TABLE OF CONTENTS After Chapter 3's examination of the output mode, we'll now turn to PIC pins used as digital input devices. Many PICs include analog-to-digital converters and we'll cover analog inputs in Chapter 11.
Digital signal levels are either a logical low (0) or a logical high (1) what could be simpler? As with the rest of this book, we assume V DD is 5 volts and V SS is 0 volts. In PIC logic, a 0 volt input corresponds to a logical low. Likewise, a 5V input is a logical high. But, suppose our input is 1.7V. Is it a logical low or is it a logical high? Does the answer to this question depend on our choice of an input pin? And, does it depend on the voltage at the input pin earlier in time? We'll find out in this chapter.
First, let's define a few terms, as illustrated in Figure 4-1:
V IL The maximum voltage on an input pin that will be read as a logical low.
V m The minimum voltage on an input pin that will be read as a logical high.
Undefined region The undefined region the voltage level between V IL and V IH. Input voltages in the undefined region may be read as a low or as a high, as the PIC's input circuitry may produce one result or the other. (Obviously the voltage will be read as either a low or a high; it's just that we...
