Logic Threshold Voltage Levels

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 -
V_CC: The voltage applied to the power pin(s). In most cases the voltage the device needs to operate at.
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.

Description of TTL, ECL and CMOS Glue Logic Families

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 Plus 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 comp., 800pS*

Back to the Logic Design Page

**Related pages on this site:**
Logic Family Slew Rate .	.Description of Company Prefixes .	.Glue Logic Speed x Power Chart .	.How do I Termination Traces .	.Ground / Power Planes .

Engineering Design key words: TTL Switching Levels, Glue Logic Families, CMOS, TTL, ECL, Speed, IC, Integrated Circuits, Propagation delay, Logic Types, logic switching levels, Output voltage, Low-Voltage, Threshold Voltage, 74xx, 74AC00, 74HC, 74AHC00, Definition for Logic family, Logic Terms, TTL: Transistor Transistor Logic, ECL: Emitter Coupled Logic, CMOS: Complementary Metal-Oxide Semiconductor, BICMOS: Bipolar Complementary Metal Oxide Semiconductor


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