Thanks again, Rongzhong! Unfortunately, it's still the same no matter I upload the OpenCat.ino first or the WriteInstinct.ino first. In fact, I just crossed checked again with the images from CodeCraft. If I use the "Flash Factory Code", the "khi" would work correctly. If I use the "Update Preloaded Skill", it would not work and the bittle just keeps shaking non-stop. Simiar to the display on Serial Monitor when I upload "WriteInstinct.ino", which keeps showing something like "0.00 t-0.5" non-stop too but no mechanical shaking. On the other hand, Dimitry of the youtube did reply me and told me that it's not due to the current/voltage drop but the UART output message which would interfere the ubuntu bootup and so I would need to modify the kernel for silent bootup and it works for me now then. When I told you that my Pi 3 worked before, it's with Rasp Pi OS, not ubuntu, and that may explain why I didn't have problem with it. Because he also did same change on his Pi 3 with ubuntu too. Here is the link for anyone who may need later too:- https://raspberrypi.stackexchange.com/questions/84175/disable-uart-such-that-it-will-not-be-on-during-the-boot-process-but-only-after Thanks.

Thanks again, Rongzhong! I tried your suggestion to modify the baudrate to 57600 and reflashed via Arduino IDE. And then it is interesting to see that the reset button would work this way but only for 2 consecutive commands, the 3rd one will always fail and then another reset will work again for 2 commands . So probably the battery power may still not be enough...?! I just posted my question to Dimitry's post to see if he will reply me on how he made it work with Pi 4 then too. In addition, I notice the github code for the "khi" hand shaking action is broken, different from the default factory image I flashed from CodeCraft. Hope that it will be fixed soon. Thanks.

Thanks again, Rongzhong. I tried to press the reset button of NyBoard after Pi 4 is up as you suggested. Unfortunately, it doesn't work well this way and gradually it would give no response to the ardSerial.py command at all. Therefore, I had concern that either my NyBoard or the servos might be damaged. Fortunately, after using CodeCraft to reflash the factory code, I can use the USB connection from Pi 4 to execute ardSerial.py again just now. I looked into the source code of the bittle ROS driver the Youtube author provided and found that it's with baud rate: 57600. And so I suspect that he's using the older Nyboard V0.2 as I read another post saying that Nyboard V0.2 is using 57600, not 115200, right?! So that may explain why he did not encounter the problem I have now with Nyboard V1 too?! Any chance that I can buy a Nyboard V0.2 board to try if you also think that there may be some difference between Nyboard V1 and Nyboard V0.2 for my issue please?! Thanks.

Thanks again, Rongzhong! After more debugging, I finally managed to have both my Pi 3's and Pi 4's serial port working. For Pi 3, I just made sure my raspi-config was reset correctly and so on and it can be mounted without problem. For Pi 4, I followed the solution mentioned in: https://raspberrypi.stackexchange.com/questions/114366/rpi4-serial-port-not-working-on-either-raspberry-os-or-ubuntu However, I find that I still can't mount the Pi 4 directly onto the 2x5 socket as the serial port will not have respond then. When I first power up the Pi 4 separately with another power source and then mount it up and then remove the power source, it would work then. It seems to me some power-up sequence from the Nyboard may not work well with the power-up sequence from the Pi 4 ubuntu somehow?! Please let me know if you would have any other suggestion too. Thanks again!!

Thanks, Rongzhong! I plan to order the Nyboard and the Bluetooth board then. I need to mount the Pi 4 as I already set it into Ubuntu with ROS and Lidar connection working and so I want to see if I can mount it to test the feature. However, it seems that Pi 4 with Ubuntu makes it difficult for the UART to work normally too. So I soldered the 2x5 socket and just wired the 3 pins TX, RX, GND and used another Rasp OS Pi 3 to test the ardSerial.py. To my surprise, it would work the first time usually and then the second time would fail. Looks like some ser.inWaiting issue or flushing issue?! And there's also difference if I modified the original Python 2 version to the Python 3 version, it could be worse with the Python 3 version. That may be another reason why Pi 4 would not work as it's Python 3 default for Pi 4 Ubuntu too. Anyway, I will keep debugging to see if I can find out better solution... Thanks.