Emulate an EPROM - How Hard Could it Be? -- Episode 517

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Emulate an EPROM - How Hard Could it Be? -- Episode 517

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The Apple IIe uses a custom microcontroller and ROM chip to put ASCII values into the computer’s memory. How hard could it be to replace that dual-chip setup with a microcontroller to emulate another kind of keyboard? Bald Engineer tries using an Arduino Mega 2560, Teensy 4.0, and a Pi Pico to replace a vintage ROM chip. See what works AND what does not!

In the attached zip file are:

• KiCad files for the ROM Interposer Board
• Arduino code for ROM test
• Teensy code to test interrupt latency
• Python code that can respond to an Output Enable signal

A couple of notes that keep coming up:

Q: Why do you need to emulate a ROM chip anyway?

A: Like most 8-bit computers, the keyboard interface eventually talks directly to the memory bus. In an Apple IIe (and IIc), that is done through a ROM chip.

Q: Why not just replace the keyboard matrix?

A: The final design will not use the original keyboard microcontroller or ROM. Ideally, I want to have an interface for a USB keyboard (and possibly Apple Desktop Bus: ADB.)

Q: The Teensy was only 200 nanoseconds, why not just use that?

A: The Mini-Apple IIe is based on the MEGA-II ASIC from an Apple IIgs. Its timing is tighter and slightly different from the Apple IIe’s MMU. The Teensy is slightly too slow.

Q: What is “Golden Delicious”?

A: Originally, I was naming this project “Golden Delicious.” So, let’s call it a code name. Now, I call it “Mini Apple IIe.” But, the final project has a different name.

Supplemental Content:

• Workbench Wednesday 18: Digilent Analog Discovery Review
• Workbench Wednesday 16: Instrument Basics: Logic Analyzer
• Keyboard ROM (26-pin) Interposer Board: https://github.com/baldengineer/Golden-Delicious/tree/master/kicad%20files/ROM%20Interposer%20board

Bill of Material:

Additional Parts:

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