Logic Threshold Voltage Levels

TTL Logic switching levels and a comparison of the different logic families.
The threshold Level, logic level or transition point is shown to the left of the voltage range.

The CMOS families [74ACxx, 74HCxx, 74AHCxx, and 74Cxx] have different input and output switching levels than the TTL logic devices [74Fxx, 74Sxx, 74ASxx, 74LSxx, and 74ALSxxx]. The output switching levels from CMOS ICs is higher then a TTL IC, which happens to result in a better design and does not have a negative effect. However the difference in input logic switching level does impact your design, the TTL output does not correctly switch a CMOS input. The switching difference between a TTL IC output and a CMOS IC input has to be accounted for.
The mixed CMOS/TTL [74ACTxx, 74HCTxx, 74AHCTxx, and 74FCTxx] logic devices have TTL logic input switching levels and CMOS output switching levels. The mixed TTL/CMOS devices are CMOS devices which just happen to have TTL input trigger levels, but they are CMOS ICs.

So the rule is, if you use a CMOS IC for reduced current consumption [for example], and a TTL IC feeds the CMOS chip, then you need to either provide a voltage translation or use one of the mixed CMOS/TTL devices [which have a "T" in the part number].

Side Note: I used 74xx part numbers above. The 74xx families [or just 74 prefix] refer to a commercial operating temperature range.
A 54xx part number [or just 54 prefix] may also be found which refers to a military operating temperature range. Some 74xx device families may also work at the Industrial temperature range [but you have to check the data sheet, there is no hard and fast rule]. So a 74xx244 works at a commercial operating temperature, while a 54xx244 which is the same device [maybe the same pinout, and package] will continue to operate to the military operating temperature range [which is wider].
The difference in operational temperature ranges is provided on the Logic Prefix page.
The logic switching level does not change between the different temperature ranges.

An important point to keep in mind is that these threshold levels are fixed. However the voltage level is not fixed if the ground is moving, the levels apply to zero volt ground. If an IC is drifting above ground because of current in the ground pin or if voltage shifts between ICs are occurring because of different ground potentials than the static voltage levels shown above no longer work. The same applies to excessive noise voltages occurring near a threshold level. The transition voltages are separated by guard bands, but they only work when not effected by noise. Voltage spikes could cause the actual logic transition to shift in time as the output of one IC becomes effected by coupled voltages from another source, changing the point i time that a voltage level is reached..

Switching Terms -
V_CC: The voltage applied to the power pin(s).
In most cases Vcc is the voltage the IC needs to operate at [see note below].
V_IH: [Voltage Input High] The minimum positive voltage applied to the input which will be accepted by the device as a logic high.
V_IL: [Voltage Input Low] The maximum positive voltage applied to the input which will be accepted by the device as a logic low.
V_OL: [Voltage Output Low] The maximum positive voltage from an output which the device considers will be accepted as the maximum positive low level.
V_OH: [Voltage Output High] The maximum positive voltage from an output which the device considers will be accepted as the minimum positive high level.
V_T: [Threshold Voltage] The voltage applied to a device which is "transition-Operated", which cause the device to switch.
May also be listed as a '+' or '-' value. Note most integrated circuits are level sensitive and not threshold sensitive.

Return to the Logic Design page.

The voltage levels depend on the value of Vcc, so if the power rail varies so would the input threshold level.

Description of TTL, ECL and CMOS Glue Logic Families

The pull-down table lists all the possible TTL and CMOS families.
However not all IC functions are available in all logic families, as some of the early TTL families are reaching obsolescence.
It's difficult to say what parts may still be purchased in some of the early logic families that were released first, but safe to say that the generic TTL family is obsolete.

Vcc Note; The power supply voltage could be called almost anything on a schematic.
The smaller the schematic the more control the designer has over the naming of the supply voltage.
In fact the absolute voltage may be identical on many ICs, but because of regulators or inductors may have a different symbol.
For example; the voltage supply net [Vcc] on one side of a filter inductor needs to have a different name [5v], so the nets don't connect to each other.
Because Vcc effects the input voltage level, the power supply should always be filtered and by-passed.

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