lvcmos lvttl
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Voltage Levels Description:
Comparison of Input and Output [I/O] logic switching levels for the CMOS, TTL, mixed CMOS/TTL, ETL, BTL, GTL, and Low voltage glue logic families. The graph above provides a comparison between the Input and Output [I/O] logic switching levels for CMOS, and TTL logic families. The graph shows 5 volt CMOS, TTL, and mixed CMOS/TTL IC devices, and 3.3 volt LVTTL LVCMOS IC devices. BTL and GTL [Bus Driver] IC are shown for comparison. Note many Low Voltage [LV] CMOS families are 5 volt tolerant [not damaged by applying 5v to the input pins]. The output logic levels above are defined by the Terms section below. For a review of Noise Margin numbers and a short description of many of the IC logic families , refer to the Logic Family Selection page. A graph for Low Voltage [LV] devices resides on the LV Logic Threshold page. An additional chart of Interface bus threshold levels is provided on the Interface Threshold Voltage Level page.
The GTLP switching levels [not shown above] follows; Output-Low is less-then 0.5v, Output-High is 1.5v, and the receiver threshold is 1.0 volts.
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 Prefix page.
Terms -
VCC: The voltage applied to the power pin(s). In most cases the voltage the device needs to operate at.
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 maximum 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. May also be listed as a '+' or '-' value.
Description of TTL, ECL and CMOS Glue Logic Families