Imported from GitHub: FoxCutter/486Dev · commit 449eb91 · license BSD-3-Clause

Description

Repo for the 486 breadboard computer project

README

486 Breakout Board

SMD parts

The decoupling capacitors for the CPU are made with surface mount parts. This is done because the gap the socket provides between board and CPU is fairly limited, and while Through-Hole parts can be used, they might not have enough clearance. Now these are using 0805/2012 size resistors, and are fairly spread out so there should be enough room even if you've never worked with surface mount parts before.

The capacitors themselves are 1uF (marked as 105), and are fairly inexpensive.

7400 ICs

The pair of 7400 NAND gate ICs provide the logic for generating the BHE#, BHL#, A1 and A0 and can be left off the board if you do not need or would rather generate those signals yourself.

CPU Socket

The components on the board and spaced such that a standard ZIF socket should fit correctly. This is not a guarantee, but it should work.

Otherwise, a 17x17 PGA socket (168 or 169 pin) should be used.

DP0-DP3

These are the data parity lines for each byte in the data bus. The spec sheet advises that if you are not using them to pull them high. This is what the J2 jumper block is for.

These lines have no effect other then on the output of the PCHK# pin.

RDY# and BRDY# jumpers

Jumper block J10 exists as a way to set the RDY# and BRDY# pins if you don't plan to dynamically set them. The recommended configuration is to pull RDY# low, to indicate that the normal data bus access as complete, and BRDY# to high to disable burst access.

Both High and Low will go through a pull-up resistor, so can be overridden by other parts of the system.

IGNNE#, KEN# and A20M# jumpers

Jumper block J6 allows you to force any of these three lines to low, as all three are pulled high on the board. IGNNE# is ignored if the NE bit is set in CR0 (which you should do), and the A20M# is the state of the A20 Line mask, and is not very useful unless you are making an IBM PC compatible system.

NOTE: This jumper is the only place where the A20M# line is exposed on the breakout board.

The KEN# line indicates the cachability of the current cycle. And probably isn't worth asserting the line to ground.

Other Versions of the 486

The breakout board is built around the 486-DX that I own, it should be usable with a later version of the DX chip. Though in that case some of the previously not connected pins will now be connected.

Make sure you cross reference the extra pins against the correct data-sheet for your CPU. I'm hardly an expert and the advice below could turn out to be wrong.

486-SX

The original version of the 486SX has a slightly different pin arrangement, and while it should work if you are careful, I wouldn't recommend it. There should be no risk of damaging the CPU, but it might not function as expected.

Later version of the 486-DX

Later version of the DX add a few new pins, SRESET, SMI#, SMIACT#, STPCLK# and UP#. While these have built in pull up/down resisters, if the cpu you are working on has them, it would be best to pull SRESET low, and SMI#, STPCLK# and UP# high.

486-DX2 And 486-SX2

The biggest change for the DX2 is that the internal clock speed is doubled. It doesn't seem to add any additional pins outside of what was already added above.

The SX2 brought the pins in line with the DX2, so should be fully compatible.

486-DX4

The DX4 adds the ability to control the clock multiplier using the CLKMUL pin durring a reset. So you need to make sure that the pin is properly pulled up/down depending on how you want to use the multiplier.

In addititon, the DX4 is a 3 volt CPU.

3 Volt CPU

This is probably the most challenging chip to work with, as it runs at 3.3 volts and not 5 volts. It can support 5v logic, but extra steps are needed to provide a 5 volt reference input to the J1 pin on the cpu.

Please double check the data sheet before doing this, to make sure things work the way you will want them to!

If you are using the 3.3 volt cpu with 5 volt logic, you must cut jumper JP1 on the back of the board, and attach a 5 volt input to TP1. Failing to do this could cause undefined results.

Test Port

The test port is available on some DX models, and appears to be on all DX3 and DX4 chips. This adds the TCK, TDI, TMS and TDO pins. The test port pins are allowed to be unconnected on some versions of the DX and DX2, but it's probably wise to pull the input lines (TCK, TDI and TMS) high unless you plan on using it.

Write-Back Enhanced

The Write-Back Enhanced version of the DX2 and DX4 add the INV, HITM#, CHACH# and WB-WT# pins. The INV pin should be pulled high, while the FLUSH# pin should be pulled low.

Can I use the 486 Symbol/Footprint in my own projects?

Of course, they are standard Kicad Symbols and Footprint, and you can ues them without credit. I tried to make them as generic as possible for all PGA versions of the DX and majority of SX cpus (the same issues with the SX from above apply). You would need to adjust and add some pins for the QFP versions.