HD74AC datasheet(33/52 Pages) HITACHI

33 / 52 page

Design Considerations

1. Interfacing

FACT devices have outputs which combine balanced CMOS outputs with high current line driving

capability. Each standard output is guaranteed to source or sink 24 mA of current at worst case conditions.

This allows FACT circuits to drive more loads than standard advanced Schottky parts; FACT can directly

drive ALS, AS, LS, HC, and HCT devices.

AC/ACT

NMOS,

CMOS or

TTL

VCC

Figure 1 Interfacing FACT to NMOS, CMOS and TTL

FACT devices can be directly driven by both NMOS and CMOS families, as shown in figure 1, operating

at the same rail potential without special considerations. This is possible due to the low input loading of

FACT product, guaranteed to be less than 1

µA per input.

Some older technologies, including all existing TTL families, will not be able to drive FACT circuits

directly; this is due to inadequate high level capability, which is guaranteed to 2.4 V. There are two simple

approaches to the TTL-to-FACT interface problem. A TTL-to-CMOS converter can be constructed

employing a resistor pull-up to V

CC of approximately 4.7 kΩ, which is depicted in figure 2. The correct

high level is seen by the CMOS device while not loading down the TTL driver.

TTL

Figure 2 V

IH Pull-Up on TTL Outputs

Unfortunately, there will be designs where including a pull-up resistor will not be acceptable. In these

cases, such as a terminated TTL bus, Hitachi has designed devices which offer thresholds that are TTL-

compatible (figure 3). These interfaces tend to be slightly slower than their CMOS-level counter-parts due

to an extra buffer stage required for level conversion.

TTL

VCC

ACT

Figure 3 TTL Interfacing to HD74ACT

HD74AC Datasheet(PDF) 33 Page - Hitachi Semiconductor