Although not many years ago logic integrated circuits in electronic circuits for the hobbyists were non-existent, these gradually made an appearance in a variety of emerging new electronic projects.
We are now in the period where there is apparently hardly any electronic circuit that don't include one of these CMOS ICs, and they can be even found in circuits which are linear in nature.
However, you find an ever larger choice of available types. This is to some extent because of entirely new units developing, but most likely the majority of the innovative ICs are ‘improved’ editions of the previous versions.
Currently you may find numerous categories of ‘improved’ CMOS ICs, of which the conventional CMOS (4000 series) and low-power Schottky (74LS00 series) are the very best known. Both of these logic families have caused the initial 7400 series to be practically outdated, and TTL is hardly ever employed in new designs. The initial competitors to the 7400 TTL family (including the DTL and RTL chips) have ever since become totally obsolete.
For ant electronic enthusiast deciding on the most appropriate logic device can be somewhat complicated matter due to the a wide range of variants available, and several amateurs basically adhere to the common types instead of taking a chance on ‘improved’ products that ultimately proves to be a costly blunder.
It is sometimes difficult to determine precisely which alternate options to be replaced for any existing CMOS IC without compromising the functionality of a circuit, or even causing it to get corrupted completely.
We are going to have a quick glimpse here at the many different logic families that exist, and just how these vary with regards to their efficiency specs.
A table of features which provides the information in a nutshell, and sets it in an easy to access manner is furnished below. Having said that, the data given below is actually a general type of info, and which will only tell you about the common things like the speed etc. of the different logic families.
Specific products within each family generally have a little bit different propagation delays, highest working frequencies, etc., and for comprehensive details the proper datasheet book needs to be checked out.
How TTL ICs Work
The conventional TTL ICs certainly give you a fair degree of working performance otherwise you might have not seen them being applied to such a huge scale. No doubt, the main issue with TTL ICs is a relatively high current intake, meaning several gates might be enough to consume approximately 50 to 100 milliamps. With a few of the more advanced ICs this rate of consumption could be at around 100 milliamps for each device. Also, they are a lot touchy with the supply voltage, and demand a precise 5 volts +/- 5%.
Probably the critical thing about TTL devices is there extremely intolerant behavior to noise spikes across their power supply track, and circuits using one of these TTL ICs usually have to include high number of supply decoupling capacitors tossed across the circuit board.
However, common TTL devices are very fast and are designed to deal with frequencies over approximately 30 to 40 MHz.
How CMOS ICs Work
CMOS (complementary metal oxide silicon) devices defeat most of the disadvantages associated with common TTL devices, but while in doing this, add a few of their own. CMOS ICs are popular due to their incredibly reduced standby current consumption. But the current usage may increase as the running frequency of the CMOS IC increases, an issue that is usually overlooked.
At higher frequencies the power consumption of a CMOS device is also going to be high just like a TTL equivalent device. However, on an average the power consumption of CMOS devices tends to be lower than TTL equivalents.
Additionally, normal 4000 series CMOS ICs are never designed to run at too high frequencies, that highest being around 5 MHz, and several of the more advanced ICs even failing to reach this humble magnitude.
CMOS ICs earlier were prone to damages due to static electricity. The first ‘A’ series units were perhaps far more vulnerable to static electricity compared to current 'B' suffix types having their significantly enhanced built-in safety circuits.
The possibility of destruction due to static charges is perhaps to some degree overstated anyway, and eventhough these devices are handled casually much like non-MOS types for countless years most of us have never seen a single CMOS getting ‘zapped’ so far.
Besides their very low current consumption, some other features of common CMOS ICs are their comparatively high fanout (each output of a CMOS IC are designed to drive around 50 inputs), their vast operating voltage range (3 to 18 volts for the ‘B’ series devices), and an excellent noise immunity around the DC supply rails.
A crucial thing to be aware of is that the excessive fanout number is accomplished by the CMOS devices that have a extremely high input impedance, and their optimum output current is, generally speaking, fairly low. Aside from possible challenges with the driving medium and high current loads like LEDs and relay coils, CMOS ICs have quite restricted fanout while handling the majority of the analogue devices.
LSTTL Chips Working
The LS (low-power Schottky) logic family is practically the most used logic IC in the present times, which have successfully substituted the standard 7400 TTL range and founded themselves as being the brand new logic standard.
These LSTTL chips work with Schottky diodes to prevent transistors at crucial points from getting pushed to saturation, which makes it possible for enhanced switching speeds with respect to a specific operating current.
The typical Schottky (74S00 series) ICs possess characteristics which are nearly the same as the conventional TTL devices, although with a highest working frequency that may be around 3 to 4 times greater. These low-power ICs are comparable with their switching speed efficiency to the normal TTL units, but with a great deal reduced power consumption.
To be precise 74LS00 series ICs are usually a little quicker compared to their 7400 TTL counterparts, although with just about 1/5th of the current intake. This specific feature, combined with their very reasonable price tag, makes low-power Schottky devices an exquisite proposal to circuit designers, and it is obvious why these have grown to be a favorite.
You will find additional Schottky TTL logic families, specifically the 74AS00 (enhanced Schottky) and 74ALS00 (enhanced low power Schottky) ranges. These enhanced Schottky types are designed as an option to the conventional Schottky types, yet featuring relatively greater speed for approximately the identical current usage.
The enhanced low-power type usually are meant to be a substitute for the conventional low-power type, and provide relatively improved working speed at only about 50 % the power consumption. Despite the fact that these chips appeared all set to substitute the 74LS00 range previously, haven't been able to accomplish this up to now, and seems to be extremely unlikely in near future also. More modern ‘improved’ families might not have a chance to push these LSTTL to obsolete until they themselves prove their abilities.
How HC CMOS Function
A lot of the latest improvements in logic device technologies have incorporated CMOS elements. They have been gone through rigorous enhancements until they were no more restrained to low speed applications, and several contemporary CMOS devices are designed as equivalents for the TTL rather than 4000 series CMOS variants.
The initial types were the 74HC00 (high-speed CMOS) variants which provide a optimum working frequency of around 60 MHz, almost two times more than that of regular TTL types. The 74HC00 chips continue to have the large input impedance facility just as the normal CMOS types, however they include increased optimum drive currents that are more close to those proposed by the many TTL versions, but not exactly as high. This enable them to have an enormous fanout functionality whenever high-speed CMOS chips are incorporated to commute some other CMOS products, and provides a decent fanout number when accustomed to run TTL inputs.
One significant difference between 74HC00 range of ICs and traditional CMOS variants is that they possess a a lot more constrained working voltage range between 2 to 6 volts. Even then these chips are still greatly superior to many other TTL logic families, combined with the fantastic protection against supply rail noises commonly involved with CMOS logic gadgets.
It is necessary to be aware of that there are actually 74HC variation for numerous 4000 line of parts, and not just for the 7400 range. As an example, the 74HC73 could be the high-speed CMOS substitute of the TTL 7473, and the 74HC4017 can be high-speed CMOS replacement of the CMOS 4017BE.
An argument much in favor of CMOS ICs which is frequently ignored is their large operating temperature range of -40 to +85 degrees Centigrade, in contrast to a 0 to 70 degree range associated with the normal type TTL devices. You can find the so-called “unbuffered” variations of common CMOS ICs, and these are mainly designed for circuits which implement the ICs in a pseudo-linear style (oscillators as an example).
Additionally, there are unbuffered variants of a number of 74HC00 gadgets, these include 74HCU00 type quantities. The unbuffered variants of standard CMOS devices possess 4000UBF type numbers as an aside. An additional variant within the 74HC00 range is the 74HCT00 series.
While the conventional high-speed CMOS collection possess input switching ranges that happen to be practically just like regular CMOS devices, the 74HCT00 range have got input switching ranges which are appropriate for the 74LS00 sequence of ICs.
The 74HCT00 range is designed to work in environments where high speed CMOS chips need to be combined with normal 74LS00 (or comparable types) TTL devices. Besides the input switching ranges their efficiency is rather identical to the regular 74HC00 range, however the tolerable supply voltage range for these chips can be far more constrained to around 4.5 to 5.5 volts, and of more importantly, these severely lack the supply noise immunity.