Disclaimer: this is a work in progress

The Processor

The Intel 80386EX processor is a successor to the 80386SX, a stripped down version of the full 80386. The SX uses a 16-bit bus, and is limited to 16Mb of memory. The EX processor is targeted at Embedded applications and, like the 80186/188, includes many of the most useful peripherals on-board. It is derived from the SX in that it supports a 16-bit bus, but it expands the address space by 2 bits to 64Mb. The on-board peripherals include: 2 interrupt controllers, 2 DMA channels, 2 serial I/O ports, 3 timers, FPU port, a synchronous I/O port, 3 parallel ports, and multiple programmable I/O and chip select pins. Many of these peripherals share chip pins, so a designer has to choose those needed in a particular application.

www.retrobrewcomputers.org_lib_plugins_ckgedit_fckeditor_userfiles_image_boards_sbc_386ex-um-cover.jpgUser Manual (4.6mb)

The Package: PQFP-132

The “millipede” surface-mount package is a serious deterrent to use in a hobby project. However, a good workaround is an adapter board, as seen below. This is the second version, smaller that the first, measuring 50mm x 50mm. If these can be supplied, a wider range of hobbyists may wish to tackle the final SBC to come out of this project.


Two 50mm X 50mm adapter boards with 80386EX in place. The pin connections are on a 2mm grid and are 2×17 pins each.

The prototype board SBC-386EX-0


This board take several short-cuts: SRAM instead of DRAM, limited peripherals. However, it is a test bed to learn …

  1. Enabling the slot-15 I/O ports (all of the on-chip goodies); and disabling them, too.
  2. Programming the I/O and memory chip selects. (critical)
  3. Using 16-bit and 8-bit mixed ports.
  4. Accessing & programming the serial I/O port 0 at 0x03F8.
  5. Getting to the timers. Possible automatic determination of CPU clock speed.
  6. Using a new access method to get to the DS1302. Three pins on one of the parallel ports should work.
  7. Enabling & using the 387SX math co-processor.
  8. Accessing peripherals on the ECB bus; i.e., other RetroBrew boards.

DRAM, synchronous serial I/O (SSIO), and DMA will have to wait for another prototype round.

Here's the board 98% built-up, lacking only FPU & RTC crystal:


Project Status

2017-Nov-17: A third round of prototype boards have arrived (see above).Theywillusetheadapterboards, and will check out a subset of the circuitry to be used on any final board. There may be additional “prototype” rounds to check out other circuitry, before committing to a fully functional SBC.

2017-Nov-20: Board running @ 20Mhz (40Mhz osc.) and passing memory test. I switched from a 64KB (28 pins) flash to 128KB flash (32 pins) to avoid positioning errors when strapped for Vcc on pin 30(28). The SST 128KB flash erases and programs faster, too.

2018-Jan-09: Three boards are running now at 25Mhz and 33Mhz. Speed limitation is the FPU, since Intel FPU's only go up to 25. Cyrix FPU runs at 33Mhz, but requires a board mod to handle the READY# line with a 74LS241. The prototype board has served well as a learning experience with this high-integration CPU chip.

–JRCoffman (johninsd at gmail dot com)

boards/sbc/sbc-386ex.txt · Last modified: 2018/05/17 18:00 by jcoffman
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