Bus Blaster v4 design overview

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You can get a Bus Blaster v4 for $45.

Bus Blaster v4 is an experimental, high-speed JTAG debugger for ARM processors, FPGAs, CPLDs, flash, and more. Thanks to a reprogrammable buffer, a simple USB update makes Bus Blaster v4 compatible with many different JTAG debugger types in the most popular open source software.

  • Based on FT2232H with high-speed USB 2.0
  • Buffered interface works with 3.3volt to 1.5volt targets
  • Reprogrammable buffer is compatible with multiple debugger types
  • Compatible with 'jtagkey', 'KT-link' programmer settings in OpenOCD, urJTAG, and more
  • Should support Serial Wire Debug when available
  • Mini-CPLD development board: self programmable, extra CPLD pins to header
  • Open source (CC-BY-SA)

Bus Blaster v4 is a redesign of v3/v2 that supports SWV, an obscure extension to a reduced pincount JTAG protocol most people will never use. Unless you need it, stick with v3 and save a few bucks!

  • Can now support the SWV feature of Cortex microcontroller for advance debugging when software support is available
  • SWV is little used and not currently supported in software, most users will be better off with Bus Blaster v4 available here
  • Fitted in a DP9056 (90x56 mm) standard PCB
  • Added series resistors to input and output pins to protect against damage and noise

Bus Blaster v4 is available now for $45. Each unit is tested with a real JTAG target before it ships.

Read about the design below.



Project Summary
Name: Bus Blaster v4 design overview
Buy it: Get one for $45 at Seeed Studio
Price: $45
Status: Mature
Manufacturing: Shipping
Forum: Bus Blaster v4 design overview Forum

The Bus Blaster is used to program and debug devices with a JTAG interface like ARM processors, CPLDs, flash memory, and more. A FT2232H USB chip gives us two high-speed JTAG interfaces. The 16 pins of the primary interface are connected to a CoolRunner-II CPLD. The CPLD is a programmable buffer that translates between the FT2232H at 3.3volts, and a 1.5volt to 3.3volt target.

Many JTAG debuggers use the FT2232 chips, but they have slightly different buffers. The CPLD can be programmed to imitate many of them, so it works out of the box with our favorite open source debugging apps. Updates are done over USB using the second JTAG interface on the FT2232H.

This project was inspired by a forum post that linked to a Texas Instruments' XDS100 programmer. TI's design is essentially the same, but we moved the CPLD JTAG connection to the second JTAG interface for easier reprogramming. Bus Blaster v4 was developed in a public forum, and progress was documented on a wiki.

Parts Layout Guide




Click for a full size schematic image. Schematic and PCB were designed with the freeware version of Cadsoft Eagle, download the latest project files from our Google Code project page.



The FT2232H (U2) is a powerful USB to serial communication chip. It has an MPSSE feature that provides a simple USB to JTAG converter (and UART, I2C, or SPI). Most DIY JTAG debuggers use this chip, as do many commercial models.

Bus Blaster v4 uses the 'H' version of the chip, the latest 3.3volt revision that supports JTAG adaptive clocking. The circuit is based on a reference design from the FT2232H datasheet.

Buffered interface


The buffer translates voltage levels between the FT2232H (3.3volts) and a JTAG device (1.5volts-3.3volts). The four main JTAG IO pins (TDI, TDO, TCK, TMS) are fixed on the FT2232, but the other reset and control pins vary among programmers.

Bus Blaster v4 is buffered by a programmable logic chip (CPLD) that can be updated to imitate many common buffer types. The CPLD programming pins are connected to the secondary JTAG interface on the FT2232, so uploading a new buffer type is done entirely from software over USB.

Reprogramming logic


Bus Blaster v4 was designed primarily to support the SWV feature of advanced Cortex microcontroller debugging. Basically the microcontroller has an extra serial UART channel to send debug or other info back to the programmer. This is implemented by using the second JTAG engine of the FT2232 as a serial UART input and passing the info to the user through a serial terminal or whatever.

The Bus Blaster v2/v3 used the secondary JTAG block to program the CPLD with different buffer interfaces. These pins cannot be recycled for use with SWV in the v2/v3 hardware. In v4 we use two 4066 analog buffers (U4/U5) to switch the secondary JTAG interface between the programming pins of the CPLD and other multipurpose pins.

When the MODE jumper is set to program, the secondary JTAG interface of the FT2232 connects to the JTAG programming interface of the CPLD. When MODE is set to normal, the secondary JTAG interface of the FT2232 is connected to multipurpose pins on the CPLD. This way we can both program the CPLD and use the secondary interface for SWV channel information. The disadvantage is that you now have to swap a jumper to change between modes, though most users will not do this very often.



An XC2C64A CoolRunner-II CPLD (U3) is used for the buffer. This is the bigger, 100 pin version of the XC2C32A used in Bus Blaster v2/v3. More CPLD pins are needed to support the SWV protocol.

The CPLD core requires a 1.8volt supply, which is conveniently available from the FT2232. The JTAG and IO pins are powered by a separate supply between 1.5 and 3.3volts. Each supply pin gets a 0.1uF capacitor.

This chip is perfect for voltage translation because the IO pins are divided into two groups that can operate from different power supplies. We connected one group to the FT2232 and the 3.3volt FT2232 power supply. The other group connects to the JTAG target, and operate from a 1.5volt to 3.3volt target supply.

You must connect the target power supply to the buffer
The buffer is powered by the target, 1.5volts to 3.3volts only
Put a header on "Target power"/JP4 to power the target from the programmer. 3.3volts max 200mA
The buffer is NOT 5volt compatible.

The IO pins on Bus Blaster v4 are buffered with 82ohm series resistors to reduce noise and prevent damage to the programmer.

We brought the extra CPLD pins to a header. The Bus Blaster v4 can also be used as a simple CoolRunner-II CPLD development board.

Buffer logic

New buffer logic is designed using simple schematic entry, Verilog, or VHDL, and the free ISE Webpack software from Xilinx.

Here are two examples of buffer logic to give you an idea how flexible the design can be.

Buffer logic for Bus Blaster v4 and Bus Blaster v2/v3 ARE NOT COMPATIBLE!

JTAGkey compatible


The JTAGkey is probably the most commonly used buffer configuration among DIY FT2232-based JTAG programmers. It is compatible with OpenOCD, urJTAG, and more.

Program the Bus Blaster with this buffer and it will work with most applications that support JTAGkey type debuggers.

KT-link compatible


OpenOCD and urJTAG will soon support new Serial Wire Debug and Serial Wire Viewer JTAG protocols via a KT-link type buffer.

Program the Bus Blaster with this buffer and it can support SWD in OpenOCD and urJTAG. Special thanks to the developers of libswd for help implementing this buffer on the Bus Blaster.



JTAG header pinout
Pin Fixed FT2232 pin Description Direction
VTGVoltage targetinput
TRST Reset outputoutput
TDIADBUS1 JTAG data in to targetoutput
TMS ADBUS3 JTAG state machine updateoutput
TCK ADBUS0 JTAG clock in to targetoutput
RTCKADBUS7 System return clock input
TDOADBUS2JTAG data out from target input
TSRST Bi-directional reset pin inout
DBGRQDebug request output
DBGACKDebug acknowledge input

9 CPLD pins are brought to the 20pin JTAG header. The pins are labeled, but the CPLD buffer makes placement totally arbitrary.

Not all pins are supported by all buffers and/or applications

The VTG pin should be connected to the power supply of the device under test. The JTAG pin outputs will then work at the same voltage.

The target must generally supply power to the VTG pin!

The JTAG pins can also operate from the 3.3volt on-board power supply by placing a jumper on header JP4. In this configuration the VTG pin can supply up to 100mA @ 3.3volts to the test device.



We used the freeware version of Cadsoft Eagle to make the schematic and PCB. Download the latest designs and firmware from the project Google Code page.



Click for a full size placement image.

Bus Blaster v4.1a
JP1,JP220.1" header1X13
JP410.1" header1X02
JTAG1Shrouded headerPINSHRD_PTH_2X10
L1,L22Ferrite beadFB603
MODE10.1" header1X03
XTAL1112MHz crystalHC-49U

The latest sources and distributors are in the master partlist. See something missing? Please let us know.

Taking it further

The reprogrammable buffer logic should future proof Bus Blaster v4. Already we were able to add Serial Wire Debug support by creating a new buffer implementation.

Bus Blaster v4 uses a 100pin CPLD to include support for SWV, another reduced pin-count JTAG protocol. Most people will never need this. Stick with v3 unless you absolutely need the SWV feature.

Get one!

Bus Blaster v4 is available now for $45. Each unit is tested with a real JTAG target before it ships.

Your purchases at Seeed Studio keep the open source projects coming, we sincerely appreciate your support!



Hardware license: CC-BY-SA