5/7/2021 0 Comments Logic Voltage Levels
The 74HCTxx series were High-Speed CMOS but were voltage-compatible with their TTL counterparts.
![]() Logic Voltage Levels Series Were HighThis instructable will briefly explain the two logic families of ICs you will most likely be encountering and using in your circuit designs and how to ensure that the HIGH or LOW output of the other is translated and interpreted correctly between a TTL and CMOS device. This process is called voltage level shifting and is the subject of this instructable. Turn the page and read up on the two logic families that you have probably already implemented in your designs. Logic Voltage Levels Download Step 1Add Tip Ask Question Comment Download Step 1: TTL Integrated Circuits TTL ICs -- members of the Bipolar Logic Family -- were first developed in the 1960s and are made with transistors, hence the name Transistor-Transistor Level (TTL) device. The original line were 74xx series and have since been replaced with better performing TTL devices, such as the 74LSxx, 74ALSxx, and 74Fxx series. The LS is a Low-power Schottky, ALS is an Advanced LS and F is Fast. The bipolar logic family, as the TTL series falls under have had consideral improvements, the first being the 74H which provided twice the speed of the original 74xx series, but at a cost of over twice the power consumption. The next improvement was the74L which increased all the internal resistances, leading to a net improvement in power consumption, but increased the propagation delay. A key improvement came with the 74S series which placed Schottky diodes across the base-to-collector junctions of the transistors. Capacitive effects were reduced and yielded a speed increase of a factor of 5 with about a two-times increase in power consumption. The above mentioned lines 74LSxx were derived from this series and reduced power consumption by about 13. The 74F series, based on a new technology, reduced the propagation delays and even further reduced the size of the IC. Add Tip Ask Question Comment Download Step 2: CMOS Integrated Circuits There was an alternative to TTL ICs being developed at around the same time and these type of ICs were called CMOS (Complementary MOSFET) because of the inclusion and use of a high-impedance MOSFET. The high-impedance of the MOS circuit reduced the power dissipation of the circuit by a significant amount. ![]() The CMOS line also had a much wider operating voltage range (3V to 18V) and had a specific requirement for the voltage meaning HIGH and LOW, for both input and output to other integrated or digital circuits (which I will cover later). The 4000 series, while offering greater power requirement reduction, were considerably slower than its TTL counterparts and were particularly susceptible to static discharges and were easily damaged by static electricity. The 74Cxx series were made to be pin-for-pin compatible with the TTL IC of the same name (ie 74138 74C138). Also, later was introduced the 74HC and 74HCTxx series of CMOS ICs. This High-speed CMOS (HC) line was also pin-compatible with the TTL series but still used the same voltage supply and logical voltage setup as the previous CMOS circuits.
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