AN-368| Application Note

An Introduction to and Comparison of 74HCT TTL Compatible CMOS Logic

An Introduction to and Comparison of 74HCT TTL Compatible CMOS Logic
The 54HC/74HC series of high speed CMOS logic is unique in that it has a sub-family of components, designated 54HCT/74HCT. Generally, when one encounters a 54/74 series number, the following letters designate some speed and power performance, usually determined by the technology used. Of course, the letters HC designate high speed CMOS with the same pinouts and functions as the 54LS/74LS series. The sub-family of HC, called HCT, is nearly identical to HC with the exception that its input levels are compatible with TTL logic levels. This simple difference can, however, lead to some confusion as to why HCT is needed; how HCT should be used; how it is implemented; when it should be used; and how its performance compares to HC or LS. This paper will attempt to answer these questions. It should also be noted that not all HCTs are the same. That is, HCTs from other vendors may have some characteristics that are different. Thus, when discussing general characteristics this paper will directly address Fairchild Semi
conductor's 54HCT/74HCT which is compatible with JEDEC standard 7. Other vendors' ICs which also meet this standard will probably have similar characteristics. WHY DOES HCT EXIST? Ideally, when a designer sits down to design a low power high speed system, he would like to use 54HC/74HC, and CMOS LSI components. Unfortunately, due to system requirements he may have to use NMOS microprocessors and their NMOS or bipolar peripherals or bipolar logic (54S/74S,

Fairchild Semiconductor Application Note 368 March 1984

54F/74F, 54ALS/74ALS, or 54AS/74AS) because either the specific function does not exist in CMOS or the CMOS device may not have adequate performance. Since the system designer still desires to use HC where possible, he will mix HC with these products. If these devices are specified to be TTL compatible, incompatibilities may result at the interface between the TTL, NMOS, etc. and HC. More specifically, in the case of where a TTL or NMOS output may drive an HC input, a specification incompatibility results. Table 1 lists the output drive specifications of TTL compatible outputs with the input specifications of 54HC/74HC. Notice that the output high level of a TTL specified device will not be guaranteed to have a logic high output voltage level that will be guaranteed to be recognized as a valid logic high input level by HC. A TTL output will be equal to or greater than 2.4V, but an HCMOS input needs at least 3.15V. It should be noted that in an actual application the TTL output will pull-up probably to about
VCC minus 2 diode voltages, and HC will accept voltages as low as 3V as a valid one level so that in almost all cases there is no problem driving HC with TTL. Even with the specified incompatibility, it is possible to improve the TTL-CMOS interface without using HCT. Figure 1 illustrates this solution. By merely tying a pull-up resistor from the TTL output to VCC, this will force the output high voltage to go to VCC. Thus, HC can be directly interfaced very easily to TTL. This works very well for systems with a few lines requiring pull-ups, but for many interfacing lines, HCT will be a better solution.

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FIGURE 1. Interfacing LS-TTL Outputs to Standard CMOS Inputs Using a Pull-Up Resistor

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1998 Fairchild Semiconductor Corporation

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