OpenOCD and FTDI Chips

To let OpenOCD correctly drive the FTDI chip, we need to tell OpenOCD the hardware configuration of the FTDI chip through the config file:

### FTDI interface configuration ###
ftdi_vid_pid                0x0403 0x6010
ftdi_channel                0
ftdi_layout_init            0x0888 0x8B8B

Reference: https://openocd.org/doc/html/Debug-Adapter-Configuration.html

We will analyze the config file line by line

ftdi_vid_pid                0x0403 0x6010

This line tells OpenOCD to find the USB device with the signature of corresponding vendor ID (VID) and product ID (PID).

0x0403: Future Technology Devices International Limited

0x6010: FT2232C/D/H Dual UART/FIFO IC

ftdi_channel                0

Selects the channel of the FTDI device to use for MPSSE operations. Here we select channel A, which is the first channel (channel 0).

ftdi_layout_init            0x0800 0x0800

Defines the initial data and IO direction of the GPIOs. The mapping is as follows:

0x	bit [15..8]	bit [7..0]
	ACBUS[7..0]	ADBUS[7..0]

As an simple example, to initialize the JTAG LED on FT-LINK, which is an open-drain LED connected to ACBUS3 pin, we use the following setting

ftdi_layout_init            0x0800 0x0800   # this initialize LED off

ftdi_layout_init            0x0000 0x0800   # this initialize LED on

We can also define the JTAG_LED variable with the following command

ftdi_layout_signal JTAG_LED -oe 0x0800

Then, in the init routine, we can use the variable to reference this pin and blink the LED

init

ftdi_set_signal JTAG_LED 0    # LED on

sleep 1000

ftdi_set_signal JTAG_LED z    # LED off

sleep 1000

ftdi_set_signal JTAG_LED 0    # LED on

note that since we are only enabling the output driver by the -oe flag, the chip can only drive it open-drain, and therefore we are only allow to set the signal state to either low (0) or high-Z (z).

If we try to set it to high (1), we will get the following error:

Error: interface can't drive 'JTAG_LED' high

If we have some normal signal that we want to drive both high and low, we can use the -data flag instead:

ftdi_layout_signal JTAG_LED -data 0x0800

init

ftdi_set_signal JTAG_LED 0    # LED on

sleep 1000

ftdi_set_signal JTAG_LED 1    # LED off

sleep 1000

ftdi_set_signal JTAG_LED 0    # LED on

If we somehow want all the states, we do the following

ftdi_layout_signal JTAG_LED -data 0x0800 -oe 0x0800