Application Note 319 Comparison of MM74HC to 74LS, 74S and 74ALS Logic

The MM54HC/MM74HC family of high speed logic components provides a combination of speed and power characteristics that is not duplicated by bipolar logic families or any other CMOS family. This CMOS family has operating speeds similar to low power Schottky (54LS/74LS) technology. MM54HC/MM74HC is approximately half as fast (delays are twice as long) as the 54ALS/74ALS and 54S/74S logic. Compared to CD4000 and 54C/74C, this is an order of magnitude improvement in speed, which is achieved by utilizing an advanced 3.5 micron silicon gate-recessed oxide CMOS process. The MM54HC/MM74HC components are designed to retain all the advantages of older metal gate CMOS, plus provide the speeds required by today’s high speed systems. Another key advantage of the MM54HC/MM74HC family is that it provides the functions and pin outs of the popular 54LS/74LS series logic components. Many functions which are unique to the CD4000 metal gate CMOS family have also been implemented in this high speed technology. In addition, the MM54HC/MM74HC family contains several special functions not previously implemented in CD4000 or 54LS/74LS. Although the functions and the speeds are the same as 54LS/74LS, some of the electrical characteristics are different from either LS-TTL, S-TTL or ALS-TTL. The following discusses these differences and highlights the advantages and disadvantages of high speed CMOS. AC PERFORMANCE As mentioned previously, the MM54HC/MM74HC logic family has been designed to have speeds equivalent to LS-TTL, and to be 8–10 times faster than CD4000B and MM54C/ MM74C logic. Table 1 compares high speed CMOS to the bipolar logic families. HC-CMOS gate delays are typically the same as LS-TTL, and ALS-TTL is two to three times faster. S-TTL is also about twice as fast as HC-CMOS. Flip-flop and counter speeds also follow the same pattern.