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 impleme nted using logic components and were usu ally lar ge, heav y box es. Lat er on, microp roce ssors were us ed an d the ent ire con troller 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 lo w.

A typical microcontroller has bit manipulation instructions, easy7 and direct access to I/O, and quick and effi cient int errupt pr ocessi ng. There fore, a m icroc ontroller is a highly integrated device which includes, on one ch ip, all or most of the parts needed to perform an application control function.

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

Figure 1. Typical MCU Block Diag ram

CONTAINS PROGRAM

AND DATA

EXECUTES PROGRAM

MANAGES EVENTS

SIGNAL GENERATION

EVENT COUNTING

APPLICATION TIMEBASE

PERIPHERAL INTERFACE

ROM

CPU

TIMER

I/O P OR T

RAM

EEPROM

A / D

CONVERTER

SERIAL

INTERFACE

CONTAINS

TEMP ORARY

DATA

CONTAINS PERMANENT

DATA

ANALOG DATA

ACQUISITION

SENDS AND

RECEIVES

DATA

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ROM (Read Only Memory) ROM is usually used to store program instructions. RO M is the l east 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 F lash memory is i ts enduranc e 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 e xec ution o f a prog ram, d ata ha ve to be sav ed for la ter use. D ata a re u sual ly

stored in RAM. EEPROM (Electrically Erasable Programmable Read Only Memory) There is anothe r way to store data in a microcon troller : EEPRO M is used to sto re 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 e xecute a program , th e C PU will re ad th e f irst i nstruct ion from pr ogram mem o ry. This instruction is decoded by t he C PU a nd exec ute d. At the co mpletio n o f the e xec ution of the instruction, the next instruction is fetched from memory and is executed. This pr ocedure is repeated until the end of the program (or an endless loop) is found.

Figure 2. Typical Microcontroller Block Organization

PROGRAM

INPUT

OUTPUT

PERIPHERAL

SYSTEMS

BUS

CENTRAL

PROCESSING

TIMING

SYSTEM

MEMORY

ROM

DATA

MEMORY

RAM

EEPROM

UNIT

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TIMERS

The timer or timing system makes it poss ible 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 l ess bi ts, usually programmable as either i nput or output bits.

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MICROCONTROLLERS MADE EASY

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 asynchro nous interfa ce is cal led a se rial com munic ation interfac e (SCI or UART) while the synchronous interface is called a serial peripheral interface (SPI). A typical SCI application is to conne ct a PC for debu gging pu rpose while a t ypical SPI appl icatio n is to co nnect 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 as ynchronous 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

CLOCK

bbb

b b

DATA

0b bbb

Start

ASYNCHRONOUS

Stop

CLOCK

DATA

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A/D CONVERTER

Converts an external analog signal (typically relative to voltage) into a digital representation. Microcont rollers that have th is featu re can be used fo r instrume ntation , envir onmen tal data logging, or any application that lives in an analog world.

Figure 4. A/D Converter Principle

Voltage

ANALOG

5 4

3 2

ANALOG

SIGNAL

A / D

CONVERTER

DIGITAL

Time

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1 TYPICAL MICROCONTROLLER APPLI CATIONS

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

CAR RADIO

REMOTE

CONTROL

BATTERY CHARGER

BODY CONTROLLER

KEYBOARD

DIMMER SWITCH

TELEPHONE SET

REMOTE

METER

MONITOR

DASHBOAD

FRONT PANEL

KEYLESS

While microproc essors targe t the maximu m proces sing performa nce, the pur pose of micro controllers is to impl ement a se t of control functi on in the most cos t effective w ay. Although

(TM)

controlling a microwave oven with a Pentium

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

KEYBOARD SCANNING

MEASURE

TEMPERATURE

FUN CTIO N

CHANGE

TEMPERATURE

INFORMATION

DISPLAY

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

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