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).Theywillusetheadapter boards, 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.

–JRCoffman (johninsd at gmail dot com)

boards/sbc/sbc-386ex.txt · Last modified: 2017/11/20 16:34 by jcoffman
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