Programming Embedded Systems, 2nd Edition

Study the External Peripherals

At this point, you’ve studied every aspect of the new hardware except the external peripherals. These are the hardware devices that reside outside the processor chip and communicate with it by way of interrupts and I/O or memory-mapped registers.

Begin by making a list of the external peripherals. Depending on your application, this list might include LCD or keyboard controllers, analog-to-digital (A/D) converters, network interface chips, or custom application-specific integrated circuits (ASICs). In the case of the Arcom board, the list contains just two items: the SMSC Ethernet controller and the parallel port.

You should obtain a copy of the user’s manual or datasheet for each device on your list. At this early stage of the project, your goal in reading these documents is to understand the basic functions of the device. What does the device do? What registers are used to issue commands and receive the results? What do the various bits and larger fields within these registers mean? When, if ever, does the device generate interrupts? How are interrupts acknowledged or cleared at the device?

When you are designing the embedded software, you should try to break the program down along device lines. It is usually a good idea to associate a software module called a device driver with each of the external peripherals. A device driver is nothing more than a collection of software routines that control the operation of a specific peripheral and isolate ...