TABLE OF CONTENTS We've seen in Chapter 26 how a PIC can help send Morse code. Now, it's time to put a PIC to work reading Morse. And, even if you're not interested in receiving Morse with a PIC, read this chapter anyway to learn a bit about phase locked loops and an adaptive algorithm to account for timing variations in hand sending.
We won't repeat the Morse code introductory material presented in Chapters 17 and 26. But, let's consider briefly how Morse code is sent and received. In Chapter 26, we learned that amateur radio operators use off/on transmitter keying to send Morse; a dot or a dash represents transmitter on; spaces are transmitter off time. In the amateur radio fraternity, this on/off modulation scheme is known as CW, or "continuous wave" transmission. In the computer and data communications world, on/off modulation is called OOK (on-off keying) or ASK (amplitude shift keying). We've seen OOK before in Chapter 22 to be precise, as it's part of the infrared remote control communications protocol. And, of course, there's nothing limiting us to sending Morse by OOK; we could use any of many modulation protocols, such as frequency shift keying (dots and dashes are represented by one frequency; spaces by a second frequency) or phase shift keying or many other modulation techniques. As a practical matter, however, we'll concern ourselves with OOK, as that's what is used in practice.
Since OOK implies turning a transmitter on and off, to detect the...
