Greta

About

Expansion board for the Amiga A500 computer providing SDRAM, Secure Digital mass storage and Ethernet interface.

Features

• Attaches to the Amiga A500 expansion bus
• 8 MiB Fast RAM
  • Zero wait state
  • Always available to the system via Amiga AUTOCONFIG protocol
• SPI controller
  • For interfacing with Secure Digital storage
• 10/100Mbit Ethernet PHY

Hardware overview

FPGA

Xilinx Spartan-3AN is used for control logic. This FPGA device is equipped with an internal bitstream flash.

Bus transceivers

The Amiga computer uses 5V signalling while the FPGA and the other expansion board components operate at 3.3V. Signal level conversion and bus isolation is solved by using 5V-tolerant 3.3V drivers for signals between the computer to the expansion board.

Bus protocol

A500 expansion bus protocol is equivalent to the MC68000 processor bus protocol. The MC68000 bus control signals are sampled by the FPGA, synchronized, and bridged to an internal custom bus protocol with time-slots allocated to on-board peripherals.

Voltage regulators

3.3V and 1.2V are generated by on-board voltage regulators, powered by the expansion bus connector.

Clocking

A 50 MHz input clock is generated by the Ethernet PHY. This clock is synchronous with the PHY RMII signals. The FPGA logic and SDRAM operate at 133 MHz.

Status

• Full support for the 8 MiB zero waitstate SDRAM is implemented.

• AmigaOS AUTOCONFIG for device discovery implemented.

• An SPI controller is implemented and is currently being used for developing an AmigaOS compliant disk controller driver with SCSI emulation.

• An Ethernet MAC with DMA is still to be implemented.

• Gerber files are readily available and can be sent to a PCB manufacturer for production. Refer to the schematic files for bill of materials (BOM).

Documentation

Details of the design is documented in the Technical description.

Schematic and layout files are available in the pcb directory. A PDF rendering of the schematic can be found in doc/schematic.pdf.

Tools

The following tools have been used in the development of hardware and HDL:

• KiCad: schematic and PCB layout
• GHDL: RTL simulation and testbenches
• Xilinx ISE: Xilinx FPGA synthesis and layout

Authors

Martin Åberg - martin@fripost.org

License

All schematics, PCB layout and VHDL code described in this document are released under the GNU General Public License.