Using FTDI FT232RL as a JTAG adapter via openOCD

One of the cool tools in embedded is openOCD. Basically, it enables us to interface between host (PC) and target (MCU) for flashing and debugging. It does so by utilizing the electrical signalling of an interface device to bit-bang a protocol.
FTDI FT232RL is a chip for converting UART signals to USB and vice-versa. It is often used in applications where the serial port is not available on host (like modern PCs) but we want to manipulate a UART device.
In this post, we are going to utilize FTDI FT232RL as a JTAG debugger/programmer via openOCD, meaning something it was not originally designed for.

1. Set up openOCD on your system.
   I’m using Liviu Ionescu’s binaries here since my distro’s binaries are a little older.
2. Follow the documentation of openOCD for your interface and target.
   The pdf is located under your_dir/liviu-openocd-0.10.0-15/share/doc/pdf. Instructions are found in chapter 8 under ft232r
3. Attach the wires from interface to target.
   Here our target is the Bluepill board (STM32F103). Your FTDI board may have come soldered only for Vcc/GND/Tx/Rx pins but we also need other pins here. So first solder a header to all break out pins.

Caution: STM32F103 is a 3v3 MCU. Remember to change your FT232R VCC jumper to 3v3 volts otherwise you will damage your chip!

Schematic Description

FT232RL : Jumper on 3V3
Bluepill : Both jumpers on 0

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11

  FT232RL     | Bluepill(STM32F103)
    VCC       -      3v3 
    GND       -      GND 
    TX        -      SWCLK(PA14)
    RX        -      A15 
    CTS       -      SWDIO(PA13)
    DCD       -      RST   
    DSR       -      NC
    RI        -      NC
    RTS       -      PB3  
    DTR       -      PB4

4. Open two terminals. On one of them execute openocd -f interface/ft232r.cfg -f board/stm32f103c8_blue_pill.cfg. On the other execute telnet localhost 4444.

Now you have a JTAG adapter! You can run these commands on the latter terminal or use GDB according to your workflow (see openOCD documentation for explanation):

• list memory banks of target
  flash banks
flash list
flash probe 0

• reading firmware from flash to disk
  flash read_bank 0 /tmp/saved.img

• erasing flash
  reset init
flash erase_sector 0 0 last

• writing firmware to flash from disk
  flash write_bank 0 /tmp/saved.img
reset

• verifying if upload was successful
  flash verify_bank 0 /tmp/saved.img

Instead of using two terminals, we can use openOCD’s program script to flash&verify the MCU in one go which is suitable for substituting with st-link st-flash utility in makefiles. Exit other instances before trying this:

1
2

openocd -f interface/ft232r.cfg -f board/stm32f103c8_blue_pill.cfg\
            -c "program $(BINARY).bin verify reset exit 0x08000000"

Flashing and debugging is slow with FT232R (FT232H would be much faster). For example reading the complete 128 KiB of bluepill’s flash takes ~50 seconds…but hey, our zombie FTDI is dancing to the tunes imposed by openOCD and it sings JTAG slowly but correctly!