Sardis Technologies -- 6809-Tiny6 SBC

(This page is still under construction.)

The 6809-Tiny6 is a powerful 6809E/6309E single board computer (SBC) in only 6 square inches! It provides most of the functionality of the ST-2900 CPU and RAM-512 boards, but in less than 12% of the board area, using less power, costing less to build, yet runs 2 or 3 times faster.

6809-Tiny6 features:

• 6x09 microprocessor:
  • Motorola MC68B09E up to 2 MHz, or ...
  • Hitachi HD63C09E up to 3 MHz
  • 40-pin .600" DIP package
• Microchip (Atmel) AT28C256 32Kx8 EEPROM:
  • can be updated in-system, write-protect jumper
  • ST-MON 3.xx monitor program
  • contains 6809 FLEX Plus 10TM and (Nitr)OS-9 Level 1 boot images, boots either operating system in seconds
  • can also boot from remote disk images via the second serial port, using any of these protocols:
    • my own DSKdrv/DSKserv for FLEX and (Nitr)OS-9
    • FLEX-OS9-Net for FLEX and (Nitr)OS-9
    • DriveWire for (Nitr)OS-9
  • 28-pin .600" DIP package
• Alliance AS6C4008 512Kx8 SRAM:
  • 63.75KB main RAM
  • 446.25KB RAM-Disk
  • 32-pin .600" DIP package
• MaxLinear (Exar) XR88C92 DUART:
  • two serial ports, each with 8-character transmit and receive FIFOs
  • automatic RTS/CTS hardware flow control (implemented correctly)
  • four baud rate generators, up to 460800 bps
  • one 16-bit counter/timer
  • 7 input and 8 output lines
  • 44-pin PLCC package, in thru-hole socket
• Microchip (Atmel) ATF750C CPLD generates:
  • E and Q clocks for 6809E/6309E microprocessor
  • reset signal and 14.7456 MHz clock for DUART
  • read and write enables and chip selects for EEPROM, SRAM, DUART
  • supplied with clock from crystal oscillator
  • 24-pin .300" DIP package
  • WinCUPL design files supplied to allow modification
• crystal oscillator:
  • frequency depends on logic design in CPLD, should be integer multiple of 14.7456 MHz
  • 4-pin .300" DIP-8 package in a half size metal can
• Analog Devices (Maxim/Dallas) DS1813 voltage monitor:
  • generates reset during powerup, powerdown, and manual reset
  • TO-92 package
  • pushbutton reset switch
• connectors:
  • one 2-pin power connector (.100" pitch) for +5.0V supply, plus green power-on LED
  • two 6-pin connectors (.100" pitch) to connect serial ports to external USB or RS-232 adapters:  TXD, RXD, RTS, CTS, +5.0V, GND
  • one 8-pin connector (.100" pitch) for bit-banged serial expansion port (such as SPI, and with external FETs also 1-Wire and I2C)
  • one 32-pin I/O expansion bus connector (two versions available: 1x32 .050" pitch, or 2x16 .100" pitch)
  • one 2-pin test point for GND
  • four mounting holes for M2 screws
• only 2.4" x 2.5" (61mm x 63.5mm), exactly 6.0 sq.in., smaller than a credit card
  (version with 2x16 expansion connector is 2.55" x 2.5", 65.8mm x 63.5mm, 6.375 sq.in.)
• quiet 4-layer board
• all components are easy-to-solder thru-hole devices
• all components (except the MC68B09E/HD63C09E) are currently in production and available at DigiKey or Mouser or Newark/element14 or Arrow or Future
• requires +5.0VDC at ???mA (via 2-pin power supply connector, or from USB-serial adapter??)

This board is probably the smallest possible for the features provided when using all thru-hole devices (or sockets) and using an actual 40-pin microprocessor instead of an FPGA emulation. In order to achieve the small size I placed several components on the bottom side.

There were a few other features I would have liked to add to the board, but every pin on the CPLD is already used, and I don't want to use a larger CPLD (in a larger package) for this board. Actually, the ATF750C CPLD is a perfect fit for this design, in terms of numbers of pins, speed, and available package. (Unfortunately, most low-cost device programmers don't support the ATF750C, only more expensive programmers such as the Xeltek SuperPro, eeTools ChipMax2, and Dataman 40Pro Universal ISP Programmer.)

I do not plan to manufacture and sell this board myself. Instead, I will eventually publish the complete set of documentation on this web page: KiCad schematics and PCB layout files, Gerber files, BOM, assembly and use instructions, firmware and software in source and binary form, and CPLD design files.

I have some ideas for a small optional daughterboard for the 6809-Tiny6 that would significantly speed up loading OS-9 memory modules by calculating the header parity byte and 24-bit module CRC in hardware rather than using the kernel's software routine. But it would require surface mount components.

(Click on a picture for a larger version.)

KiCad 3D renders of tentative board layouts. (I used FreeCAD to add text to the IC images.)

Current Status (updated 2024-May-09)

I started creating the initial schematics and PCB layout for the 6809-Tiny6 design in 2019 using ExpressPCB Classic, since I was most familiar with this tool. I had hoped to entirely avoid any surface mount components, since I haven't acquired the skills to solder them. But the ExpressPCB 4 Layer MiniBoardPro Service, which I was planning to use for the initial prototype, only allows 350 holes, and my layout had 370. In order to eliminate some holes I replaced two 8-pin thru-hole resistor SIPs with eight 0805 SMT resistors, and a thru-hole pushbutton switch with an SMT version -- it now had exactly 350 holes.

But in November 2023 I started seriously learning and using KiCad, and have now ported the Tiny6 design to KiCad 7.0. This will allow me to use other PCB manufacturers who allow more holes, which in turn has allowed me to replace the SMT parts with all thru-hole parts, and to add features such as the I/O expansion bus, power-on LED, and four mounting holes.

The KiCad schematics and PCB layout are now mostly finished. The next step is to create the CPLD design in WinCUPL. If the required functionality doesn't fit into the ATF750C, then the entire project is doomed! But I'm confident that it will all fit, even if only barely.

History

I actually started designing several tiny 6809 single-board computers over 35 years ago. One of them (2.7MB .zip) was exactly the same size as the new 6809-Tiny6: 2.4" x 2.5". But it only had 32KB SRAM instead of 512KB, 16KB EPROM instead of 32KB EEPROM, 1 serial port instead of 2, although it was going to have a full expansion bus, which the initial Tiny6 design didn't have. I never built any prototypes, and didn't create any PCB layout, so don't know whether I could actually have routed all the signals, especially with a 2-layer PCB, since 4-layer boards were still rather expensive back then. Also, I didn't have any PCB CAD software at that time, so don't know how I would have generated the PCB layout. Today's availability of powerful, yet free, PCB CAD software, and of fast, low cost PCB prototype manufacturing, even for 4-layer boards, is wonderful.

"FLEX" was a trademark of Technical System Consultants (TSC).
"6809 FLEX Plus 10" is a trademark of David C. Wiens, dba Sardis Technologies.
"OS-9" is a registered trademark of Microware LP.
"1-Wire" is a registered trademark of Maxim Integrated.

Last revised 2024-May-24 13:13 PDT.
Copyright 2017- David C. Wiens.