Which 6502?

The original 6502 was made in 1975, by MOS Technology. It used NMOS technology, which was an improvement on PMOS technology. PMOS used more energy, as power always runs through them. Also, PMOS circuits often needed two positive voltages, a negative voltage and a ground! Also, PMOS circuits required a strong clock circuit and are slower than NMOS. The original 6502 had a maximum speed of 1 MHz.

Later versions used CMOS (65C02), and the latest iteration can work up to speeds of 14 MHz.

I will be using a modern 6502 from WDC (Western Design Centre). These are known as 65C02, the C denoting the use of CMOS technology. One reason for choosing the current 6502 is that it is possible to single-step it, allowing me to see what’s on the data bus. This is possible because this 6502 is ‘static’, the original is ‘dynamic’. With a static 6502 you can stop the clock and single-step it, allowing you to see what’s on the data bus. This cannot be done with a dynamic 6502 because its internal registers would lose data.

A further reason is that it has the potential to be used up to 14MHz.

Also, the original 6502 contained a number of bugs and undocumented instructions. Although these may not cause any problems because these were fixed in the CMOS versions, it’s an added benefit – reassurance of no anomalies from these.

There are so many fakes out there! This is not a problem just with 6502s. Places like ebay are a great place to buy electronics but please be careful, there are many fake ICs out there! You can buy the 6502 from WDC, as well as from many other electronics sites, which will remove the chance of ordering a fake. If you have a device (such as a BBC Master) that you can pull one from there and safely use that. However, you wouldn’t be able to benefit from the ability to stop and single-step the clock, as well as having to running the computer at a much lower clock speed.

BE

I will not be using this pin, so I will be connecting it to +5V, via a 3.3K resistor.

MILB

I will not be using this pin. Since this is an output pin, it can be left unconnected.

SET OVERFLOW

This is a rarely used pin and is not recommended for modern designs. I will not be using this pin and I will connect it to +5V via a 3.3K resistor.

READY

I will not be using this pin, so I will be connecting it to +5V, via a 3.3K resistor.

SYNCH

I will not be using this pin. Since this is an output pin, it can be left unconnected.