Maybe you want to develop something. How do you access the internal microcontroller?
!!!!WARNING!!!! This most likely will not brick your device. I have made all checks I can to make sure this procedure is safe but this discovery is new. This is a "Follow this at your own risk" kind of thing. Connecting to the microcontroller will erase the internal flash. Your radio will not boot until reflashed with a firmware. Also, your settings will be gone. If you want to save your settings, make a backup on an SD card. Also, there isn't much point in doing this right now unless you want to test or possibly develop something. !!!!WARNING!!!
Accessing the microcontroller:
Step 1: Get the required tools. First, you'll need the Renesas Flash Development Toolkit. Unfortunately, Renesas forces you to login with an account to download it. Check BugMeNot. Second, you'll need the latest Yaesu firmware update for the FT1DR. You'll need to pick the fw update for your model on that page. You may need the SCU-* cable driver from the firmware update zip file and you'll need the updater to reflash your firmware because upon connecting, the Renesas microcontroller will wipe its internal flash and will not start the radio until reflashed.
Step 2: Install the Renesas Flash Dev Toolkit and drivers for the cable if nessessary.
Step 3: Remove the battery from the radio and pull off the rubber seal under it. Then move the program switch into the upper position as shown in this PDF. Then, connect the radio using the included AC adaptor and plug the SCU-* programming cable into your radio and connect the other end into your PC.
Step 4: Open the Flash Development Toolkit and configure a new profile. When it asks for a microcontroller model, find and select the model titled "Generic BOOT Device". In the next screen, select the COM port your radio is on. You can check device manager for this information. If it's not there, check your drivers.
Step 5: THIS IS THE LAST STEP YOU CAN TURN BACK ON. Click OK on the next screen that pops up. It will try to connect to the microcontroller. Select clock mode 0 if you're asked.
Step 6: On the next screen, enter 8.000 mHz as the clock and set all multipliers to one. This is the only setting I got to work in my short tests. I'll try more later.
Step 7: In the next screen, uncheck "Use Recommended Settings" on the baud rate and select "19200" in the drop down.
Step 8: Next screen is up to you! Click "Finish" when you're done.
Step 9: If all went well you should have an interface to the microcontroller! From here is up to you for now. You can flash different firmwares onto the microcontroller as well as many other things. If you want to develop something, go get the Renesas toolchains and start coding in their IDE!