Make It With Matty

6502 Part 3 — Adding ROM In  Part 1  we powered up the 6502 and watched it running a NOP test with just the CPU and LEDs on the address bus. In Part 2 we created a reset circuit and a simple 555 timer as a clock for the CPU. Now it’s time to let the 6502 actually run code from ROM . In this part you’ll: Add a 32KB ROM (AT28C256). Use a 74LS00 NAND chip for simple address decoding so you’re ready for RAM + VIA later. Capture the bus with a logic analyser to prove the CPU is fetching and executing your program. No inputs, outputs, or RAM yet just a clean, minimal “CPU + ROM + decode” core. You can watch the video of this build followed by a description and analysis of what has been built. The video is at the end of this post. 1. The Plan We’ll use a very simple memory map that matches what you’ll want later: $0000–7FFF → future RAM $8000–FFFF → ROM Right now you only populate the ROM, but the 74LS00 will generate both selects so RAM drops in easily in Part...

6502 Retro Console Part 2 Add a Reset and Clock Circuit In Part 1 we powered up the 6502 and watched it running a NOP test with just the CPU and LEDs on the address bus. Now we’ll give it a clock and a clean start every time you power on, rather than relying on a function generator. In this post we will cover a simple 555 Timer Clock circuit and reset circuit. 555 Timer The 6502 needs a steady pulse on its Φ0 (pin 37) to step through instructions. A NE555 timer is perfect for low-speed testing reliable, simple, and easy to wire. Parts List NE555 timer 1 kΩ resistor 10 kΩ resistor 100 nF capacitor (timing) 0.1 µF capacitor (decoupling) Breadboard + jumper wires Circuit Diagram How to Wire It Add a 0.1 µF decoupling cap across the +5V and GND rails stability. Typical speed: ≈ 7–10 kHz perfect for a visible NOP test. Use larger resistors or capacitors to slow it down. Tip: Swap the timing capacitor for 1 µF and the 10 kΩ for 100 kΩ to step slowly enough ...

 The Simplest Working 8-bit CPU Circuit Before we start building a full retro console, let’s do something beautifully simple get a 6502 CPU to run,  no RAM, no ROM, no glue logic, just the chip, a few LEDs, and a clock. This is called a NOP test (No Operation). It’s the digital equivalent of “does it breathe?” What You’ll Need Part Purpose MOS 6502 CPU The heart of our system Bench power supply (5 V) Power for the CPU Function generator Clock source 8 × LEDs To watch address lines change 8 × 470 Ω resistors Current-limiters for the LEDs 1 × 0.1 µF capacitor Decoupling between VCC and GND Breadboard and jumper wires For connections MOS 6502 Cpu Pin Out Check your IC and its datasheet to ensure that you have the correct pin out map as the steps below may vary slightly. Step 1 — Power and Decoupling Give the 6502 a clean 5 V supply: VCC (pin 8) → +5 V VSS (pins 1 and 21) → Ground Place the 0.1 µF capacitor between VCC and VSS close to the chip. That tiny capacitor...