6502 traces of 6501

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(Back to 6502 Observations)

The 6501 was the original product from MOS Technology (who, as a team, had recently left Motorola where they worked on the 6800.) It was pin-compatible but not code-compatible, and was withdrawn in a deal which left MOS free to sell the 6502 (even though it has many resemblances to the 6800)

There are several places on the NMOS 6502 where the layout shows the history: it is a modified 6501 and not an independent design.

data pads

Data Bus Enable

The 6502 doesn't have the DBE input found as pin 36 on 6501/6800. But the signal exists on the chip - it's tied to cclk, which is the chip-internal version of phi2. (This is why the 6502 tristates the data bus for approximately the first half of each clock cycle)

Here's the location of the via which ties the two signals together.

Here's a picture showing the two signals running in parallel, even though they are on 6502 the same signal. See how they even interleave. This would represent a waste of space and effort if it the 6502 was an original design, but it's a minimal change to the 6501.

Here's a location where the Rockwell version of the chip has an extra short, compared to the MOS 6502D (and also showing that Nintendo's 2A03 has the MOS version of the layout) Compare-r6502-balazs-2a03-dbe-short.png


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