Cumbayah wrote:
I am measuring a solid 5V logic H level from the address bus output of a R65C02 that I am doing a project with, but the chip is designed to work at TTL levels and the Rockwell datasheet promises only at least 2.4V for a logic H.
Firstly let's be clear that a chip's input characteristics and its output characteristics are two separate subjects.
Chips from families such as 74 and 74LS fall within the TTL spec for both their inputs and outputs. Their inputs are allowed to require a certain current in order to pull low, they require virtually no current to go high, and the transition point for inputs is around 1.5 volts. Their outputs, as you'd expect, are adequate for driving such inputs.
But they don't over-deliver. That is, the outputs are quite strong (low impedance) when pulling low, but their ability to pull high is rather limited, both current-wise and voltage-wise. And that's OK, because 74 and 74LS series inputs don't require much voltage or current to perceive a high.
Chips from the 74HCT logic family also fall within the TTL spec for both inputs and outputs. The input transition point is around 1.5 volts, and that's what enables an HCT chip to reliably "hear" the wimpy logic High that an actual LS chip will output. (You may wish to refer to my thread,
TTL Compatible... NOT! ( modern WDC CPU's ). I'm attaching one of the illustrations below.)
Output-wise, chips from the 74HCT logic family not only meet the TTL spec; they actually exceed it, and are documented as doing so. That is, you can expect an HCT output in the High state to go pretty well all the way to Vcc, and even source a fair amount of current while doing so. Clearly this is an output that requires no pullup resistor.
You've asked for advice about your R65C02, and I suggest you deal with it just as you would a 74HCT series device, both input-wise and output-wise. To achieve (near) rail-to-rail swings, the R65C02 outputs require no pullups or logic-level converter ICs, even though the datasheet fails to offer you assurances in this regard (ie, the output logic High level). The R65C02 was made at a time when one could safely assume that any CMOS device would have outputs that offer (near) rail-to-rail swings.
Be warned that many
modern CMOS devices powered by 5V
don't feature rail-to-rail output swings. Regarding modern 5V CMOS I
don't recommend that anyone assume that the outputs will over-deliver. Certainly I and other forum members commonly see modern 5V CMOS RAMs and PLDs whose output-High is 3-volts-ish even with no load applied. The datasheet warns you not to expect rail-to-rail swings, and that's the straight poop -- many modern 5 Volt CMOS chips
won't give rail to rail output swings.
It's as if the chip internally runs on approximately 3 volts, and I suspect that is actually the case. Certainly something about the internals is different compared to old school CMOS. To be clear, it's specifically the
5 Volt modern CMOS you need to watch out for. In regard to modern CMOS chips intended to operate on 3.3V or less, I'm not aware of any that don't deliver rail to rail outputs swings.
Despite what the R65C02 datasheet says (actually
fails to say), I'm sure you
can expect rail-to-rail output swings (and you yourself have measured a solid 5V logic H). They just didn't bother to document it (probably because it was a 74LS world back then, and 74LS doesn't require rail-to-rail voltage swings).
-- Jeff