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# Macintosh Classic II FPU card
Add an FPU to your Macintosh Classic II. Switchable clock source between the system clock (16 MHz) and a crystal oscillator on the board (40 MHz should generally work).
This repo contains a KiCAD design of an expansion card that adds an MC68882 FPU to your Macintosh Classic II. The board has two layers to keep the PCB cost down. It offers a switchable clock source between the system clock (16 MHz, synchronous operation) and a crystal oscillator on the board (40 MHz, asynchronous operation; tested with MC68882FN40A, the rumours claim 50 MHz will work too).
### Results
The board dramatically improves the Floating Point performance of the computer.
MacBench 1.0 results running @16 MHz (system clock):
### Parts
- 5-532955-8 (A34313-ND)
When using an external 40 MHz oscillator, the Floating Point results improves a bit more (it's not proportional to the clock frequency as at some point the slow 16-bit bus becomes a limiting factor):
### Order a PCB
If you want to order a PCB directly, I have created a model on PCBWay:
### Bill of Materials
| Qty | Value | Device | Package | Parts | Digikey reference |
|-----|-----------------|--------------------|----------------------|--------------------------------------------------------|-------------------------------|
| 1 | | MC68882FN40A | PLCC68 | IC1 | |
| 1 | | PLCC68 socket | PLCC68 | IC1 | |
| 1 | | TE 5-532955-8 | | J1 | A34313-ND |
| 1 | | Pin Header 2.54mm | 3 pins | J2 | |
| 5 | 100nF X7R | SMD capacitor | 0805 | C1-C5 | |
| 1 | 10uF 10V | Tantalum Cap | A (3216) | C6 | 399-4788-1-ND |
| 1 | 51 ohm | SMD resistor | 0805 | R1 | |
| 1 | 40 MHz | Crystal oscillator | DIP-8 | X1 | |
(sorry, I had most components at hand, so I only have Digikey references for the rather specific ones)
### References