Category: Bare Metal

First, we’ll walk through the HW setup. More specifically, the portion of the diagram below that connects the two green boxes and the USB connection from the Host System to the FT232. This is a fairly simple setup. The HW consists of the Raspberry Pi 4, a FT232H breakout board, 6 jumper wires and a USB-C cable.

** WARNING **: There should be no power connected to the Raspberry Pi 4 and NO USB connected to the FT232 board while hooking up the Jumper wires. Hook these up first. This will be safer for you and the HW. There are no high voltages here, but doing this with the power off is always a good idea.

Probably the most complicated part of this is soldering the pin headers onto the FT232 board. However, I will not cover the soldering of the pin headers.

The headers can be oriented toward either side of the board. The picture below is my setup. I put the headers ‘under side’ of the board and mounted the board in a ‘dead roach’ configuration (on its back with the pins/legs up). Installing the FT232 board on a breadboard is another option you might consider. In this photo, the raspberry pi is mounted on an oak plank and the FT232 board is held in place with standoffs.

The HW setup is simply connecting the 6 jumper wires from the FT232 Breakout board to the Raspberry Pi 4. From the Table below we are hooking up the following

• D0 to Pin 22 RPI4 J8
• D1 to Pin 37 RPI4 J8
• D2 to Pin 18 RPI4 J8
• D3 to Pin 13 RPI4 J8
• C0 to Pin 15 RPI4 J8
• GND to Pin 39 RPI4 J8

Once again, you should do this before connecting power or a USB cable.

AdaFruit Board PIN Name	FT232H Pin Name	JTAG Function	RPI 4 GPIO	RPI 4 J8 Pin
D0	ADBUS0	TCK	GPIO 25	Pin 22
D1	ADBUS1	TDI	GPIO 26	Pin 37
D2	ADBUS2	TD0	GPIO 24	Pin 18
D3	ADBUS3	TMS	GPIO 27	Pin 13
D4	ADBUS4	GPIO	Not Connected
D5	ADBUS5	GPIO	Not Connected
D6	ADBUS6	GPIO	Not Connected
D7	ADBUS7	GPIO	Not Connected
C0	ACBUS0	/TRST	GPIO 22	Pin 15
C1	ACBUS1	/SRST	No Connected
C2	ACBUS2	GPIO	Not Connected
C3	ACBUS3	GPIO	Not Connected
C4	ACBUS4	GPIO	Not Connected
C5	ACBUS5	GPIO	Not Connected
C6	ACBUS6	GPIO	Not Connected
C7	ACBUS7	GPIO	Not Connected
C8	ACBUS8	func defined	Not Connected
C9	ACBUS9	func defined	Not Connected
GND	GND	GND	GND	Pin 39

Once these 6 wires are connected, you can connect USB-C cable between the FT232 and your PC. The AdaFruit FT232 board has two sockets on it, the image blow is of the USB-C connector.

Although the FT232 uses a USB-C connector, this is just a regular USB connection and you can use a USB-C to USB-C or USB-C to USB-A cable. Connecting the cable to your computer should be something you are already comfortable with.

Here is another image of my setup with the USB attached.

For information about connecting power to the Raspberry Pi 4, please consult the documentation that came with your Raspberry Pi 4.

In our Tools series, we will discuss software and hardware tools that facilitate embedded systems development. For each tool, we will discuss the primary purpose, typical installations, and demonstrate a common use.

JTAG (Joint Test Action Group) is a standard system for verifying and testing circuits after fabrication. Originally developed in 1985 as a validation tool, engineers quickly realized that its ability to control a CPU and communicate system state information could be used in conjunction with a debugger to create a powerful development and debug environment that could rival an ICE. In fact, many of these setups were and are still referred to as JTAG ICE setups. This series of articles will describe how to create a JTAG debug setup for bare metal and OS development and testing on a Raspberry Pi 4 board.

Specifically, we will cover:

• • Setting up the HW, FT232H breakout board, Raspberry Pi 4
  • Setting up “openocd”
  • Setting up gdb
  • Setting up VSCode to use gdb so you have a full IDE for debug of HW.

The hardware consists of a FT232H breakout board from Adafruit (under $20 at the time of writing this article), your Raspberry Pi 4, some jumper wires and a USB-C cable. This hardware is combined with several Open Source software tools to create complete development system.

The software stack for this article includes openOCD (Open On Chip Debugger), GDB (Gnu Debugger) and Visual Studio Code. In part 3 we will cover turning on the jtag in the Raspberry Pi 4 and a “Null” boot image for the Raspberry Pi. Then we will cover setting up the software in Parts 4, 5 and 6

LINK TO PART 2