Edit:jmp(FFFA) wrote:
most CMOS hardware will actually switch much closer to the 50% mark (good experiment to try, BTW) meaning you can often get away with driving CMOS logic with TTL logic at 5 volts.
Six years later, I *did* try this experiment! In 2021 I actually measured the switch point of the inputs on three 65xx microprocessors.
As jmp(FFFA) pointed out, attaching TTL outputs to CMOS inputs may often work in practice. But it isn't a robust combination unless the CMOS inputs have a switch point (aka input transition point) well below 50%, as with the inputs of 74(A)HCT parts. Although the Rockwell 'C02 I tested did have a lowered switch point suitable for TTL levels, the experiment indicated that the WDC CPU's under discussion in this thread did not.
WDC datasheets do certainly have a reputation for errors, but in this case I believe there is no mistake. The inputs of the WDC 'C02 and the WDC '816 I tested both switched at about 50%, corresponding to 74(A)HC parts, not 74(A)HCT. For details, see TTL Compatible... NOT! ( modern WDC CPU's ).
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It's maybe helpful to think about the simplest case, which is just Chip A driving Chip B. A's output will switch between logic-high and logic-low. The logic-high voltage and the logic-low voltage must satisfy the input of Chip B -- they must be "loud enough" for B to "hear." Unfortunately TTL outputs typically put out less than 3 volts when high. But TTL inputs are engineered to receive this -- TTL inputs have extra-good hearing. (Sorry about the corny analogy!
)
CMOS driving TTL presents no voltage problem, because CMOS outputs switch virtually rail to rail, and TTL inputs respond to less than 3 volts.
TTL driving CMOS does present a voltage problem, because as noted TTL outputs may supply 3 volts or less when high, and ordinary CMOS inputs require about VDD × 0.8 -- ie; 4 volts when VDD is 5. (A pullup resistor will raise the voltage as required, but that's OT.) To recap, CMOS driving TTL presents no voltage problem. But TTL driving CMOS
does present a voltage problem because ordinary CMOS inputs are somewhat deaf (as compared to TTL inputs).
Luckily it's possible for a manufacturer to alter the response of a MOS or CMOS input, and that accounts for the difference between logic chips from 74HC and 74AC families as compared to 74HC
T and 74AC
T families. Inputs for the latter are altered to accept TTL voltage levels.
The same altered input response is commonly featured on CPU chips, including the original NMOS 6502 --
and BTW all 65c02's except WDC. For example Rockwell 65c02's accept TTL compatible levels. WDC is an outlier in this regard. I suspect WDC omitted TTL-compatible inputs because of conflicting factors pertaining to battery operation (ie, Vdd below 5 Volts) but my guess could be wrong. In any case the need for TTL-compatible inputs has largely disappeared, thanks to 74LS being replaced by CMOS equivalents whose outputs switch virtually rail to rail.
jmp(FFFA) wrote:
In practice, however, most CMOS hardware will actually switch much closer to the 50% mark (good experiment to try, BTW) meaning you can often get away with driving CMOS logic with TTL logic at 5 volts.
If you want to build a more robust system incorporating both logic families, have a look at using (A)HCT parts to interface from TTL outputs to CMOS inputs.
Agree. Sorry, jmp(FFFA), I missed your post while I was typing my own.
jmp(FFFA) wrote:
Speaking of 6809s, isn't the 63C09 a CMOS variant, or is it merely an HMOS variant?
The
6309 (no "C") is a Hitachi super-6809, implemented in CMOS. It eliminates many of the 6809's all-too-numerous dead cycles and features a far more powerful programming model -- including extra registers and several 32-bit operations (!).
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In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
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