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6502 -> 65816




What is it?

This projects tells you how you to make a very small module so you can replace a 6502 with a 65816. Except de-soldering your 6502 (if needed at all), no other soldering to the original board is needed.


The (dis)advantages of replacing your 6502

I received emails like: "I replaced my 6502 but my system didn't become faster?" and "How can I address 16 MB?" So before you start to grep your soldering iron, ask yourself: "Do I need this modification?"

First remark: Replacing a 1 MHz 6502 with a 4 MHz 65816 does not mean that the system suddenly becomes four times faster: a 65816 is as fast as the clock it is fed with!
I have studied the specs of the 65816 to see if it did optimize some opcodes. But IMHO it didn't so it is as fast as a 6502. So no gain here either.

Second remark: The 65816 has more opcodes. And the new ones replace some so called "illegal" opcodes. So if you run programs using these illegal opcodes, you certainly will run into trouble sooner or later.

Third remark: The 65816 is capable of addressing 16 MB. But it needs extra hardware to be able to do this trick! And I'm very sure that a regular 6502 system does not have this hardware.

Disadvantage:
- except not to be able to run the original illegal opcodes, none AFAIK.

Advantage:
- You are able to learn how to use the extra features and opcodes on a system you're familiar with.


The story

In 1985 I could lay my hands on a 65816 processor. During my first experiments to replace the 6502 of a VIC-20 with this uP, it broke down :( In 1997 I could lay my hands on several 65816's for peanuts. So after 12 years I could resume my experiments. This resulted in a small module that you simply put in the place of the original 6502.


The differences

The main differences between both processors are:
Pin     65816   6502
---   -------  --------
 1      VP      GROUND
 3      ABORT   CLK1
 5      ML      NC
 7      VPA     SYNC
35      E       NC
36      BE      NC
38      M/X     SO
39      VDA     CLK2
The 65816 lacks the CLK1 and CLK2 clocksignals so we have to provide them ourselves. That's where two 74F04 invertors comes in view.
ABORT and BE are 816-specific inputs and only need a pull-up resistor. These resistors enable you to use these inputs later. If you are sure you don't need them, just ommit the resistors and connect these inputs directly to +5V (pin 8) when the resistors are mentioned during the construction.
The 65816 has no SYNC or equivalent but we have to supply this output with a level and IMHO the safest level is (L). AFAIK the only system using SYNC is the KIM-1 and the Elector Junior.
The 65816 also lacks the SO-input. The only systems I know of using the SO-input are some of the C= drives like the 1541.


The module

You only need: One of the sockets has to be of the type with the rocket-shape pins (R-socket). If you understand Dutch: "gedraaide voetjes". The other one can be of the type with the two V-shaped metal tongues which grep the pin of the IC (V-socket). The pins of this socket are as thick as the pins of an IC. It also can be of R-type as well but the intention is to use this socket to replace the original 6502 and the rather thick pins of the R-socket could stress the original socket the 6502 was placed in. If you have to desolder the 6502 and want to solder the module to the board, then there is no problem. An advantage of using two R-types is that they are easier to solder on top of each other in a later stadium of the project. Time needed: 2 hours.


Schematics


Using the complete 16 MB range

Please see: 65SC816.





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