Embedded Micro Controller Systems

8/8/2019 Embedded Micro Controller Systems

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Prof. Cherrice Traver EE/CS-152: Microprocessors and Microcontrollers

Embedded Microcontroller

SystemsMore interfacing and examples
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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 is ready

and when it is actually latched into uC output device time between when device is

ready 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 (bytes per second) that can be processed from theI/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 whenmore than two or more devices request at the sametime. 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 maximumlatency.

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 status

flag in a loop that does not exit until thestatus flag is set.

waiting for input - busy

new input isready - 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 doneInput Device:

waiting for new

input

read

data

process

datawaiting read

data

uC software: I/O bound

busy done

Input Device:

readdata

waiting

uC software: processdata

CPU bound

readdata

processdata

busy done waiting

processdata

busy


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Synchronization Gadfly ScenariosBuffering

busy done

Input Device:

readdata

uC software:

processdata

readdata

processdata

busy done

processdata

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 doneInput Device:

processdata

waiting readdata

uC software: processdata

waiting readdata

fixed delay


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Synchronization - Interrupts

Interrupts hardware causes software toexecute 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 main program loop!


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Buffering FIFO Queue

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

As long as FIFO is not full or empty, both producer andconsumer operate at their own rate.

Need a way for producer and consumer to know if FIFO isfull or empty.

Producer (uC or I/O)

Consumer (uC or I/O)FIFO

Put Get


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Synchronization Periodic Polling

Periodic polling uses a clock/timer interrupt to periodically check the I/Ostatus.

Used in cases where interrupts are desirable(there is much to do in the main program) butthe I/O device does not support interrupts.

Keypad is an example will investigate in thenext 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 duringcycles when the uC is not using the bus. used when uC rate is faster than I/O

uC

DMA Controller

Memory

I/Odeviceaddr

data


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Slow I/O Interface

Keypad how slow can we go?


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I/O Devices and hardware interfaceissues

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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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 mechanicalenergy.

For some range of voltage, the torque of themotor shaft is proportional to value of voltage.


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DC Motor

Current required bymotor depends onhow it is loaded.

Current is almostalways more than theuC can provide.

Need an interfacecircuit between uCand motor.

Applied voltage (volts)

no load

loaded s t a l l e

d

Motor current(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 inductor though, so it storescurrent.

Flyback diode used to

route current away fromswitch when switchopens to avoid damageto switch.

motor

External Voltage+

Controlsignal from

uC

flyback diode

s w i t c h c l o s e d

c u r r e n t

switch open current


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Interfacing MotorsDigital Outputs

H-Bridge circuit topology that allows bi-directional control of motor.

motor

+externalvoltage

-

Each switch

controlled by anoutput of the uC.

Switchesimplemented byrelays, 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 currentamplification 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 andvelocity

Used in disk drives, printers, etc.

Small, fixed rotation per change in controlsignals
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Basic Operation

Simplified stepper motor

N 1

N 2 N 3

N 4

S 1

S 2 S 3

S 4

S 5 N

5 + phase 1

-

- phase 2

+

5 teeth360 5 = 72 moves 72 per step

Changing polarity of stator magnets causesstep.

Typical stepper motors have 200 steps

per revolution, with1.8 per step.stator

rotor

electromagnets


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Stepper Motor Interface

8051

port pins

AABB

VDD

Inverting buffers

+Vmotor

10101001010101101010

4-phasestepper 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 Interfacing

Simple parallel interface similar to LED:

8051

7-segmentLCD

Driver/Decoder

port pins

ABC

D

a bcdef

g

60 HzOscillator

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 tocontrol (position, velocity, temperature, etc)

Control commands output to actuators

Driving forces the actuator forces thatcause 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

Desiredcontrol

variablesX*(t)

Controlcommands

U(t)

DrivingForcesV(t)

Real

controlvariablesX(t)


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Open-loop Control Example

Stepper motor


Plant

Controlcommands

U(t)

DrivingForces

V(t)

Realcontrol

variablesX(t)

uC

Desiredcontrol

variablesX*(t)

Invertingdriving buffers

stepper motor shaft position

desired

shaft positionspecified

in program


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Open-loop Control Example

Traffic light controller

uC

Invertingdriving buffers

desired light patternin software


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Closed-loop Control

Feedback from plant to controller


Plant

Desiredcontrol

variablesX*(t)

Controlcommands

U(t)

DrivingForcesV(t)

Real


Sensor

Cl d l C l


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Closed-loop ControlBang-bang control

Bang-bang control output can only turn something ONor OFF. No variable control.

Requires a deadband or hysteresis which defines a

range of acceptable values for output - otherwise thecontrol system components can wear out from too manyswitching cycles. (Relays, for example, have a limitedlifetime).

Works well with physical plant with a slow responsetime.


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Closed-loop Control Systems Bang-bang control temperature control

example.

Heater

Temperaturesensor

uC

plantDesired 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 smallconstant from the output control command, U(t),in response to X(t) sensed.

uC+1or -1

Actuators PhysicalPlant

Desiredcontrol

variablesX*t

ControlcommandsU(t)

DrivingForcesV(t)

Real


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 willnot keep up with plant.

Rule of thumb for rate: control executionrate is 10x the step response of the plant.

Must check for underflow and overflowafter increment or decrement.

l d l l


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Closed-loop ControlPID Controller

Faster and more accurate than previous systems. Based on linear control theory. Three components sometimes fewer are used.

P roportional 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.


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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 Actuator

PhysicalPlant

Desiredoutput

x*u(n) p(t)

Real

controlvariablesx(t)

Sensor x(n)

e(n)

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

+

-


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PID Controller

uCPID

controller

Actuator Physical

Plant

Desiredoutput

x* u(n) p(t)

ActualOutput

x(t)

Sensor x(n)

e(n)+

-

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

Proportional output is proportional to error inputContinuous time: P(t) = Kp * E(t)Discrete time: P(n) = Kp * E(n)


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

Integral output 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


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

Derivative output is proportional to derivative of error signal

Continuous: D(t) = Kd*dEdt

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 DCMotor

uf (n)

p(t)

Actual position

2(t)

Desired position

1*

(potentiometer)

potentiometer

2(n) u b(n)

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


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PI Controller Hardware Setup

8051Microcontroller

ADC0

Inputvoltage

conversion

interface

ADC0.0

ADC0.1

setpointpotentiometer

outputpotentiometer

0 - 2.45V

0 - 2.45V-15 to +15V

-15 to +15V

Output

voltageconversioninterface

DAC0

DAC1

0 - 2.45V

0 - 2.45V

m u l t i p l e x o r

DAC0

DAC1

to SA1SODInput 1

to SA1SODInput 2


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Circuit Schematics for Interface Circuits

Input voltage conversion interface Output voltage conversioninterface


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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 outputdiffers from the input after it has settled.

For position controller DEADBAND is ameasure 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

Embedded Micro Controller Systems

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