TABLE OF CONTENTS Chapter 4 looked at reading individual switches and a matrix keypad. We'll extend our switch reading to look at additional pin saving techniques. We'll also see how rotary encoders both internally decoded and incremental versions can be read with a PIC.
It's a common requirement to read a configuration switch during start-up. Perhaps the switch defines the baud rate for a serial output, or it may set some other parameter. Regardless, to dedicate several pins to sense a start-up configuration is a waste of scarce pin resources and may force us to use a larger, more expensive PIC than otherwise desired.
How might we read a 16-state switch? We'll immediately discard hooking 16 individual switches to 16 input pins. We recall that 4-bits give us 16 unique combinations, so we can use four switch contacts to generate 16 states. This might be either a 4-section DIP switch with individual switch selections, or it might be a rotary DIP switch with positions 0 F internally coded into one common connection and four switched poles, such as the simple connection is shown in Figure 6-1. A closed switch places a high on the corresponding input pin, and an open switch places a low on the corresponding pin. Reading RB7 RB4 as PortB.HighNib, the switch sections correspond to the 4 bits in the nibble.
To determine the switch closure pattern corresponding to any particular value, we look at the binary combinations. For example, if PortB.HighNib is...
