The STM32F723 can't use the USB2.0 High Speed PHY with the built in Bootloader (Boot0 = High). This is very problematic becuase it forces the engineer to either use USB 2.0 FS to get a built in DFU mode or build a USB 2.0 High Speed compatible bootloader with DFU support.

This repo includes the source code to build the bootloader make or you can use the include binary in the binaries directory.

Fist to use this successfully you need to change the option bytes on your STM32F723. You will be pushing the BOOT0 (LOW) entry point up by 32KB. Then setting the BOOT0 (HIGH) entry point to the initial entry point of 0x08000000. This is done because of the awkward layout of memory sectors on the STM32F723 [16KB,16KB,16KB,16KB,64KB,128KB,128KB,128KB]. The best optimization of these sectors is to place the 22KB bootloader in the first 32KB of space. This leaves 480KB for the rest of the firmware.

DANGER These sectors are protected in DFU mode so it's very unlikely that you will be able to overwrite these sectors. However you can most certianly use the ST-LINK programmer and wipe out the entire memory contents of the MCU. I don't belive in locking down my products, so if you do this sorry. I can help you fix this if you still have the STM32 programmer or a STM32 Discovery board to bootstrap the ST-Link off of.

The schematic below links pin E0 to BOOT0 with a big 22uF capacitor on the line, the theory here is that you set EO to output mode and write a high bit. Then use current from that pin to charge the cape for HAL_Delay(1000) and then use a call to NVIC_SystemReset() to reset the STM32, the resistor to ground limits the flow of current so that the line holds a logic high value for the time needed for BOOT0 to read high after the reset and activate the Nyan Keys DFU Bootloader.

Now you need to setup your option bytes like such BOOT_ADDR0 = 0082 and BOOT_ADDR1 = 0080

After you are done with that step clone this repo - NYAN KEYS Keyboards ONLY

git clone https://github.com/russeree/nyan-keys-dfu-bootloader.git
cd nyan-keys-dfu-bootloader
make clean; make -j16; st-flash --connect-under-reset write build/nyan_keys_dfu.bin 0x08000000
cd ..
git clone https://github.com/russeree/nyan-keys-stm32-firmware
cd nyan-keys-stm32-firmware
make clean; make -j16; st-flash --connect-under-reset write build/nyan_keys.bin 0x08008000

Now you can visit the Nyan Keys web configurator

connect to your Nyan Keys and then use the comman dfu-mode

Now you can use the DFU mode - Check with lsusb on linux you should see 483:df11 STMicroelectronics STM Device in DFU Mode

Now ...