There are two lines, called logic families,of analog IC's and while they can generally be interoperable "off the shelf," they work best with ICs of the same family and sometimes IC's of mixed logic families can require a voltage translation to get working correctly. Often, though, our designs will have need to intermix different logic families. This isn't always a problem when there is no inherent dependency on signaling between the logic IC's, and in some cases even signaling between them won't pose a problem. However, in some cases, like when signaling between a CMOS IC and a TTL IC, there can be a problem because each logic family defines the valid range of voltage that make up a valid HIGH and valid LOW and the logic families don't agree on the range.
This instructable will briefly explain the two logic families of IC's 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.
This instructable will briefly explain the two logic families of IC's 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.
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Signing UpStep 1: TTL Integrated Circuits
TTL IC's -- members of the Bipolar Logic Family -- were first developed in the 1960's 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 1/3. The 74ALSxx improved performance even further. The 74F series, based on a new technology, reduced the propagation delays and even further reduced the size of the IC.
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 1/3. The 74ALSxx improved performance even further. The 74F series, based on a new technology, reduced the propagation delays and even further reduced the size of the IC.
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Also, I think you're confusing two issues. One is that 5V TTL output levels (1 is > 2.7V as you say) are not necessarily compatible with CMOS input levels. This turned out to be rarely a problem, and more with supposedly "ttl compatible" devices that output ~3V than with actual TTL chips (which have an output pretty close to 5V, unless heavily loaded.) The other issue is that CMOS logic families can frequently operate off extended power supply levels., while TTL was strictly a 5V family. And there isn't any way that an HC part running on a 2V supply or a cmos microcontroller running at 1.8V is going to produce logic pulse compatible with other chips running at 5V, regardless of logic family. So there you MUST have level shifting.
The reason CMOS has taken over is power consumption. A small 74xx IC consumed as much power as a modern cmos microcontroller, and did a whole lot less.
I appreciate those two issues and they definitely are distinct problems. However, I don't touch on 1.8V ICs except with the maxim IC, which is powered at 5V anyway and intercepts all 1.8V signals. The main issue I discuss here between TTL and CMOS is that by their standards, there is a ~ 1V gap between what they each consider to be HIGH levels, and if a signal falls within that gap it is undefined and behavior is unpredictable.