RFP doesn't work correctly in serial mode with EK-RA4M3

Hi all,

I've played a little with RFP v3.14 with my eval board EK-RA4M3. 

I wanted to check some communication over UART (by using J-Link on eval board). 

With baudrate 9600 everything is fine, I can read memory option bytes etc., but when I change to 115200 I cannot do anything and some error occurs:

I've checked communication with logic analyzer and I didn't find anything special, setting baudrate with command 0x34 looks exactly the same for 9600 like for 115200. With 9600 after ~118ms programmer continues communication, with 115200 there is no communication after 0x34 command. 

I've tried with different USB cables, different Windows (11 and 10 checked), I also upgraded J-link firmware to the newest one (from 12.2023).

I don't know what else I could check.

Also I don't know why only 9600 and 115200 speeds are supported if AN R01AN5372EU0110 (System Specifications for Standard Boot Firmware) indicates that we can set up to 2Mbit (Table 24).

If you have any idea what could be wrong or how I can fix it or show more, please let me know.

Thanks in advance,


  • Hello Adam,

    After checking your issue internally, your issue is caused due to the fact that you are using the J-Link OB-S124 debug interface. And the J-Link- OB- S124 cannot work properly and keep up with 115200 bps.

    You could either use a USB to UART adapter and select COM port and 2-wire UART interface in RFP  by making the connections according to app note mentioned here:


    Check TYPE C connections for COM port.

    Or either you can connect your USB cable to USB Full Speed Port available.

    Make sure that you have  closed jumper J16 to drive the MD pin to low to enter the boot mode and also close jumper J15 to provide power to the MCU from the USB port and set jumper 12 to pins 2-3.

    Reset once your board by pressing the reset button and go to device manager and you will see the COM port get recognized as:

    And then in RFP connect with these settings:

    And then you will be able to communicate through USB at 115200 bps via RFP.

    I have tested this with EK-RA4M2 but Boot Firmware through USBFS is also supported on RA4M3.

    Best Regards,


  • Hi again,

    I've make connection with simple external serial converter (with setting all jumpers as you suggested) - I was able to reach 1Mbit communication :) WIth some better converter I will also check real limit here. Thank you very much!

    About SWD - I've checked Jeremy response but it just about limitation on protocol/commands level (cannot set IDAU regions etc.) but in my case the issue is that regardless of chosen SWD speed, it always run on ~1.7MHz. I've checked external SWD programmer and it correctly changes SWD communication speed - probably issue sits in firmware in built-in jlink.

  • The JLink-OB S124 is limited in the SWD speed it can achieve, I think it is 4MHz. You would need an external JLink model to run at a faster SWD speed.

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