Beagle FPGA Expansion Board

Assembly Phase 2

Assembled board w/ Beagle

What is it?

This is a small (3.1" x 3" - same size as Beagle) board with a Xilinx Spartan 3A FPGA connected directly to the Beagle expansion connector. Designed to mount back-to-back with the Beagle using 3/8" stand-offs, it provides the following features:



The impetus for this board is to experiment with FPGA-based audio synthesis algorithms, Software-Defined Radio and miscellaneous interfacing.


The original version of the Tracker (pictured above) is a one-off personal project intended as a proof-of-concept. A commercial version is being considered which will have a few minor modifications to allow more I/O flexibility. Follow its progress at

Design Details

Expansion Connector

All GPIO/Muxed signals on the Beagle Expansion connector are routed to the FPGA, with the exception of the I2C signals which are used for ID and control on-board. Configuration of the FPGA uses SPI port 4, so your kernel will have to support this and alternate functions for these pins are probably not practical without some serious kernel code changes. That shouldn't be a big issue though, because the FPGA bitstream can use SPI port 4 for control, or route it through to the I/O connectors after configuration. All other GPIO pins connect without restriction, aside from direction controls which must be setup via I2C for GPIO_130-GPIO_132 and GPIO_157.

The 1.8V, Reset and Regen pins are not used.


This board has no on-board flash for the configuration of the FPGA - the configuration is written to the FPGA by the Beagle board via SPI port 4. This allows the board to be re-configured at any time merely by restarting the configuration and downloading a new bitstream from the Beagle. A PCF8754 I2C port expander controls the configuration process and also provides direction control for some signals that had to be level shifted. The port expander is at I2C address 0x38.

A bitstream download application which uses kernel I2C and SPIDEV drivers to manage the process will be provided for userland setup. After configuration these interfaces can also be used to control the FPGA.


A 6-pin JTAG header is provided for external configuration and debug. It conforms to the Digilent JTAG pinout, but can also be used with Xilinx download cables if flywires are used. This is handy for testing the FPGA without the Beagle attached, or for debugging designs using Xilinx's Chipscope application.


A 128-byte I2C EEPROM is available for conformance with the Beagle Expansion board ID standard. While normally write-protected, a Write-Enable jumper is provided to set the ID and any other data stored in the EEPROM. This device uses address 0x50h


A 3.3V level shifted version of the Beagle I2C2 bus is available, along with 3.3V supply and GND on a 4-pin header. This can be used to add I2C peripherals off-board. Bear in mind that there alread 3 devices (ID EEPROM, Port Expander and Clock Oscillator) on the bus, so check that any additional devices don't collide with those already on the bus.

Digilent Ports

These are 12-pin right-angle 0.1" receptacles which conform to the Digilent PMOD standard. Each connector provides 8 bits of direct connection to the FPGA, along with two power and two ground connections. Power is jumper selectable as either 3.3V or 5V on a per-connector basis. Digilent (and others) sell a variety of I/O boards for this standard that are inexpensive, small and provide a wide range of interfacing opportunities, including Ethernet, RF/Wireless, motor control, audio, etc. Find out more at Digilent's site.

There are two of these connectors on two sides of the board, spaced at the same interval as is found on Digilent FPGA boards, so standard two-connector boards with up to 16 I/O signals are possible.


The Xilinx XC3S200A Spartan 3A FPGA on this board is inexpensive (about $10 in single quantity) and provides sufficient I/O and logic resources to perform a wide range of interfacing and processing tasks. Simple signal processing, sequencing and control are certainly possible, as well as some buffering and reformatting using on-chip RAM.


Two LEDs are on-board. A blue one labled "DONE" lights when the FPGA has a valid configuration. A green one labled "ACT" is connected to an I/O pin on he FPGA and can be used as a visual annunciator of internal activity.


No dedicated clocks are provided on the Beagle expansion header, so this board provides two clock sources. A crystal oscillator generates a stable 50MHz clock suitable for high-accuracy timing, while an I2C-programmable oscillator supplies a variable clock in the 1kHz to 68MHz range with less accuracy and more jitter. This device is at I2C address 0x17.


On-board 3.3V, 1.8V and 1.2V LDO regulators derive all the supplies required from the Beagle 5V supply provided on the expansion connector. While the 3.3V and 1.2V regulators which supply the FPGA I/O and Core voltages are rated for close to 1A, it is recommended that FPGA designs which require high power are not considered for this board. Although the Spartan 3A chip used here can operate at fairly high internal frequencies approaching 200MHz, the practical limitations of power supply and heat management will constrain the designs that can safely be realized on this board.



Minimal kernel driver support required for the Tracker includes I2C2 and SPI port 4 with SPIDEV, as well as pin muxing to make these interfaces available on the Beagle expansion port. Since the Tracker conforms to the Beagle IDPROM standard it is already automatically recognized by recent versions of u-boot and pin-muxes are correctly set. Kernel patches to enable I2C2 and SPI4 may be required however - find out more about how to do this at emeb's Beagle page.

A complete patch against the Angstrom stable 2.6.29 kernel is here: bfpga.patch

A patch against the Angstrom unstable 2.6.32 kernel is here: 0045-ARM-OMAP-beagle-add-support-for-beagleFPGA-expansion.patch


A Beagle user-space application and access library is provided which supports command-line bitstream loading and control of the features on the board. The application can be used to download FPGA configuration files, control the programmable clock oscillator and test the SPI control port. The access library provides a low-level API for control of the I2C and SPI ports, as well as higher level functions for bitstream download and may be used to construct complex user-space applications for interaction with the FPGA design. Full source for this is provided below, but you can also get it rom here:


A "Manufacturing Test" program is also available which exercises all the expansion port pins between the Beagle and the FPGA, as well as the Digilent output ports and the on-board clock oscillators. The program can be used as an example of accessing GPIO, controlling the level shifter directions and talking to the FPGA's designed-in SPI control port. Note that this requires some kernel hacking to make Beagle GPIO pins 144-146 available via SYSFS. More details on that later.

FPGA Design

Synthesizing an FPGA design requires the Xilinx ISE Webpack which is a free download from the Xilinx website. This suite of tools includes a GUI IDE as well as command-line applications that can be run under both Linux and WinXX. Designs can be created with either Verilog or VHDL. Demo designs will be provided to showcase the capabilities of the Tracker board and will provide enough detail for starting on custom designs.

Design Documentation


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Last Updated
Comments to:
Eric Brombaugh

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