radio, and it did allow me to test the code at a range of a few meters, but it is no good for actually flying.

    Cheers, Tridge

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