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PostPosted: Wed Sep 05, 2018 11:13 am 
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Joined: Tue Sep 03, 2002 12:58 pm
Posts: 325
Atlantis wrote:
Is it specified somewhere in the manufacturer documentation?


Yes, but not in those words. Ioh and Iol in the "DC Characteristics" table is the information you want. They show how much current you can pull from an output while keeping its voltage above/below the given threshold. Look up the corresponding input currents on the chips you want to drive, and their voltage thresholds, and if the 6502's output can provide sufficient current without dropping below the required voltage, that part is OK.

Then you need to look at the input capacitance of the devices you're driving, and make sure the output current can charge them to the threshold voltages in sufficient time.

Or, if everything in your design is CMOS, you can ignore all of that, because for small low frequency designs, CMOS is a close enough approximation to infinite drive and zero load that it doesn't matter.


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PostPosted: Wed Sep 05, 2018 7:02 pm 
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Joined: Fri Aug 30, 2002 1:09 am
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Location: Southern California
Atlantis wrote:
GARTHWILSON wrote:
    You will not need bus transceivers with a CMOS 6502, especially WDC's current production W65C02S.

Is it specified somewhere in the manufacturer documentation? I can't find any mention in the WDC65C02S datasheet.
In R65C02 datasheet it was mentioned, that address bus line can drive single TTL input, which is basically the same as in original NMOS version.

The data sheets don't let on how strong the outputs really are. They have the usual boilerplate language of driving a standard TTL input or whatever, like 1.6mA low at .4V and 400uA high at 2.4V, even for the 65c22; but related, on the 65c22 VIA, I have tested one I/O pin at a time of Rockwell's R65c22 and the WDC W65C22S outputs here (note the WDC ones having the "S" ending), and they are many times as strong as the data sheet says (and I suspect the processor has the same pin driver circuits). In my experiment, WDC's outputs were able to pull to within 0.8V of either rail with a 220-ohm resistor to the opposite rail, meaning a 19mA load, even pulling up, and give 50mA into a dead short. Rockwell's could pull down with 100mA into a dead short, but could not pull up as hard, not being symmetrical like WDC's. WDC's data sheets have always been poor, but fortunately they always err on the conservative side, sometimes to the extreme. As we've said elsewhere, just because the parts do actually do better than specified does not mean they're guaranteed to. They're not. However, unless WDC changes their pin driver circuits (which I'm confident they never will), I think we can depend on the outputs being much, much stronger than the data sheet lets on.

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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?


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