Nuts & Volts - July 2004

Just For Starters

decode logic. A state machine may not always increment between consecutive states. Instead, it can jump around in its state space (i.e., counter value) similar to the way a computer program branches back and forth between routines. State machines can be as simple as our LED blinker or complex enough to rival a small microprocessor.

State Machines in Software

Hardware state machines are appropriate in many circumstances. Simple tasks — such as blinking an LED — are readily solved with a few logic gates. Complex tasks that must be performed with utmost speed are natural applications for hardware state machines. Yet, there is a large middle ground of algorithms that have relaxed speed requirements. Software running on a microprocessor can implement these state machines. Software implementations allow state machines and algorithms to be readily changed without having to connect new logic gates as with hardware.

General purpose microprocessors are often too bulky for small projects. Your microwave oven doesn’t have a Pentium in it and likely makes do with a microcontroller that costs around $1.00. Microcontrollers contain small microprocessors along with some memory and a few basic peripherals. They are often self-contained devices that require only power and a clock to function. As such, microcontrollers are perfect devices for implementing control algorithms that do not require high speed processing.

Microcontroller Timing

Practically every microcontroller contains at least one timer peripheral that can be programmed to generate an interrupt at regular time intervals. The timer is a counter that is similar to the discrete counter discussed previously. The discrete counter provides eight blinking states with a 62. 5 ms increment period. Therefore, the microcontroller’s timer would be programmed to generate an interrupt every 62. 5 ms.

Programming a timer varies among specific micro-controllers. Generally speaking, a timer is clocked using a derivative of the microcontroller’s main clock. If the microcontroller runs at 8 MHz, the timer may be clocked at 1 MHz. In this case, the timer would be programmed to roll-over and generate an interrupt every 62,500 cycles.

Microcontrollers also contain input/output (I/O) pins that software can read and write. These pins are used to query status from the outside world and to control actions such as blinking an LED. Figure 3 shows a general purpose micro-controller that would be used in our application. Note the on-chip, read only memory (ROM) for software storage and the random access memory (RAM) for general software variables.

Blinking With Software

You can design software to blink an LED after creating

JULY 2004

State Value

LED On?

000

Yes

001

No

010

Yes

011

No

100

Yes

101

Yes

110

No

111

No

Table 1. Three-bit blinker truth table.

ISR Begin

Get Next LED State From Current State Value Lookup

Drive LED Control Pin With LED State

Increment Current State Value

ISR End

Figure 4. LED blinking interrupt service routine.

a regular interrupt with the microcontroller’s timer. The microcontroller invokes software called an interrupt service routine (ISR) whenever the timer generates an interrupt. Between interrupts, the microcontroller executes a main routine. The main routine initializes the system and then remains in an idle loop for the rest of the time. All of the blinking functionality is time-driven and implemented in the ISR.