Mickey Mouse logic, address decoding, power management, etc.

[No True Scotsman]

No worries. Critique leads to a better product. Unfortunately, in the DIY world as well as in the corporate world, the eternal battle between engineer and bean counter continues apace.

[No True Scotsman]

No True Scotsman wrote:

Mouser doesn't have the 74FCT521 in DIP, Jameco doesn't list it period, and it's insanely expensive coming from the UK on Ebay, at ~23 USD for one chip.them yet

DigiKey has the 74FCT521APC in DIP, if you're willing to pay $314 for 198 of them.

[No True Scotsman]

Now that I'm buying the WDC chip, this bit of advice comes into play:

GARTHWILSON wrote:

All the 65c02's can be stopped indefinitely with Φ2 high, unlike the NMOS 6502's.  WDC's can be stopped in either phase, and stopping with Φ2 low would probably be good for keeping things de-selected (or at least outputs disabled), thus taking less power when you want to get down to microamps.

Referring to Figure 8 from the Rockwell datasheet again, I would need to remove the inverter between the clock and the NAND gate to stop the clock on Φ2 low, correct? (I'm using a 1MHz oscillator in a can, not the clock shown in Figure 6.)

[BigDumbDinosaur]

GARTHWILSON wrote:

...WDC's can be stopped in either phase, and stopping with Φ2 low would probably be good for keeping things de-selected (or at least outputs disabled), thus taking less power when you want to get down to microamps.

I suspect that wouldn’t quite work as described, due to the WDC MPU’s static core.

The 65C02 stabilizes the address bus during Ø2 low—on the 65C816, for example, that happens right around 12ns after the clock’s fall.  Hence the addressed device will see a chip select despite Ø2 having been halted in the low phase.  In order to not generate a chip select with Ø2 continuously low, logic would have to be added to either include Ø2 in address qualification (definitely not recommended, and won’t work with 65xx I/O hardware), or to have the glue logic stay quiescent when the clock is intentionally halted low.

While either method would somewhat reduce power dissipation, I have to question whether it would be enough to offset the power that would be consumed by the logic that is monitoring the clock state.

[No True Scotsman]

BigDumbDinosaur wrote:

While either method would somewhat reduce power dissipation, I have to question whether it would be enough to offset the power that would be consumed by the logic that is monitoring the clock state.

I presume at this stage that the Arduino Nano ESP32, a 3.3V device, will both stop and restart the clock in response to conditions to be decided. (At this point, I'm leaning towards the user manually putting the system in standby mode via pushbutton, as that would eliminate complex code that second-guesses, and thereby annoys, the user.) The Arduino will be constantly running while the system is powered up, regardless of whether the 65C02 clock is stopped or not.

As an aside, I've been thinking we shouldn't refer to this as "stopping the clock," since the clock isn't actually being stopped, but rather blocked.

[No True Scotsman]

I forgot to mention that the Arduino will also dim the TFT backlight and has its own power management features. So it won't be running full blast all the time.

I placed my Mouser order for the parts needed to facilitate a successful build. I take advantage of quantity discounts on 74xx and the like to get the most from the shipping fees. My total order was $88.88 for the following items:

1 x W65C02 (replaces suspect R65C02)
1 x Arduino Nano ESP32 (replaces older DEVKIT V1)
10 x 74F521 comparator (replaces 74HC866)
10 x 74HCT138 IO decoder (vs. HC138, per recommendation)
10 x 74HCT139 RAM/ROM & RD/WR decoder (vs. HC139, per recommendation)
10 x 6N138 opto-isolator (for plug-in MIDI adapter)
10 x CD74HC193 counter (not related to this project)
10 x CD74HC165 shift register (not related to this project)

I'm buying the Saronix oscillator for my DUART from a US seller on Ebay who has quite the selection of unusual parts. The listings of sellers like these are like going to a swap meet. You never know what cool stuff you might find.

[BigDumbDinosaur]

No True Scotsman wrote:

10 x 74F521 comparator (replaces 74HC866)

Not a good choice, in my opinion.  74F logic is power-hungry and has TTL outputs, not CMOS.  You need to be careful about mixing logic families.  It might work...or not...

[No True Scotsman]

It's either that or the HC866. I'm not buying any more comparators, nor making any more substitutions.

I was under the impression that when people throw recommendations for individual parts out there, they're taking into account the other parts it will have to play with.

[AndrewP]

The /P=Q output from the F521 looks like it's only connected to the HCT138 and HCT139. They're both TTL level inputs so it will be fine.

[BigDumbDinosaur]

No True Scotsman wrote:

It's either that or the HC866.  I'm not buying any more comparators, nor making any more substitutions.

How about rethinking the glue logic to see if there is another way to accomplish what you want?  It’s been my observation that parts that are only available in the older or obsolete logic families are not used much because other, more efficient, ways have been devised to accomplish the desired task.

Quote:

I was under the impression that when people throw recommendations for individual parts out there, they're taking into account the other parts it will have to play with.

Presumably, but it’s the old “trust, but verify” thing.

Sometimes recommendations don’t take into account the subtle differences between logic families or a particular logic family’s characteristic behavior in certain situations.  For example, I could recommend, “use a 74AC... something or other” in your unit.  I may be presuming you know about the aggressive outputs of 74AC devices and their possibly deleterious effect on overall circuit behavior.  You, in taking my recommendation, should do your due diligence to determine if that aggressive behavior will be a problem in your design.

AndrewP wrote:

The /P=Q output from the F521 looks like it's only connected to the HCT138 and HCT139.  They're both TTL level inputs so it will be fine.

Yes, that is true.  However, the 74F series is power-hungry, yet has weak fanout, limited to sourcing about 1 mA (sinking is much better, as is typical of TTL devices).  Weak fanout can cause trouble with high-going signals, due to the need to charge parasitic capacitance.  The result is the high-going signal tends to be rounded, not sharp, something that CMOS devices without Schmitt action on their inputs might not like.

As I siggested above, the logic should be re-evaluated to see if the identity comparator can be designed out.

[No True Scotsman]

Whether the comparator can be factored out or not, I'm done buying parts. It would be more cost-effective to buy a kit at this point. It's not as though I need to "learn how computers work" or anything like that. I maintained a transistor computer in the Navy. Troubleshooting faults at the transistor level was my job. I poked programs into the registers with my fingers, three bits at a time. I have a pretty good idea of what's gong on under the hood.

I really just want something to write on that doesn't get on my nerves.