Introduction

This little interface module allows the programming and debugging of many JTAG enabled devices using a PC with a USB port. Traditional JTAG programmer modules, like the CPLD-based programmer presented on this site attach to the parallel port of the PC. While this method allows for easy connectivity, it has many drawbacks. Many current PCs, especially laptops don't even have a parallel printer port any more. In most cases programming speeds are rather slow as well, which is problematic in interactive environments and with large devices.

This module solves both issues. It attaches to the USB port, which is available on almost every PC in use today. It works well with both laptop and desktop computer. The USB interface chip used on the board allows for JTAG datarates up to 6Mbps, which is significantly higher than most parallel port based solutions can offer.

The module is compatible with the OpenOCD project, which enables great integration with tools such as GDB and Eclipse CDT.

Features

• Standard USB interface
• Integrated 3.3V power supply, running from the USB port
• Can provide (3.3V) power for the target device
• Supports programming port voltages down to 1.2V
• Supported by GDB and Eclipse through the OpenOCD project

Design description

USB interface

The heart of the circuit is the FT2232 dual-port USB-to-serial bridge from FTDI. While this chip was originally designed to provide standard RS-232 ports for PCs with only USB interfaces, it also has many additional features. One of these features is the support for the JTAG serial interface. This operating mode is used in this design to provide a USB to JTAG bridge.

JTAG interface

The outputs of the FT2232 chip are buffered by level-shifter gates to prodvide wide-range logic-level support. These devices, the SN74AVC4T245 and the SN74AVC2T45 can manage logical level-shifting between two power domains in the 1.2V to 3.3V range. One of the power domains is fixed 3.3V, and is connected to the power supply running the FT2232. The other domain can run from the same power supply or potentially run from an external power supply provided by the target board. In this latter operating mode arbitrary logic-levels can be used from 1.2V to 3.3V. A jumper can select between the internal or external power supply for the second power domain.

The outputs of the level-shifters can be disabled through software control, using some GPIO pins on the FT2232 device. The pins used make the device compatible with the JTagKey from Amontec.

Power supply

While the FT2232 chip contains an internal 3.3V regulator to allow for bus-powered operation, an external LDO regulator is used. This device, the IRU1207-33, can easily provide a couple of hundred mA-s, much more current than the built-in regulator of the USB bridge chip can deal with. This additional current-delivery capability allows the circuit to power the target circuit as well in many situations.

Other features

Two feedback LEDs are provided on the board. One shows the presence of the 3.3V power supply of the FTDI chip. Since this power is generated from the USB power, this LED pretty much shows if the device is connected to the PC.

Another LED shows the presence of the target power supply. If this supply is provided by the programmer, than this LED will always be turned on. However if the target provides its own power and powers the second power domain of the level shifters, than the LED monitors the presence of that power and can be used to visually verify that a target device is hooked up to the programmer. The LED is driven by a low turn-on level MOSFET, so it can accurately detect the presence of power supplies down to 1.2V. The power detection line is also hooked up to a GPIO pin of the FT2232 device so the presence of the target power can be verified programmatically.

Design files

Schematic and PCB in PDF format (HSNCL)