Super-newb here. Been devouring anything I can 6502 related. Really want to build a simple Homebrew for all the usual reasons. I would like to eventually expand the computer so it has a built in "display" created from LED matrices. Initially just an 8x8 multiplexed with a MAX7219 or similar. I would like this display to have two properties:

1) it displays a portion of memory. If a byte of memory that it's displaying changes, that change is reflected on the display immediately.

2) The portion of memory being displayed can be changed so that any address range can be displayed.

I'm.wonderingif anyone has run into similar projects, or might point me in the right direction.

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http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

Welcome! To add, if I can, to Garth's good points... what you propose is something like a front panel, but more so. It's quite complex, and as such, possibly an ideal job for a computer to do! In effect, Garth is saying the same - use an interrupt service routine to update the display. As it mostly makes sense to view memory when the CPU is stopped or is running very slowly, you might write a monitor program which can single-step your main program and update the display each time.

Or, you could decouple the 6502 design from the front panel, and use a second CPU to drive the display - it would be common these days to use an Arduino or a microcontroller, and to write that display update program in C or similar. Or you could use a second 6502... but in this case you'd need to arrange that the second CPU can view the memory of the main system, or snoop the bus of the main system.

If you do away with the idea of a multiplexed display with a smart driver, maybe it's not too hard to hook up an 8 bit latch to the databus and use that to drive 8 LEDs. The latch is enabled by an address decoder, and you'd need some interface to adjust the address decoder. Perhaps use an 8 bit magnitude comparator? If you can do that once, you can do it 8 times to give yourself the 8 byte window you mentioned. The insight here is that you can make an 8 bit output port very simply, and it's OK if the output port is mapped to a location which is also mapped to another device, such as RAM, ROM, or a peripheral.

Reminds me to whygee's DYPLED module for displaying what's on a 16 Bit bus...

If you just wanted a matrix of 64 LEDs (8x8) then 8 74xx164 serial to parallel shift registers is probably the easiest way to do it (software wise). It would appear to the CPU as a single-byte output port. 8 consecutive writes would completely refresh the display. As others have noted this could be accomplished via an interrupt routine or on demand.

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Bill

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

Thanks for all the replies. My initial desire for this display was to be able to examine memory without relying on the computer to manage display, so I was hoping to implement this in a way that runs separately from the 6502 and pulls the led data from the same memory that the computer is using - though I'm not sure how to do that without colliding data on the buses...

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

Ah, that makes sense. Hmm, so if I just invert the clock signal for a second processor to handle the matrix? How important would timing be for something like that? I was planning on 1 mhz for the 6502. 6502 does all of it's read/write in φ2 high?

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

You could manage an 8-byte snooping window with a handful of logic chips, though as has been suggested an Arduino-based snooper would probably be both easier and more scalable.

The simplest thing would be to only snoop the bus writes, in which case each of the eight bytes displayed wouldn't update until written to after boot or after moving the window.
You could add some additional LEDs though to mark which of your bytes are currently valid.

I find the logic chip approach to be a fun exercise though, so here's a few that you could use for this:

• 74x521 Identity comparator
  Using a couple of these you'd match on all but the three lowest bits of the address bus to generate a chip-select for your register.
• 74x138 Decoder
  This is what you'd use for the lowest three bits of the address bus to select which of your 8 bytes is being written to.
• 74x573 8-bit Latch
  Eight of these will store the bits that your LEDs show.
• 74x259 Addressable 8-bit Latch
  If you do use an additional LED per byte to indicate the validity of each byte, this would be convenient for storing that flag.
• DIP Switches
  Not a chip, but you'd need some way to specify the higher address bits that position your viewing window. Though you could also use another 74x573 and control it from your computer directly.

As for connecting the LEDs, you *could* wire them up to the outputs of the 573's but it would be a LOT of wires/traces. Multiplexing the LED matrix would be a good idea here. You'd use another 138 decoder, a counter (74x161 or 74x163 for instance), a multiplexing switch like a CD4051, and some clock source like a 555 timer circuit to rapidly cycle which row is being lit.


If you wanted to show more than 8 bytes you could connect fewer of the address bus bits to the identity comparator and use bigger decoders, more 573's. That gets pretty expensive and unwieldy fast though.