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:
* Xilinx XC3S200AVQ100 Spartan 3A FPGA with 200kgates, 16 Multipliers and 288kb RAM. * 50 MHz crystal oscillator * Linear Tech LTC6904 I2C 10kHz-68MHz programmable CMOS clock oscillator * Beagle community standard I2C ID EEPROM * 4 x Digilent-compatible I/O ports with 8 FPGA I/O, VDD (selectable 5V or 3.3V) and GND. * 3.3V I2C port for off-board serial expansion. * JTAG port for FPGA development and debugging.
The impetus for this board is to experiment with FPGA-based audio synthesis algorithms, Software-Defined Radio and miscellaneous interfacing. 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. Configuration 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. 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 Beagle attached, or for debugging designs using Xilinx's Chipscope application. ID 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 I2C 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. 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 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. Power 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.
* Schematic Diagram * Bill of Materials * Bitstream Loader (v0.1) * more to come...
* 04-04-10 - SPI Loopback works. * 04-06-10 - Start Schematic. * 04-07-10 - Start Layout. * 04-09-10 - Preliminary Layout Complete. * 04-12-10 - Revised Layout for proper footprints and backside connector. * 04-14-10 - Cleanups, fix exp. connector location to match. * 04-14-10 - Prototype boards ordered. Wait ~3 weeks... * 05-06-10 - Prototype boards arrive * 05-09-10 - Partial assembly - all I2C slaves and level shifts work * 05-10-10 - Assembly complete - FPGA works, I2C reset works * 05-12-10 - Bitstream download from Beagle works - app source posted.