TABLE OF CONTENTS Tim Williams
The interfaces between logic integrated circuits including signal, clock and power supply lines must be considered to achieve a reliable digital design. This applies whether the devices concerned are microprocessors, their support chips, application specific ICs (ASICs), programmable logic arrays (PLAs) or standard "glue" logic.
A logic input can take any value of voltage, nominally from one supply rail to the other, although due to transmission line effects the actual voltage can exceed either supply rail on transitions. Each input is designed so that any voltage below one level, conventionally V IL, is regarded as a logic "0" and any voltage above another level, V IH, is regarded as logic "1" (Figure 14.1).
These levels are characterized for each logic or microprocessor family, and worst case values of V IL and V IH can be found on any data sheet. Note that, as with any hardware-determined parameter, they may vary with temperature and you should ensure that the values you use are guaranteed across the device's temperature range. They are also a function of supply voltage. If all ICs are fed from the same supply this is not a problem, but it becomes more significant if you are interfacing logic circuits which may be fed from different supply rails.
The significance of the band between V IL and V IH is that the input logic state (and therefore...
