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gdbproxy: An open source GDB stub for Blackfin


  • Introduction
    • History
    • My Motivation
  • JTAG
    • Overview of the JTAG standard
    • The JTAG Chain
  • Blackfin Processor Debug and Emulation
    • Blackfin Scan Chains
    • Blackfin Watchpoint Unit
  • gdbproxy
    • The Big Picture
    • How It Works
    • Features
  • The Hardware
  • Future Development
  • Resources
  • Thanks

The Original rproxy

  • Developed by Aleksey of Quality Quorum around 1999 – 2000.
  • Original released under GPL, but later changed to BSD License.
  • Not maintained now.

The Original gdbproxy

  • Developed by Steve Underwood for TI MSP430.
  • Based on rproxy.
  • It was used to embed a proprietary library from TI.
  • Only generic code is in CVS, with a skeleton target.
  • Not actively maintained now.
  • fetproxy: an open source replacement.

The Original gdbproxy for Blackfin

  • Developed by Martin Strubel of section5.
  • Use a proprietary library for JTAG operation: libbfemu.
  • Support IGLOO parallel port JTAG cable and ICEbear USB JTAG cable.

My Motivation

  • Free as in Freedom.
  • Our customers need a JTAG debugging solution: Fast, Cheap, Linux.
  • VisualDSP++: Extremely expensive. Only available on Windows.
  • IGLOO parallel port JTAG cable: Too slow. No parallel port at all.
  • ICEbear USB JTAG cable: Too expensive (US$ 299). Not open source.
  • I want to learn something about JTAG.
  • We CAN do it.

An Overview of JTAG (1)

  • Joint Test Action Group. IEEE 1149.1-2001.
  • Formed in 1985 to develop a method to test populated circuit boards after manufacture.
  • Now used for: Debugging. Storing Firmware. Boundary Scan Testing.

An Overview of JTAG (2)

JTAG pins:

  • TDI (Test Data In)
  • TDO (Test Data Out)
  • TCK (Test Clock)
  • TMS (Test Mode Select)
  • TRST (Test Reset) optional
  • Vendor pins

An Overview of JTAG (3)

  • Instruction register
  • Instructions: BYPASS, SAMPLE/PRELOAD, IDCODE (optional).
  • Data registers: Boundary Scan Register, Bypass Register,
  • Device Identification Register.
  • Driven by a state machine.

The JTAG Chain

Blackfin Scan Chains

Scan Chain Name Size (in Bits) Contents
BOUNDARY SCAN N Chip Boundary scan

Blackfin Watchpoint Unit (1)

Watchpoint Unit registers, accessible in Supervisor and Emulator modes:

  • The Watchpoint Status register
  • 6 Instruction Watchpoint Address registers (can be grouped)
  • 6 Instruction Watchpoint Address Count registers
  • The Instruction Watchpoint Address Control register
  • 2 Data Watchpoint Address registers (can be grouped)
  • 2 Data Watchpoint Address Count registers
  • The Data Watchpoint Address Control register

Blackfin Watchpoint Unit (2)

Two operations implement instruction watchpoints:

  • The values in the six Instruction Watchpoint Address registers, are compared to the address on the instruction bus.
  • Corresponding count values in the Instruction Watchpoint Address Count registers, are decremented on each match.

Blackfin Watchpoint Unit (3)

Two operations implement data watchpoints:

  • The values in the two Data Watchpoint Address registers, are compared to the address on the data buses.
  • Corresponding count values in the Data Watchpoint Address Count registers, are decremented on each match.

The Big Picture

gdbproxy: Generic Code

  • Provide stubs for target part of GDB Remote Serial Protocol.
  • Parse incoming packets and construct outgoing packets.
  • Call corresponding target functions.

gdbproxy: Blackfin Target

  • Provide implementation for each stub.
  • Use UrJTAG for low level JTAG operations.
  • Written from scratch.
  • Released under GPLv2.

How It Works

  • Take single register reading for an example:
  • GDB sends a read register packet, p n, to gdbproxy.
  • gdbproxy receives this packet, parses, calls into handle_read_single_register_command.
  • handle_read_single_register command calls into bfin_read_single_register.
  • An instruction EMUDAT = Rn; is generated and scanned into EMUIR register.
  • Run the JTAG state machine to a specific state so the instruction in EMUIR register is executed.
  • EMUDAT SCAN is selected to shift out the bits in EMUDAT register.
  • gdbproxy constructs a reply packet and sends it back to GDB.


  • Free software.
  • Supported host OS: Linux, Mac OS X, Windows.
  • Actively maintained.
  • Support multi-core processor: Dual-core BF561.
  • Support multi-processor (partially): Multiple Blackfin processors. Blackfin processor chained with other chips, like FPGA.
  • Potential to support many JTAG adapters. All JTAG adapters supported by UrJTAG can be used.
  • Stable. Well tested.

IGLOO Parallel Port Cable



  • Based on FT2232D/L.
  • Support full speed (12Mb/s) of USB 2.0.
  • Cheap (about e60).
  • Open source hardware.



  • Based on FT2232H.
  • Support high speed (480Mb/s) of USB 2.0.
  • Also cheap (no price now).
  • Open source hardware.

Future Development

  • Make libftdi work with libusb-1.0.
    • Linux: Done.
    • Windows: Use libusb-0.1.
  • Support non-stop mode for multi-core debugging.
  • Fully support multi-processor debugging.


Source Code of Blackfin gdbproxy

Document of Blackfin gdbproxy

Another very interesting JTAG application


Thank you for your attention.

Any questions?