Practical Problems and Applications

Abstract

Many of the input signals needed by microprocessor systems are derived from mechanical switches or contacts. At first sight there appears to be no difficulty in connecting these devices to a PIA port; for example all port inputs can be connected via pull-up resistors to the +5 V supply, and the switch can be connected between a data line and earth. The normal condition is then that all inputs are high, and when one or more goes low, a switch has been operated. If processor action is required, all inputs could be connected to an AND gate as shown in figure 6.1, and the output used either to set a flag or create an interrupt. A service routine could then read the port, determine which line was earthed, store this fact and return from the service routine. One possible action would be to increment a counter keeping a running total of the number of operations of a set of contacts.