The CPU Board
Photos by @dwbrite
Step 1: Before we begin
- This page will walk you through building the CPU Board, which holds the main system bus, blitter, cartridge ports, gamepad ports, and GPIO ports.
- The power board is also attached to the CPU board.
- In general we'll be placing the shorter components first.
- It will help to have a flat board handy, about the same size as the PCB. This can be used to keep parts from falling out as you flip the whole board to solder components.
Step 2: Bypass Capacitors
- The bypass capacitors smooth out the power supply to each chip.
- Every IC gets a 0.1uF capacitor next to it.
- There should be 63 of these
Step 3: Resistors
- 7 x 3.3k resistors (Orange Orange Red)
- 150 Ohm resistor next to LED1
- Polarity doesn't matter but they should be nice and close to the board to leave room for nearby components.
- After soldering and trimming resistors, set a couple of the trimmed leads aside to help with a later step.
Step 4: Big Capacitors
- 220 Ohm capacitor near the power entry pads
- Be sure to align the stripe indicating the cathode (negative side) with the pad that does NOT have a "plus" + symbol
Step 5: DIP Sockets (Prelude)
- The next several steps will insert sockets for chips.
- The "wood board" method will still help if the sockets are still the tallest thing on the board.
- Otherwise, you can bend the socket pins outward to help them stay in the holes before soldering.
- BE SURE TO match the divot on the socket to the divot on the printed socket outline!
Step 12: Mezzanine Connector (Audio Side)
- TX24-40P-12ST-H1E
- It is IMPORTANT that the beveled corner is on the top left to mate with the A/V board (assuming controller ports are oriented downward)
- There are a lot of vias on this board, so be careful not to short any of this connector's small pins to a via.
Step 14: Cartridge Slot
- 36 pin edge connector, with 0.1 inch spacing
- The connection is not symmetrical (don't put carts in backwards) but the connector itself is symmetrical and can be installed in either direction.
Step 15: Molex Headers
- Two locking molex 1x2 header pins
- For RESET_SW_HDR the orientation should't matter.
- For LED1 just make sure that when you attach the opposite connector to the LED wire, that the flat side of the circle aligns with the minus side of the connection.
Step 16: Reset Button Wiring
- 1 x shorter wire with molex plug attached
- 1 x Yellow button (momentary switch)
- Solder one of the wires with a molex plug to the switching pins
- The plus and minus pins aren't needed for this button.
Step 17: Power Button Wiring
- 1 x shorter wire with molex plug attached
- 1 x longer wire with molex plug attached
- 1 x Red button (on/off toggle)
- The shorter wire connects to the plus and minus LED terminals
- Check the orientation of the header from the earlier Molex Headers step. Ensure that the minus side connects to the grounded flat side of the circular LED marker.
- The longer wire is used for the other two pins (switch) and its polarity doesn't really matter
