mini2440 FPGA Expansion Board

PCB Rendering

PCB Rendering

PCB Rendering

Partially Assembled

What is it?

This is a small (3.1" x 3.9" - same size as mini2440 LCD) board with a Xilinx Spartan 3A FPGA connected directly to the mini2440 expansion connector. Designed to mount back-to-back with the mini2440 or nearby, it provides the following features:

Motivation

The main goals for this board are to experiment with FPGA-based audio synthesis algorithms and miscellaneous interfacing.

Design Details

Expansion Connector

All GPIO/Muxed signals on the mini2440 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 0, 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 0 for control, or route it through to the I/O connectors after configuration. All other GPIO pins connect without restriction.

Configuration

This board has no on-board flash for the configuration of the FPGA - the configuration is written to the FPGA by the mini2440 via SPI port 0. This allows the board to be re-configured at any time merely by restarting the configuration and downloading a new bitstream from the mini2440. An PCF8754 I2C port expander controls the configuration process and also provides additional I/O to the FPGA. 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.

JTAG

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 mini2440 attached, or for debugging designs using Xilinx's Chipscope application.

ID

A 128-byte I2C EEPROM is available for ID and key storage. 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 0x51h so as not to conflict with the EEPROM on the mini2440.

I2C

The 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 2 devices (ID EEPROM and Port Expander) on the bus, so check that any additional devices don't collide with those already present.

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 one side 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.

FPGA

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.

Clocks

The mini2440 expansion connector provides two clocks directly from the ARM SoC which can be configured at kernel initializtion. In addition to these, a stable crystal oscillator generates a clean 24.576MHz clock suitable for high-accuracy timing.

Power

On-board 3.3V and 1.2V LDO regulators derive all the supplies required from the mini2440 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.

Software

A 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 and control the I2C port expander that manages configuration. 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 will be provided once it's working.

Design Documentation

Status

Return to Synth page.

Last Updated
:2010-07-10
Comments to:
Eric Brombaugh

Valid HTML 4.01 Transitional