VCR board: M50950 = MELPS 740 ~= W65C02++

A project log for Repurposing OTP/mask-ROM microcontrollers

Many microcontrollers with internal ROM can be configured to run from external ROM!

Eric HertzEric Hertz 12/04/2022 at 11:432 Comments

Well, sheesh. Apparently I only found *two* such uCs in all those years' scavengings? Can't be...


Well, this one's apparently got a core that's an extension of the 6502.

This, as I recall (or deduce), came from a VCR.

I was pretty stoked finding out its mask-ROM could be bypassed... Had to write it on the board! 

(I coulda sworn I'd done similar on at least a couple other boards. Where'd they go?)

In a way, I guess, that makes this a 6502 in a weird package...

Finding out /that/ it could be bypassed wasn't as simple as looking at the pinout... I'll give you a hint, it's one of these pins.

...Some more hints...

...but, yahknow... that could be for external /RAM/... which wouldn't be uncommon for microcontrollers ala 8051 or ATmega8515...

BAM! "Microprocessor Mode"

Alls yahs gots tahs does is connect the pin called CNVSS (intuitively: "Connect-to-VSS"?) to VCC. Of Course!

So, now we've got a souped-up 6502 to install behind a switch-panel and LEDs.

I gotta say, though, I'm a bit jealous of "Memory Expanding Mode's" multiplexing of the Address/Data lines with GPIO... hmmm...

No wait! They thought of that, too!

So that's that. A 6502 with a slightly wonky pinout, and a few extra features, if you aren't interested in compatibility.


I thought it a bit interesting to see how it was ordered from Mitsubishi:


Bubblewrap wrote 12/06/2022 at 06:51 point

Another 6502 based microcontroller is the Rockwell L39 (and later L28), used in a whole bunch of modems. It's based on the R6500, with some opcode additions and changes compared to a regular 6502, but it comes with a test pin that lets you bypass the mask ROM. You can even sometimes find them socketed rather than soldered down.

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Eric Hertz wrote 12/06/2022 at 08:39 point

You Rock! [Well!]

This sort of info is exactly what'll keep me looking closely at boards. Thanks!

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