Post by @gloriouscow@oldbytes.space

This is a Harris 80c286 CPU connected to an Arduino Giga.

Many of my followers have seen this before, but I've gained about a thousand since I last posted about this stuff, so this is new to a lot of you.

Decoding of the 286 microcode has hit the point where we need to start doing some actual hardware research to figure out the missing pieces.

This rig allows us to single-step the CPU and control all the relevant pins.

We can set up CPU state with the 286's LOADALL instruction, and we can dump the CPU state with its STOREALL instruction.

#retrocomputing #reverseengineering #arduino #arduinox86

a photo of an Arduino Giga connected to a breadboard containing a Harris 80c286 CPU on a PLCC68 to DIP-68 adapter.

#arduinox86 #arduino #reverseengineering #retrocomputing

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gloriouscow

@gloriouscow@oldbytes.space 13 hr. ago

So on the off chance anyone ever finds this thread in the future, let me save you the trouble. Here's a 286 to DIP68 pinout.

a diagram showing the 80c286 to DIP68 pinout as placed in a chipquik PLCC68 -> DIP68 adapater

gloriouscow

@gloriouscow@oldbytes.space 13 hr. ago

I did eventually design a fixed PCB, which I might even order someday.

This design incorporates the 82c284 as well, since we've learned how crucial that is.

Also please appreciate my 'idiot arrow' so I don't put the socket in the wrong direction.

I've been averse to ordering any more ArduinoX86 shields because I thought I'd be moving to FPGAs, but my adventures in ground bounce on the DE-10 nano have me reconsidering.

a 3d rendering of an Arduino X86 286 shield

gloriouscow

@gloriouscow@oldbytes.space 11 hr. ago

Here's a project I will eventually share on my GitHub - Kicad symbols for the Arduino Giga's GPIOs broken out by GPIO register.

This can help you plan your traces so that you can hopefully minimize the number of port reads you need and the amount of bit-twiddling you have to do.

The annoying thing is that there's no full, clean 16-bit GPIO register to read, which would have sure been nice since we have a 16-bit data bus.

The best you get is register J which has 15 GPIO. Arduino just had to steal one of them for the green channel of the onboard RGB LED. why :(

a screenshot of Kicad's schematic editor showing the Arduino Giga broken into individual symbols by GPIO register

~2 more replies

gloriouscow

@gloriouscow@oldbytes.space 9 hr. ago

Let's get real stupid with this.

a screenshot of KiCad's PCB editor showing a 45 degree rotated PLCC68 socket and curved traces connecting it to Giga GPIO headers

gloriouscow

@gloriouscow@oldbytes.space 9 hr. ago

Optimizing to register instead of pin location makes for a bit of a mess, so we might as well have fun with it