Embedded Microcontroller Systems

7/30/2019 Embedded Microcontroller Systems

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Embedded Microcontroller Systems


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Overview

Performance metrics

Synchronization methods

I/O Devices and hardware interface issues

Microcontroller in control systems

Control block diagrams

Actuators, plant, sensors

Open loop, closed loop control

Proportional and integral controllers


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Performance Metrics

Latency time delay between when I/Odevice is ready for service and when uCresponds.

input device time between when data isready and when it is actually latched into uC

output device time between when device isready for new data and when it is sent.

uCI/O Device


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Performance Metrics

Latency Hardware delays in uC subsystems

Software delays

uCI/O Device


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Performance Metrics

Throughput maximum data flow (bytesper second) that can be processed from the

I/O device.

can be limited by uC or by I/O device

can be reported as long term average or short

term maximum

uC I/O Device


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Performance Metrics

Priority determines the order of service when

more than two or more devices request at the

same time.

determines if a high-priority device can suspend a low-

priority request that is currently being processed. may want to implement equal priority so that no

device monopolizes the uC.

uC I/O Device

I/O Device


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Real time systems

Hard real-time guarantees a maximum

latency.

Soft real-time system supports priority.


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Synchronization Methods

I/O devices can be in one of 3 states

idle disabled or inactive, no I/O occurs

busy working on generating an input (input I/O)

or accepting an output (output I/O)

done ready for a new transaction.

Busy to done transitions cause status flags to

become true.


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Synchronization Gadfly Loop

Polling loop

Gadfly loop

Busy-waiting loop software checks statusflag in a loop that does not exit until the status

flag is set.

waiting for

input - busy

new input is

ready - done

new data, gadfly loop completes

software reads data, asks for another

INPUTDEVICE:


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Synchronization Gadfly Scenarios

No Buffering

busy done busy done

Input Device:

waiting for new

input

read

data

process

data

waiting read

data

uC software: I/O bound

busy done

Input Device:

read

data

waiting

uC software:

process

data

CPU bound

read

data

process

data

busy done waiting

process

data

busy


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Synchronization Gadfly Scenarios

Buffering

busy done

Input Device:

read

data

uC software:

process

data

read

data

process

data

busy done

process

data

busy

BUFFER

done busy

As long as buffer

is large enough,

both software

and I/O canoperate at their

maximum rate


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Synchronization Blind Cycle

Software waits a fixed amount of time andassumes I/O will complete within the delay.

No status flag from I/O device.

Used for I/O that has predictable delays.

busy done busy done

Input Device:

process

datawaiting read

data

uC software:process

datawaiting read

data

fixed delay


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Synchronization - Interrupts

Interrupts hardware causes software to

execute ISR.

Global data structures used to communicate data

between main program and ISR.

Timer interrupts used to execute specific functions at

regular intervals.


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Synchronization - Interrupts

Use interrupts when:

Arrival times of input is variable.

There are other things to do in main program.

I/O is important (alarm, hardware failure) butinfrequent.

Buffering can also be used with interrupts toallow for better throughput.

Must not forget that interrupts slow the mainprogram loop!


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Buffering FIFO Queue

Data is received (Get) in the same order that it

was transmitted (Put)

As long as FIFO is not full or empty, both producer

and consumer operate at their own rate. Need a way for producer and consumer to know if

FIFO is full or empty.

Producer

(uC or I/O)

Consumer

(uC or I/O)FIFOPut Get


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Synchronization Periodic Polling

Periodic polling uses a clock/timer interrupt

to periodically check the I/O status.

Used in cases where interrupts are desirable

(there is much to do in the main program) but the

I/O device does not support interrupts.

Keypad is an example will investigate in the next

exercise.


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Synchronization - DMA

Direct Memory Access I/O transfers datadirectly to/from memory.

Requires a DMA controller between memory

and I/O device. Used when bandwidth and latency are

important parameters.

Data transfer only no processing of data.


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DMA Burst mode DMA a block transferred while the

uC is halted used when uC and DMA rates are similar

Cycle-stealing DMA data is transferred during

cycles when the uC is not using the bus.

used when uC rate is faster than I/O

uC

DMA Controller

Memory

I/O

deviceaddr

data


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Slow I/O Interface

Keypad how slow can we go?


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I/O Devices and hardware interface

issues

Overview:

Categories of I/O Input and Output examples

Output actuator examples DC Motor (analog and digital control)

Stepper Motor

Output display example

LCD display (parallel and serial)


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I/O Categories Input devices

Sensors, User-input

Output devices Actuators, Displays

Complex I/O devices (printers, faxes,

coprocessors, etc)

Analog I/O Digital I/O Voltage levels - Voltage levels

Current draw - Synchronization Sampling frequency - Throughput

Noise - Noise


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Input Examples

Sensors light

force

sound

position

orientation proximity

tactile

temperature

pressure

humidity speed

acceleration

displacement

User input

keyboards

joysticks

mouse

keypad switches

touchpad

dial

slider


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Output Examples

Actuators

motors

solenoids

relays

heaters

lights

piezoelectric materials

(buzzers, linear actuator)

speakers

Displays

LED displays

LCD displays

CRT displays

indicator lights

indicator gauges


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Example DC Motor

Important in LOTS of applications cameras, drives, elevators, trains, robots

Many types, but all work similarly:

Apply voltage across + and leads,

electrical energy is converted to

mechanical energy.

For some range of voltage, the torque of

the motor shaft is proportional to value of

voltage.


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DC Motor

Current required bymotor depends on

how it is loaded.

Current is almostalways more than the

uC can provide.

Need an interfacecircuit between uC

and motor.

Applied voltage (volts)

no load

loaded

Motorcurren

t(A)


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Interfacing MotorsDigital Outputs

Basic idea is to use aswitch of some kind toisolate current in uCfrom motor current.

Motor is an inductorthough, so it storescurrent.

Flyback diode used toroute current away

from switch whenswitch opens to avoiddamage to switch.

motor

External Voltage

+

Controlsignal from

uC

flyback diode

switch open current


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Interfacing MotorsDigital Outputs

H-Bridge circuit topology that allows bi-

directional control of motor.

motor

+

external

voltage

-

Each switch

controlled by anoutput of the uC.

Switches

implemented by

relays, solid-stateswitches, or transistors

Diodes omitted for

simplicity.


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Interfacing MotorsAnalog Output

8051 DAC can provide up to 15 mA of

current, up to 3.3V voltage.

Must provide both voltage and current

amplification to drive a DC motor.

Amplifiers

Power MOSFETs

Motor driver ICs

Relays

used in the upcoming control lab


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Stepper Motors

Inherent digitalinterface

Can easily control

both position and

velocity

Used in disk drives,

printers, etc.

Small, fixed rotationper change in control

signals
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Basic Operation

Simplified stepper motor

+

phase 1

-

-

phase 2

+

5 teeth

360 5 = 72

moves 72per step

Changing polarity of

stator magnets causes

step.

Typical stepper motors

have 200 steps per

revolution, with 1.8

per step.stator

rotor

electromagnets


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Stepper Motor Interface

8051

port

pins

A

A

B

B

VDD

Inverting

buffers

+Vmotor

1010

1001

0101

0110

1010

4-phase

stepper

motor

1.8

1.8

1.8

1.8


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Output Display Example:

LCD Display

LCD Liquid Crystal Display Lower power than LED display

More flexible in size and shape

Slower response time
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LCD Operation AC voltage required DC voltage damages LCD

Control changes reflectivity of the liquid crystal

material.

Actual light energy supplied by room light or back

light. front plane

liquid crystal material

back plane

60 Hz

Oscillator

control

CMOS


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LCD Interfacing

Simple parallel interface similar to LED:

8051

7-segment

LCD

Driver/Decoder

port

pins

A

B

C

D

ab

c

d

e

f

g

60 Hz

Oscillator

Common Back Plane

Separate Front Planes

VDD


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LCD Interfacing

Serial driver interface

48 bit shift register

48 bit latch register

MC145000 LCD Driver

BP1 BP2 BP3 BP4 FP1 FP2 FP3 FP4 FP5 FP6 FP7 FP8 FP9 FP10 FP11 FP12

48 segment LCD display

data in

clock

data out


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Using Microcontrollers for Control

Overview

Open-loop control systems

Simple closed-loop control systems

Closed-loop position control

PID controllers


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Some Terminology

Control variables properties we want to

control (position, velocity, temperature, etc)

Control commands output to actuators

Driving forces the actuator forces that cause

the control variables to change (heat, force,

etc)

Physical plant the thing being controlled


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Open-loop Control Systems

No feedback path from the plant

Note that these are all a function of time

uC ActuatorsPhysical

Plant

Desired

control

variables

X*(t)

Controlcommands

U(t)

DrivingForces

V(t)

Real

controlvariables

X(t)


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Open-loop Control Example

Stepper motor


Plant

Control

commands

U(t)

Driving

Forces

V(t)

Real

control

variables

X(t)

uC

Desired

control

variables

X*(t)Inverting

driving

buffers

steppermotor shaft position

desired

shaftposition

specified

in

program


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Open-loop Control Example

Traffic light controller

uC

Inverting

driving

buffers

desired light pattern

in software


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Closed-loop Control

Feedback from plant to controller


Plant

Desired

control

variables

X*(t)

Controlcommands

U(t)

DrivingForces

V(t)

Real

controlvariables

X(t)

Sensor

Closed loop Control


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Closed-loop Control

Bang-bang control

Bang-bang control output can only turn somethingON or OFF. No variable control.

Requires a deadband or hysteresis which definesa range of acceptable values for output - otherwisethe control system components can wear out fromtoo many switching cycles. (Relays, for example, havea limited lifetime).

Works well with physical plant with a slow responsetime.


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Closed-loop Control Systems Bang-bang control temperature control

example.

Heater

Temperature

sensor

uC

plant

Desired temperature,

Tlow < T < Thigh

estimate T

TT

Turn off Leave Turn on

T > Thigh T < Tlow

Flowchart of control algorithm


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Closed-loop Position Control

Incremental control adds or subtracts a small

constant from the output control command, U(t), in

response to X(t) sensed.

uC+1

or

-1

ActuatorsPhysical

Plant

Desired

control

variables

X*t

Controlcommands

U(t)

DrivingForces

V(t)

Real

controlvariables

X(t)

Sensor


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Incremental Control

Rate of sampling is very important

If sampling rate is too fast, actuators are saturated

and a bang-bang system results.

If sampling rate is too slow, then controller will notkeep up with plant.

Rule of thumb for rate: control execution rate

is 10x the step response of the plant. Must check for underflow and overflow after

increment or decrement.

Cl d l C t l


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Closed-loop Control

PID Controller

Faster and more accurate than previous systems.

Based on linear control theory.

Three components sometimes fewer are used.

Proportional output is linearly related to error signal.

Integral output is related to integral of the error signal.

Derivative output is related to derivative of the error.

PID C ll


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PID Controller To understand, must transform parts of control diagram

into discrete time domain.

Very important to have periodic sampling and processing.

In the figure below, n is the sample number

uCPID

controller

ActuatorPhysical

Plant

Desired

output

x*u(n) p(t)

Real

controlvariables

x(t)

Sensor

x(n)

e(n)

error signal: e(n) = x*(n) - x(n)

+

-

PID C ll


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PID Controller

uCPID

controller

ActuatorPhysical

Plant

Desired

output

x*u(n) p(t)

Actual

Output

x(t)

Sensor

x(n)

e(n)+

-

u(t) = P(t) + I(t) + D(t)

Proportionaloutput is proportional to error input

Continuous time: P(t) = Kp * E(t)

Discrete time: P(n) = Kp * E(n)

PID C t ll


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PID Controller

u(t) = P(t) + I(t) + D(t)

Integraloutput is proportional to integral of error signal

Continuous time: I(t) = Ki E(t) dt

Discrete time: I(n) = Ki E(n) t = Ki t E(n)

t is sampling period

e(t)

t

I(t)

t

I(n)

n

t

1 2 3 4 5

I(n)

n

Ki large

I(n)

n

Ki small


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Equation for Integral Component

Discrete time: I(n) = Ki t E(n)

integral += errorsig; //integral = integral + errorsig

//integral is sum of errorsignals

//integral includes: M_MEAS samples of error//multiplied by GAIN_PRECISION

...

output = Kp*errorsig / M_MEAS/GAIN_PRECISION +

Ki * integral / M_MEAS / SAMPLERATE /GAIN_PRECISION

= 1/t

PID C t ll


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PID Controller

u(t) = P(t) + I(t) + D(t)

Derivativeoutput is proportional to derivative of error signal

Continuous: D(t) = Kd*dE

dt

E(n)E(n-1)t

Discrete: D(n) = Kd *

t is sampling period


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PI Controller for Position Control

uCPI

controller

ServoAmplifier

DC

Motor

uf

(n)p(t)

Actual

position

2(t)

Desired

position

1*

(potentiometer)

potentiometer

2(n) ub(n)

(we will not use the derivative term, which can cause instability)


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PI Controller Hardware Setup

8051Microcontroller

ADC0

Input

voltage

conversion

interface

ADC0.0

ADC0.1

setpoint

potentiometer

output

potentiometer

0 - 2.45V

0 - 2.45V-15 to +15V

-15 to +15V

Output

voltageconversion

interface

DAC0

DAC1

0 - 2.45V

0 - 2.45V

multiplexor

DAC0

DAC1

to SA1SOD

Input 1

to SA1SOD

Input 2


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Circuit Schematics for Interface Circuits

Input voltage conversion interface Output voltage conversion interface


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PI Control Algorithm

Controller must execute the following tasks:

Sample inputs

Compute error value

Compute integral value

Compute output signal from error value, integral

value and preset Kp and Ki.

Send computed output signals to amplifier


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Controller Performance Metrics

Stability a requirement

Response time how fast the output responds

to the input.

Steady state error how much the output

differs from the input after it has settled.

For position controller DEADBAND is a

measure of the steady state error.


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Total DeadBand Measurements

TDB(1) = |DB(1)| + | DB(-1)|

input position output position

1 2

DB (1) = 1- 2

repeat for a negative angle, - 1 DB (-1) = 2- 1


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Control your position!

Prelab software only

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Embedded Microcontroller Systems