Chapter 10 Microcontrollezrs

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Computing Machinery

Chapter 10: Microcontrollers


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Microprocessors vs. Microcontrollers

The primary difference between a microprocessor and a microcontroller is that the

microcontroller includes more supporting functions such as on-board memory and I/O

than the microprocessor.

Until recently, microcontroller units (MCU's) were considered less powerful than

microprocessors, however, continued development has lead to MCU's that meet or

exceed the throughput (number of equivalent instructions per second) of high-end

microprocessors.

The power of some modern

microcontrollers is equal to high-

end CPU's but more specific in

their designs and applications than

general-purpose microprocessors.

Microcontrollers offer many

application specific features such

as I2C and CAN bus

communications and video

controllers.

Microcontroller


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Embedded Systems

An embedded systemis a device or module designed for a specific purpose and based on

some computational (software modifiable) element. One of the key features of an

embedded system is real-time operation.

http://www.ece.cmu.edu/~koopman/iccd96/iccd96.html


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The Harvard Architecture separates the data and instructions into separate memory

units each with their own bus. Besides speeding up the data transfer rate, separating

the data from the instructions permits us to implement different bus widths and wordsizes for program and data memory space.

Harvard Architecture


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Digital Signal Processors

Digital Signal Processors (DSP's) are types of microcontrollers that are more specialized

for a particular application. DSP represent around 20% of the total MCU market.

DSP's are particularly well suited to real-time operations in which a data stream, video,

audio, etc. is modified in someway as it is passed along to another device or

component.

Most DSP's are based on the Harvard Architecture. DSP's commonly include analog-to-

digital (A/D) conversion for their inputs and digital-to-analog (D/A) conversion of theiroutputs.


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Pulse Width Modulation

Pulse Width Modulation (PWM)- Many MCU are designed to generate variable width

squarewave signals as shown below. A pulse width modulated (PWM) signal is a

squarewave with a varying ratio of high and low logic levels, called the duty cycle.

The PWM signal is defined by two parameters, the frequency and the duty cycle.

A 10% duty cycle signal is at a logical high 10% of the time and low 90% of the

time. The average rate at which the signal oscillates between logic levels is given

by the frequency in cycles per second (Hertz).


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Digital to Analog Conversion

A PWM signal can be filtered using passive electronic components (e.g. an RC

[resistor-capacitor] network) to generate a smoothed average voltage level. A

controlled voltage level or power level can be delivered using PWM signals.


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Serial I/O Interface (UART)

The Universal Asnychronous Receiver/Transmitter (UART) is a popular form of serial

communication between digital devices.

A number of bits are presented to the UART in parallel and are then serialized,transmitted to another UART where they are converted back into a binary vector.

The UART specification does not include details of the communication details.

In fact, there are several interface standards in common use (e.g. RS232, RS422 and

RS485). For embedded systems the RS232 standard is the most popular.


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The Serial Peripheral Interface (SPI)

This is a communications protocol used on most MCU's as an inexpensive

alternative to multiple pin parallel communications. SPI is used for many

applications in MCU including memory data block transfers. The SPI interface

include a clock line, a data-in, a data-out and a chip enable line, named as,

SCLK - serial clock

MISO - master input, slave output

MOSI - master output, slave input

CS - chip select (optional, usually inverted polarity)


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Basic-X35 Schematic

AT25256


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The Inter-Integrated Circuit (I2C) computer bus is used to interface many different

types of peripherals to MCU's or other embedded systems when a high data throughput

is not required.

The I2C uses bidirectional data and clock lines with open-collector logic. The standard

data rate for the I2C is 100 kilobits/second. It also support a low-speed 10 Kbits/s or

lower.

The I2C protocol is ideal for sending control signals to remote devices, collecting A/D

signals from sensors and accessing slow-speed non-volatile RAM.

One of the most valuable features is that periperals can be connected and removed from

the I2C bus while powered up. This is possible because of the open-collector logic in

which a bus line is either set to ground or no-connection.

In open-collector circuits, a logical high is achieved by placing pull-up resistor on each

open-collector line.

I2C Communications


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Controller Area Nework (CAN) Bus

This is a multicast shared serial

bus standard. The CAN bus wasdesigned to operate in high noise

conditions, which makes it a

preferred communications standard

for applications involving

electrical motors and internal

combustion engines.

The CAN protocol uses error-

correcting codes to automatically

detect and recover bit errors in data

words. The the full data transfer

rate of 1Mbit/s the total length ofthe CAN network is limited to less

than 50 meters.


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Analog-to-Digital (ADC)

Many of the devices and sensors interfaced to a MCU produce varying voltage levels

(analogsignals) rather than binary vectors (digitalsignals). For this reason most modern

MCU's include built-in analog-to-digital (A/D)conversion circuits. These circuits accept

a voltage level on an input line and generate an internal binary encoded value that

corresponds to the magnitude of the voltage.


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Aliasing

When we choose a sampling rate, we are setting a limit on the maximum frequency (or

time rate of change) of the signal that we will be able to measure.

Shown below are two signals of different frequencies. The lower frequency signal is

being sampled at a rate of 12 samples per cycle of the signal. If we replace this signal

with one with a frequency that is 11 times higher we will be sampling this signal a little

more that once per cycle. As shown the sampled voltage levels will match those of the

lower frequency signal. This effect is called aliasing.

To prevent aliasing we need to sample at a rate at least twice as high (in samples per

second) at the highest frequency signal we want to measure (in cycles/second).


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Digital to Analog (D/A)

This is the process of converting a digital (binary encoded) value into an analog voltage

level. This operation is much less common on an MCU, but can be implemented using a

varying duty-cycle signal and some passive electronics for filtering (see Pulse Width

Modulation above). A much better aproach is to use a linear amplifier as a summing

circuit, as shown in Figure 10-6. This is an example of a digital to analog (D/A) circuit

called anR2R ladder network.

http://www.play-hookey.com/analog/d2a_converter.html


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A prescaler is a circuit that divides an incoming clock signal by a predetermined

amount. An N level prescaler is sometimes called a divide-by-N counter.

Prescalar


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AVR A/D

The AVR MCU includes 8 A/D input circuits. These cicuits can operate in two different

modes, either single conversion and free run.

When in single conversion mode, each conversion must be prompted by the runningprogramming.

In free-run mode the A/D conversion is started and will run continuously with updated

values being placed in the ADC data register.


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BasicX Microcontrollers

The BasicX system is the MCU and the software development environment for a PC-

compatible desktop computer running the Microsoft Windows operating system.

BASIC programs are written and compiled on the PC and downloaded onto theBasicX MCU through a serial interface to an EEPROM or a parallel USB interface

(which needs some additional circuitry).

As with the AVR and PIC18, the BasicX is in-circuit/FLASH programmable. The

BX-24 and BX-35 IC's are depicted in Figure 10-22. The BX-35 is also available as a

surface mount IC.

BasicX-35 BasicX-24


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The BasicX MCU's offer the following features:

25 I/O Lines BX-35 (16 I/O Lines BX-24)

- (17 TTL I/O BX-35) (8 TTL I/O BX-24)

- 8 Channel Analog-to-Digital Converters (10-bit) or TTL

32 KB EEPROM (external for BX-35) for program and data storage

On-chip RAM 400 bytes

Program execution speed 60 microseconds per 16 bit integer add/subtract

Serial I/O speed 2400 baud to 460.8 Kbaud on Com1

300 baud to 19 200 baud on any I/O pin (Com3)

Operating voltage range - 4.0 VDC to 6.0 VDC

I/O internal pull-up resistors 120 k maximumFloating point math

On-chip multitasking

On-chip clock/calendar

Built-in SPI interface

Parallel or serial FLASH Programmable


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Chapter 10 Microcontrollezrs

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