Threshold Levels Description:
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, 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 Part Number Prefix page.
However the temperature range of the IC has nothing to due with the switch levels shown above.
Low-Voltage Switching Terms -
VCC: The voltage applied to the power pin(s). In most cases the voltage the device needs to operate at.
Depending on the schematic, Vcc could be shown as any symbol to represent the power supply.
VIH: [Voltage Input High] The minimum positive voltage applied to the input which will be accepted by the device as a logic high.
VIL: [Voltage Input Low] The maximum positive voltage applied to the input which will be accepted by the device as a logic low.
VOL: [Voltage Output Low] The maximum positive voltage from an output which the device considers will be accepted as the maximum positive low level.
VOH: [Voltage Output High] The minimum positive voltage from an output which the device considers will be accepted as the minimum positive high level.
VT: [Threshold Voltage] The voltage applied to a device which is "transition-Operated", which cause the device to switch.
The Threshold Voltage may also be listed as a '+' or '-' value.
Normally a Threshold Voltage applies to an edge triggered clock or even a Schmitt Trigger.
Description of TTL, ECL and CMOS Glue Logic Families
Note that all the logic families are listed, not just the Low Voltage ones.
However normally the part number will indicate if the IC operate at a lower voltage by indicating 'LV';
As in 74LVxx, for TTL compatible low voltage operation.
|Standard Logic Voltage Thresholds||Glue Logic Logic Speed x Power Chart||How to Termination Traces||Ground and Power Planes|
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