It's a tempting possibility, I agree!

BTW it's perhaps worthwhile to comment on how the SPDT mux/demux generates /OE and /WE in the schematic in my previous post. (For reference I also posted an equivalent that doesn't use a mux/demux.) Here's an excerpt, with the area of interest highlighted.Basically the SPDT "steers" the active-low version of Phi2 to either /OE or /WE.

• This is NOT "open collector" logic, at least not in the usual sense. The pullup resistors are not responsible for returning /OE and /WE to the high state after the end of each cycle. They are actively driven back to the high state during the delay that exists before the CPU then the mux/demux select input can respond. The pullups are only intended to keep /OE and /WE high thereafter. (We don't want them to drift low during single-step operations.) I used 2k2 pullups but a much higher value would've been OK.
  
  
• The mux/demux has, in effect, almost zero propagation delay, and that can make a big difference in some cases. Signals pass through the contacts of the SPDT "relay" as if they were a 10-ohm resistor -- almost the same as a piece of wire. (However, the select input -- the "coil" of the relay -- is subject to normal propagation delay.)
  
  
• A mux/demux can often reduce the package count in a discrete-logic design -- and that's why I used it here. It saves space, and there are fewer connections for me to hook up!

( MAX4619 datasheet attached. There are LOTS of mux/demuxes on the market. 10 ohms is a rather good figure for on resistance; some products are even lower, some are higher. )

_________________
In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
https://laughtonelectronics.com/Arcana/ ... mmary.html

That mux/demux chip does look very nice. Forgive me for not saying so earlier (no one ever accused me of having the best social skills.).

So tell me, please... How do you use your CPLD-ish boot loader? Does TTC (Throw Together Computer) have ROM in it or do you need a PC and the CPLD boot-loader to boot it up? Also, does TTC have an address decoder and any I/O chips?

Um.. ok, if you say so! As for the chip, credit belongs to the folks who originated its ancestor, the 4053. BTW it and the MAX4619 can be used for switching analog signals, including those that swing above and below ground. I don't need that capability, but I do like how they put three SPDT elements in one package. You're guaranteed to find a use for all three because they're so versatile -- configurable as a wide range of gates (Inverter, AND, AND with one inverting input, OR, OR with one inverting input, etc) as well as a decoder or data selector.

No -- no address decoder and no I/O chips. And the CPLD is mounted on a breakout board that mounts onto a pair of wire-wrap headers on TTC itself. Besides providing a wiring interface, this also makes the breakout detachable; ie, recyclable for another project. The 8-pin header on top is for programming, and there's also a small prototyping area -- with fewer holes than I would prefer, but holes are budgeted, and the proto area (a low priority) is where I went broke! The entire breakout project was an afterthought, riding on the coattails of an unrelated PCB order.

Presently TTC does rely on a host PC for bootup, but that could change since the breakout board has a serial EEPROM (and an oscillator footprint, not yet populated when the photo was taken). The CPLD ( CY37064 ) is an obsolete part but I found some locally; also I have the Cypress programming software.

_________________
In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
https://laughtonelectronics.com/Arcana/ ... mmary.html