Bus Blaster v1 design

From DP

High-speed JTAG debugger, flash ROM programmer.

• compatible with OpenOCD and urJTAG as 'jtagkey'
• 3.3volt-1.2volt IO buffer
• open source

You can get a [Bus Blaster v1 for $30].

Read about the design below.

Contents 1 Overview 2 Hardware 2.1 FT2232H 2.2 Buffer interface 2.2.1 Outputs 2.2.2 Inputs 3 PCB 3.1 Partslist 4 Taking it further 5 Get one! 6 Links 7 License

Overview

The Bus Blaster is used to program and debug devices with a JTAG interface. It has a Amontec JTAGkey-compatible interface that supported by the most popular open source JTAG apps.

This project was developed in a public forum, and progress was documented on a [Ricet:_Bus_Blaster_v1|wiki]].

Hardware

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.

FT2232H

• Chip, crystal, reset, power supply
• See FT2232H breakout board documentation

Buffer interface

The four main JTAG IO pins () are fixed on the FT2232.

The buffer control pins vary by programmer type. We wanted the Bus Blaster to work 'out of the box', so we make our buffer interface compatible with the Amontec JTAGkey.

We reverse engineered the JTAGkey connections from the OpenOCD source code and the Openmoko debug board v3 schematics.

Outputs

Outputs (FT2232->buffer)

pin	IO pin	FT2232 pin	description	Enable pin	notes
TDI			Data in to target	JTAG_BUF_EN	Shared output enable
TCK			Clock	JTAG_BUF_EN	Shared output enable
TMS			Machine state	JTAG_BUF_EN	Shared output enable
TSRST				TSRST_BUF_EN	Individual output enable, connected to TSRST input
TRST				TRST_BUF_EN	Individual output enable
DGBRQ			Debug request

This buffer is difficult to choose because it requires three different output enables. A chip like the 74LVC8T245 (1.6volt to 5volts) has a single output enable for 8bits - we'd need three and 19 pins would be wasted.

The 74xx4T245 commonly used on other FT2232 programmers to give 1.6-5volt output won't work for us because the new FT2232H is 3.3volts instead of 5volts.

We used the SN74AVC4T245, which gives 1.2volt-3.6volt output range. Ideally, a 5volt part can be found in the future.

• 1.2-3.6volt output with SN74AVC4T245

Inputs

Inputs (buffer->FT2232)

pin	IO pin	FT2232 pin	description	notes
TDO			Data out from target
TSRST				Connected to TSRST output
RTCK			Return clock	For adaptive clocking
DGBACK			Debug acknowledge
VTG			Voltage target detect	Simple transistor with pullup resistor, inverts signal

Two interchangable buffers:

• 1.2-3.6 volt range with SN74AVC2T45
• 1.6-5.0 volt range with SN74LVC2T45

PCB

File:Project-render-490.jpg

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.

• PCB and placement notes
• soldering advise

Partslist

490px

Click for a full size placement image.

partlist

heading	heading
cell	cell
cell	cell

Taking it further

We're already hard at work on Bus Blaster v2. The updated design uses a CPLD instead of discrete buffer chips, it will be cheaper and more flexible.

We'll post the most recent firmware updates on our blog. You can also join the discussion in the [forum].

Get one!

You can [get one for $00], including worldwide shipping.

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

Links

• Bus Blaster manual
• [Discussion forum]
• FT2232H
• FTDI2232 datasheet

License

Hardware license: CC-BY-SA