Open collector wired or chip select

[pzkpfw]

I'm looking at options for address decoding and came up with a question, would appreciate any comment.

Say I used an n to m decoder (e.g. 74LS156), with open collector outputs, but I wanted the decoding to be asymmetric (e.g. to get blocks of 8k, 8k, and 16k, out of 32k), would it be practical to do a wired-or with pull up resistor to combine some decoder outputs to use as a chip select?

Very hypothetical right now, but for background: I just bought a stack of Commodore 1541 disk drives, which I'll borrow the 6502's and 6522's out of, so I believe the context will be 1MHz NMOS stuff.

Cheers!

(And on forum etiquette: should I have one topic for my project, or start a new topic for each distinct question?)

[BigDumbDinosaur]

pzkpfw wrote:

I'm looking at options for address decoding and came up with a question, would appreciate any comment.

Say I used an n to m decoder (e.g. 74LS156), with open collector outputs, but I wanted the decoding to be asymmetric (e.g. to get blocks of 8k, 8k, and 16k, out of 32k), would it be practical to do a wired-or with pull up resistor to combine some decoder outputs to use as a chip select?

You should be able to use the wired-OR method, as long as you understand parasitic capacitance will limit how quickly the chip selects will return to the logic 1 state. My practice has been to use the smallest value pull-up resistor consistent with the sinking capabilities of the device—which isn't much for any 74LS hardware. As an aside, you may be better served here with a 74HCT equivalent to get the stronger fanout of CMOS.

Quote:

(And on forum etiquette: should I have one topic for my project, or start a new topic for each distinct question?)

It's best to start one topic and add to it as your project progresses. That will make it much easier for others to follow along.

[GARTHWILSON]

Welcome!

As BDD said, open-collector outputs are extremely slow to rise back to a logic 1. Take for example a 3.3K resistor pulling up a line that has only 22pF capacitance on it, including all the inputs, traces, and sockets. It will have a time constant of over 72ns!!!

Be sure you go through the 6502 primer, at http://wilsonminesco.com/6502primer/ .

[pzkpfw]

Thank you both.

I've been reading a lot (including this forum, and also the primer) before joining, so both of you have already contributed to my thinking.

The HCT comment even predicted one of my next questions.

Now I need to re-check what's available from local (New Zealand) suppliers. Will surely have more questions.

[BigEd]

Welcome! I would say, it's great to have a (long-running) thread to track project progress, but it's also good, when questions arise which might be interesting to future adventurers, to ask them in a new thread with a good subject line - this forum is a great resource when searching for solutions and ideas, and even posts which are many years old can still be of interest and very useful.

That was a long sentence.

[BigDumbDinosaur]

BigEd wrote:

That was a long sentence.

Not nearly as long as some I've seen.

[Chromatix]

Depending on language, you may occasionally see words about as long!

[BillO]

German comes to mind.

[floobydust]

Ja, das ist richtig!

Then again, there's this:

Donaudampfschiffahrtsgesellschaftskapitän

[Chromatix]

Finnish is also rather word-compounding-fond. One example - which is decidedly not the longest! - is "kuumailmapallolentolahjakortti", which means "hot air balloon flight gift certificate".

[floobydust]

How true.... and of course, there's always the fun of reading German technical manuals, where a word can start on one page and end on the next.

[drogon]

Well, if we're wandering this far, then I give you:

https://www.youtube.com/watch?v=gG62zay3kck

Enjoy!

-Gordon

[joanlluch]

Chromatix wrote:

Finnish is also rather word-compounding-fond. One example - which is decidedly not the longest! - is "kuumailmapallolentolahjakortti", which means "hot air balloon flight gift certificate".

Seen from other languages perspectives, English also sounds as a sort of "word compounding" language, except that 'space' characters are used as glues in the written form. The example above is the perfect example. If we remove spaces we get: "hotairballoonflightgiftcertificate". That's of course not correct written English, but it may well sound ok upon reading. On the contrary, there are languages such as the Latin derived ones that require explicit use of prepositions and connecting words to express the same, so compounding is not possible on these languages, not even by removing spaces. (Sorry this was off topic for the forums )

[ttlworks]

Nice movie, but it can give you quite of a headache.

Hmm... an old song from Reinhard Mey:
Einen Antrag auf Erteilung eines Antragsformulars...

For the non_native German speakers:
The Twelve Tasks of Asterix: that part where Asterix just wants to get 'permit document A38' in a multi-storey bureaucratic building.

;---

He receives a registered letter from the "main document administration north" to send them at once
"A request for issuing an application form for confirmation of invalidness of a carbon copy exemplar which validation annotation origins from the acquisition authority for behoof of the submission at the responsible granting office."
//That's also the refrain of the song. Don't know if my translation is correct.

So he muck-rakes together what he happens to have at home in a shoe box: an X-ray picture, a swimming certificate, a parking permit and a laundry voucher.
Then he unwittingly stumbles into an epic quest which leads him through three bureaucratic buildings, asking questions like:
"An application formular that declares invalidness for submission of validity... no, no, stop, that was wrong...
which validity administration is in a state of issuance... oh well, such a sheet, you know where to get it ?"

[ttlworks]

Well, we happen to have some native German speakers in the forum, so one could ask for help with the "decoding".
//Reading German technical manuals requires having a bigger "stack frame" in your backhead,
//because the meaning of a sentence sometimes only could be "evaluated" after reading it to the end.

What really freaks me out is Chinese text:
You have a block of 5 odd looking symbols, and just setting a tiny dot there into a different position could make the difference
between "my dear valued husband" and "hepatitis". //I'm not making this up.

Considering that a lot of the electronics stuff tends to be made in China nowaday,
we should try to get some native Chinese speakers into the forum, but I'm getting _way_ off topic...

;---

For 1..2MHz PHI2, using open collector outputs with pullup resistors would do,
but sooner or later one would want to rise the PHI2 frequency of his 6502 system,
and then the resistance of the pullup resistors would have to become too small for making open collector outputs a practical thing.
//That problem with the ghost interrupts at 20MHz PHI2 with open collector /IRQ pins.

So I would suggest to start with an address decoding logic like in Garth's 6502 primer. //The link is up in the thread.



If things are getting more complicated, there still is 74HCT138\74HCT139.

Note, that the 6522 has a high_active and a low_active chip select input,
creatively making use of this could simplify the address decoding when building a 6502 computer.

//Ed: you could try to use line feeds for breaking long sentences into more digestible chunks, could you ?