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When prototyping, I often use spare 7400-series chips I have lying around. I have a good variety of chips, including many 74HC and 74AHC parts, as well as older varieties like 74LS.

From time to time I like to implement "quick and dirty" 5V to 3V3 level conversion, and one easy way to do this is with by interfacing a 5V signal to a 3V3-powered chip with a current-limiting resistor. The idea here is that the "input protection" clamp diodes present on the inputs of a logic chip will conduct any excess current on the input to VDD, limiting the input voltage to the supply voltage (3V3), accomplishing 5V-to-3V3 conversion on the input with only a single resistor.

I've looked around the web, and I haven't been able to find any resource indicating what 7400-series variants include these diodes on their inputs. Of course I can look at individual datasheets, but I have a suspicion that certain variants (e.g. 74HC) usually include them, while other variants (e.g. 74AHC) may not. (these just are made-up examples)

Does anyone know what the general situation is, or if there is any definitive answer for this? And does it depend on the manufacturer?

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    \$\begingroup\$ BTW, I am aware of the myriad of ways of doing level shifting, as well as the fact that the method proposed in this question is often an undesirable approach. Never mind that. I just want to know if certain variants generally have "input protection" (clamp) diodes, and other variants generally do not. TI "Designing With Logic (1997)" seems to indicate that all CMOS devices have them, but I'm not so sure. I seem to recall some 'AHC datasheets warning against any input over VDD, even at minute currents. \$\endgroup\$ Jan 29, 2017 at 21:41
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    \$\begingroup\$ I didn't want to bloat the original question with (actually) irrelevant details that I suspect some people will nevertheless get hung up on. \$\endgroup\$ Jan 29, 2017 at 21:47
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    \$\begingroup\$ @AliChen I have checked specific datasheets, but I really want to know if it's something that can be expected (or not) within certain variants. IOW e.g., all 'HC chips but not in 'AHC chips. That would simply things when deciding which ones to reach for. I suppose I could try to do an exhaustive search of all the TI SN74HCxx datasheets, and compare them with the SN74AHCxx datasheets, but I thought to myself "somebody on stackexchange probably already knows the answer". Besides, that would be for just one manufacturer... or maybe it's "standard", but I don't know how to be sure. \$\endgroup\$ Jan 29, 2017 at 21:49
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    \$\begingroup\$ BTW, for those of you who doubt the possibility that some CMOS 74xx logic variants might lack full input protection (clamp) diodes, see the TI AHC/AHCT Designer’s Guide, page 10. The input circuit does not include a clamp diode to VDD. \$\endgroup\$ Jan 29, 2017 at 22:29
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    \$\begingroup\$ Actually, there are even more sophisticated protection circuits, using dynamic floating references instead of hard VSS and VDD connections, see this patent for example: google.ch/patents/US6674622 \$\endgroup\$ Jan 29, 2017 at 22:39

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NXP's Logic selection guide says (p. 18):

Devices with overvoltage-tolerant inputs do not have input clamp diodes on digital inputs, and can be used to interface to higher-voltage systems without using external current-limiting resistors.

The following families, in alphabetical order, have digital inputs that are overvoltage-tolerant and can be used to interface with subsystems operating at a higher supply voltage: AHC, ALVC, ALVT, AUP, AVC, AXP, CBTLV(D), LVC, and LVT.

over-voltage-tolerant CMOS input

(So AC(T) and HC(T) do have ESD diodes to VCC.)

TI's Logic Guide, Toshiba's Logic ICs catalog, and Fairchild's Logic Selection Guide also mention voltage tolerance or downtranslation as features of these families.

TI's application report Input and Output Characteristics of Digital Integrated Circuits at 3.3-V Supply Voltage shows what actually happens when you go above VCC (apparently, (A)LVC have Zener diodes that clamp at a higher voltage).

For devices with ESD diodes, TI's application report 3.3 V I/O Considerations for Hercules™ Safety MCUs in Automotive and Industrial Environments suggests to use not a simple resistor but a voltage divider (section 3.2).

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    \$\begingroup\$ Thank you, @CL, for finding a definitive reference (at least for TI), rather than relying on the assumptions of "great personal experience". I suspected that something like this was the case... \$\endgroup\$ Jan 29, 2017 at 22:37
  • \$\begingroup\$ Also,the 2018 TI "Little Logic Guide" indicates that the SN74LV1Txx parts include built in level translation (no resistors needed going up or down). ti.com/lit/sg/scyt129g/scyt129g.pdf \$\endgroup\$
    – user4574
    May 19, 2019 at 1:04
  • \$\begingroup\$ @user4574 LV1T devices are just AHCT devices tested for more supply voltages. \$\endgroup\$
    – CL.
    May 19, 2019 at 6:32

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