ST AN887 Application note

AN887

APPLICATION NOTE

MICROCONTROLLERS MADE EASY

by Microcontroller Division Applications

WHAT IS A MICROCONTROLLER?

A few years ago, system control functions were implemented using logic components and were usually large, heavy boxes. Later on, microprocessors were used and the entire controller could fit onto a small circuit board. As the process of miniaturization continued, all of the components needed for a controller were built right onto one chip. By only including the features specific to the task, cost is relatively low.

A typical microcontroller has bit manipulation instructions, easy7 and direct access to I/O, and quick and efficient interrupt processing. Therefore, a microcontroller is a highly integrated device which includes, on one chip, all or most of the parts needed to perform an application control function.

Microcontrollers come in many varieties. Depending on the power and features that are needed, customers might choose a 4, 8, 16, or 32 bit microcontroller.

Figure 1. Typical MCU Block Diagram

CONTAINS

PROGRAM

AND DATA

EXECUTES PROGRAM MANAGES EVENTS

SIGNAL GENERATION EVENTCOUNTING

APPLICATION TIMEBASE

PERIPHERAL

INTERFACE

		ROM						CONTAINS
				RAM				TEMPORARY
								DATA
								CONTAINS
		CPU
				EEPROM				PERMANENT
								DATA
		TIMER		A/ D				ANALOG DATA
				CONVERTER				ACQUISITION
								SENDS AND
		I/O PORT		SERIAL
				INTERFACE				RECEIVES
								DATA
								VR02101A

ROM (Read Only Memory)

ROM is usually used to store program instructions. ROM is the least expensive mean of storing a program in a microcontroller, especially for high volume manufacturing.

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MICROCONTROLLERS MADE EASY

Flash

Flash is an electrically erasable and programmable memory. It can be used instead of ROM to store program instructions and data. A key parameter of Flash memory is its endurance or the number of times it can be erased or reprogrammed. Depending on the technology used, flash endurance can be either 100 or 300,000 program/erase cycles.

RAM (Random Access Memory)

During in the execution of a program, data have to be saved for later use. Data are usually stored in RAM.

EEPROM (Electrically Erasable Programmable Read Only Memory)

There is another way to store data in a microcontroller: EEPROM is used to store data that must be saved through a power down cycle.

CPU (Central Processing Unit)

It is the brain of the system that processes all data and their travel along the bus. For example, in order to execute a program, the CPU will read the first instruction from program memory. This instruction is decoded by the CPU and executed. At the completion of the execution of the instruction, the next instruction is fetched from memory and is executed. This procedure is repeated until the end of the program (or an endless loop) is found.

Figure 2. Typical Microcontroller Block Organization

				PROGRAM
	INPUT			MEMORY
		BUS		ROM
	OUTPUT			DATA
	PERIPHERAL			MEMORY
		CENTRAL		RAM
	SYSTEMS
			TIMING	EEPROM
		PROCESSING
			SYSTEM
		UNIT
				VR02101B
	TIMERS

The timer or timing system makes it possible to measure and time external and internal events. The power of the timers varies greatly between different MCUs.

I / O (INPUT / OUTPUT) PORTS

Most microcontrollers have several digital I/O ports that are used to drive a LED or get a keyboard input. Usually, a port consists of eight or less bits, usually programmable as either input or output bits.

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SERIAL INTERFACE

Serial interface are used to exchange data with the external world. Many microcontrollers have both asynchronous and synchronous communications peripherals built in. Usually, an asynchronous interface is called a serial communication interface (SCI or UART) while the synchronous interface is called a serial peripheral interface (SPI). A typical SCI application is to connect a PC for debugging purpose while a typical SPI application is to connect an external EEPROM.

A synchronous bus includes a separate line for the clock signal which simplifies the transmitter and receiver but is more susceptible to noise when used over long distances. With an asynchronous bus the transmitter and receiver clocks are independent, and a resynchronization is performed for each byte at the start bit.

Figure 3. Synchronous and Asynchronous Communication Principles

SYNCHRONOUS

ASYNCHRONOUS

CLOCK

b

b

b

b

b

b

b

DATA

							CLOCK
0	b	b	b	b	b	1	+
Start						Stop	DATA

VR02101C

A/D CONVERTER

Converts an external analog signal (typically relative to voltage) into a digital representation. Microcontrollers that have this feature can be used for instrumentation, environmental data logging, or any application that lives in an analog world.

Figure 4. A/D Converter Principle

Voltage
	ANALOG	A / D	DIGITAL
		CONVERTER
5
4	ANALOG
3	SIGNAL
2		5	4	2	1	1	2
1
	Time				VR02101D
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MICROCONTROLLERS MADE EASY

1 TYPICAL MICROCONTROLLER APPLICATIONS

Microcontrollers are frequently found in home appliances (microwave oven, refrigerators, television and VCRs, stereos), computers and computer equipment (laser printers, modems, disk drives), cars (engine control, diagnostics, climate control), environmental control (greenhouse, factory, home), instrumentation, aerospace, and thousands of other uses. In many items, more than one processor can be found.

Figure 5. Typical MCU Applications

TV SET	BODY CONTROLLER		TELEPHONE SET
				MONITOR
CAR RADIO		KEYBOARD
				DASHBOAD
				FRONT PANEL
REMOTE	BATTERY CHARGER	DIMMER	REMOTE
			METER	KEYLESS

CONTROL	SWITCH

While microprocessors target the maximum processing performance, the purpose of microcontrollers is to implement a set of control function in the most cost effective way. Although controlling a microwave oven with a Pentium(TM) might seem an attractive idea, it can be easily accomplished with an ST6.

In a typical application, the MCU has to manage several tasks according to their priority or to the occurrence of external events (new command send by the keyboard, external temperature rise,...)

Figure 6. Example of MCU Task Management.

				CENTRAL MCU
					FUNCTION		INFORMATION
KEYBOARD
SCANNING							DISPLAY
MEASURE					CHANGE
TEMPERATURE					TEMPERATURE

VR02101E

The ability to manage control tasks by hardware or by software is the main performance indicator for MCUs.

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