All This to Get Back Where It Started

 All This to Get Back Where It Started

Storing scrap PCBs is all well and good but what do you do with them? After buying a vacuum desoldering machine (wonderful piece of kit), I decided to dismantle the scrap PCBs that I'd been holding on to for years. One of the PCBs I removed components from was a VCR which had a large vacuum fluorescent display on it:

Underneath the display was a controller chip:

It is a Mitsubishi M50957-199SP. I didn't know what that was, but fortunately there is data on the web about it, and it turns out to be a 6502 micro-controller that goes by the family name of MELP. It's a mask-ROM device (the 199SP denotes the actual code loaded into the ROM) and has several peripheral devices embedded in the package as well as the 6502 processor. It's in a slightly odd 64 pin package with a smaller than 0.1 inch pin pitch. It has built in high voltage drivers for the VFD, which would be nice, if it wasn't mask-ROM based. That is a real show stopper if you want to do anything with these chips. Mitsubishi, though, built in a 'microprocessor mode' which can be set up at reset by pulling a pin high. That then causes the address and data busses to be sent out of what would have been IO ports, together with a couple of control lines (RW and SYNC). That allows the device to run off entirely external memory. OK, we can do something with this device now...

I really only wanted to fiddle with the chip, so started thinking along the lines of a 'trainer' or development board, something like the Sinclair MK14 microprofessor or AIM 65. After a few days of musing and then a couple of weeks of PCB layout I came up with this:

Well, yes, there's two of them. The one of the left is the first populated PCB, and it has the VFD and MELP from the VCR. There's a keypad made from MX style keys, they are much nicer to type on than tactile switches. The VFD requires a high voltage to run, so there's a DC-DC converter for that. The VFD filament requires a driver circuit so I included that, but ended up just using 3V3 on this VFD. The high voltage is switched to the VFD using two MAX6921 driver chips. There's an SD card reader for storing programs and an OLED display for extra information when a VFD is fitted and as the only display if a VFD isn't fitted. I only have one of these VFDs, after all. The VFD is mounted on a plug-in adaptor board (perfboard here) so in theory you could attach a completely different VFD if you had one. 

The whole thing is pulled together with an ARM STM32H742, which attaches to the external address and data bus of the MELP and emulates external ROM and RAM (I suppose it actually only emulates RAM, but the MELP doesn't know that). This ARM is a bit of a beast, it runs at 480MHz, has 2MB of flash and 1MB of RAM. It was a bit of a pain to get going as it doesn't have very good (or indeed any) support in the Arduino IDE. I ended up adding my own board definition to the IDE for this PCB. It's odd that there's no generic STM32H7xx board as there is for the STM32Fxxx devices. I suppose they are more common due to the blue pills. This device is also quite pricey, at about £9 each. I noticed the price when I inadvertently ran one chip at 5V for a few days. It turns out that they will run at 5V, but not for long... Then you have to spend an hour or so desoldering a 100pin QFP before fitting a new one.

The VFD has a lot of segments, and so I had to fit two MAX6921s. 

It means there is plenty of scope for displaying useful information about the address and data buses.

So far I've built some code that drives the 6502 through a short program. This program sets some configuration bits and then dumps memory to a fixed address. I wanted to see if I could change the MELP mode to one that would allow access to the mask ROM within the device, but it does look like the internal ROM is not accessible in any mode that 'microprocessor mode' can access.

The code does, however, drive the MELP and supply data from emulated memory when it needs to. You can see the program running in this video:

As I had five PCBs I had a look on eBay for more MELP chips, and did find some. They are slightly different, M50959-133SP, which has more on-board RAM and a different mask ROM. They do work though, so I have enough for the other PCBs.

I can also use this PCB to test VFDs as it has segment/digit drivers and a high voltage adjustable supply and a filament driver. All I need to do is make an adaptor board for each different display type.

As well as a trainer for the 6502/MELP, I'm thinking of adding code that allows this to be an ARM trainer as well. It's probably not going to be as simple to look at ARM peripherals and do a spot of machine code programming as the 6502, but could be fun fiddling with a 480MHz processor at machine code level. The SD card reader means that you don't lose anything you've done, and you can even build large programs on a PC then transfer it over on a card.