status = areWeThereYet();
} while (status == NO);
The second communication technique uses interrupts. An interrupt is an asynchronous electrical signal from a peripheral to the processor. When interrupts are used, the processor issues commands to the peripheral exactly as before, but then waits for an interrupt to signal completion of the assigned work. While the processor is waiting for the interrupt to arrive, it is free to continue working on other things. When the interrupt signal is finally asserted, the processor temporarily sets aside its current work and executes a small piece of software called the interrupt service routine (isr). When the ISR completes, the processor returns to the work that was interrupted.
Of course, this isn't all automatic. The programmer must write the ISR himself and "install" and enable it so that it will be executed when the relevant interrupt occurs. The first few times you do this, it will be a significant challenge. But, even so, the use of interrupts generally decreases the complexity of one's overall code by giving it a better structure. Rather than device polling being embedded within an unrelated part of the program, the two pieces of code remain appropriately separate.
On the whole, interrupts are a much more efficient use of the processor than polling. The processor is able to use a larger percentage of its waiting time to
perform useful work. However, there is some overhead associated with each interrupt. It takes a good bit of time-relative to the length of time it takes to execute an opcode to put aside the processor's current work and transfer control to the interrupt service routine. Many of the processor's registers must be saved in memory, and lower-priority interrupts must be disabled. So in practice both methods are used frequently. Interrupts are used when efficiency is paramount or multiple devices must be monitored simultaneously. Polling is used when the processor must respond to some event more quickly than is possible using interrupts.
5.3.1 Interrupt Map
It is important to initialize the interrupt vector table correctly. (If it is done incorrectly, the ISR might be executed in response to the wrong interrupt or never executed at all.) The first part of this process is to create an interrupt map that organizes the relevant information. An interrupt map is a table that contains a list of interrupt types and the devices to which they refer. This information should be included in the documentation provided with the board. Table 5-1 shows the interrupt map for the arcom board.
Table 5-1. Interrupt Map for the Arcom Board
| Interrupt Type | Generating Device |
|---|---|
| 8 | Timer/Counter #0 |
| 17 | Zilog 85230 SCC |
| 18 | Timer/Counter #1 |
| 19 | Timer/Counter #2 |
| 20 | Serial Port Receive |
| 21 | Serial Port Transmit |
/**********************************************************************
*
* Interrupt Map
*
**********************************************************************/
/*
* Zilog 85230 SCC
*/
#define SCC_INT 17
/*
* On-Chip Timer/Counters
*/
#define TIMER0_INT 8
#define TIMER1_INT 18
#define TIMER2_INT 19
/*
* On-Chip Serial Ports
*/
#define RX_INT 20
#define TX_INT 21
5.4 Get to Know the Processor
Everything you need to know about the processor can be found in the databooks provided by the manufacturer. If you don't have a databook or programmer's guide for your processor already, you should obtain one immediately. If you are going to be a successful embedded systems programmer, you must be able to read databooks and get something out of them. Processor databooks are usually well written as databooks go so they are an ideal place to start. Begin by flipping through the databook and noting the sections that are most relevant to the tasks at hand. Then go back and begin reading the processor overview section.
5.4.1 Processors in General
As it is used in this book, the term processor refers to any of three types of devices known as microprocessors, microcontrollers, and digital signal processors. The name microprocessor is usually reserved for a chip that contains a powerful CPU that has not been designed with any particular computation in mind. These chips are usually the foundation of personal computers and high-end workstations. The most common microprocessors are members of Motorola's 68k found in older Macintosh computers and the ubiquitous 80x86 families.
A microcontroller is very much like a microprocessor, except that it has been designed specifically for use in embedded systems. Microcontrollers typically include a CPU, memory (a small amount of RAM, ROM, or both), and other peripherals in the same integrated circuit. If you purchase all of these items on a single chip, it is possible to reduce the cost of an embedded system substantially. Among the most popular microcontrollers are the 8051 and its many imitators and Motorola's 68HCxx series. It is also common to find microcontroller versions of popular microprocessors. For example, Intel's 386EX is a microcontroller version of the very successful 80386 microprocessor.