Low Voltage Logic Threshold Levels

Low Voltage Switching Levels

Threshold Levels Description:
Comparison of Input and Output [I/O] logic switching levels for Low Voltage CMOS, TTL logic families. The graph above provides a comparison between the Input and Output [I/O] logic switching levels for Low Voltage CMOS, and TTL logic families. Low Voltage IC families normally run off of 3.3 volts [3.6 volts], 2.5 volts, or 1.8 volts. The graph shows 5 volt CMOS, TTL, and mixed CMOS/TTL IC devices, and 3.3 volt LVTTL and LVCMOS IC devices. Note many Low Voltage [LV] 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 Standard voltage devices resides on the Logic Voltage 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 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 -
V_CC: 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.

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 minimum 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.
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.

Device Families: TTL (74xx) True TTL 74L Low power 74S Schottky 74H High speed 74LS Low power - Schottky 74AS Advanced - Schottky 74ALS Advanced - Low power - Schottky 74F(AST) Fast - (Advanced - Schottky) 74C CMOS...................check Vcc levels 74HC (U) High speed - CMOS (Unbuffered output) 74HCT High speed - CMOS - TTL inputs 74AHC Advanced - High speed - CMOS 74AHCT Advanced - High speed - CMOS - TTL inputs 74FCT (-A) Fast - CMOS - TTL inputs (speed variations) 74FCT (-T, -AT) Fast - CMOS - TTL inputs (speed variations) 74AC Advanced - CMOS 74ACT Advanced - CMOS - TTL inputs 74FACT AC, ACT (Q) series 74ACQ Advanced - CMOS - Quiet outputs 74ACTQ Advanced - CMOS - TTL inputs - Quiet outputs Bus Driver Families 74ABT Advanced - BiCMOS - Technology 74ABTE ABT - Enhanced Transceiver Logic 74ABTH Advanced - BiCMOS - Technology - bus Hold 74BCT BiCMOS - TTL inputs 74BTL Backplane - Transceiver - Logic 74GTL Gunning - Transceiver - Logic 74GTLP GTL (Motorola) Low Voltage Families 74ALB Advanced - Low Voltage - BiCMOS 74LV (U) Low - Voltage (Unbuffered output) 74LVC (R) (U) LV - CMOS (damping Resistor)(Unbuffered output) 74LVCH Low - Voltage - CMOS - bus Hold 74ALVC Advanced - Low - Voltage - CMOS 74LVT (R) (U) LV - TTL (damping Resistor(Unbuffered output) 74LVTZ Low - Voltage - TTL - High Impedance power-up 74ALVC (R) ALV - CMOS (bus Hold) (damping Resistor) 74ALVCH Advanced - Low - Voltage - CMOS - bus Hold 74LCX LV - CMOS (operates with 3v & 5v supplies) 74VCX LV - CMOS (operates with 1.8v & 3.6v supplies 4000 True CMOS (non-TTL levels) ECL Device Families: MEC I 8nS* MEC II 2nS* MEC III (16XX) 1nS* .......* = Rise & Fall Times 101xx 100 series 10K ECL, 3.5nS* 102xx 200 series 10K ECL, 2.5nS* 108xx 800 series 10K ECL, voltage compensated, 3.5nS* 10Hxxx 10K - High speed, voltage compensated, 1.8nS* 10Exxx 10K - ECLinPS, voltage compensated, 800pS* 100xxx 100K, temperature compensated 100Hxxx 100K - High speed, temperature compensated 100Exxx 100K - ECLinPS, temp, voltage compensated, 800pS*

**Related topics:**
Standard Logic Voltage Thresholds	Glue Logic Logic Speed x Power Chart	How to Termination Traces	Ground and Power Planes

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