Background Lecture

Microcontroller-based Smart House for Improved Energy Efficiency

Background

The Problem

Reducing the energy footprint at the individual level can have a major impact on total energy and environmental costs:

A typical U.S. household could save 25% of its $1,900 yearly energy bill through improved energy efficiency.1

The average U.S. household produces 22,880 lbs of CO2 per year2 – a 25% reduction would correspond to a savings of over 2.5 metric tons.

In this project you will program a microcontroller to measure statusand control heating and cooling components to maintain a comfortable temperature in a foam smart house.

1US Dept. of Energy “Energy Savers Booklet,” http://www1.eere.energy.gov/consumer/tips/pdfs/energy_savers.pdf2http://www.epa.gov/climatechange/emissions/downloads/emissionsfactorsbrochure2004.pdf

Energy Efficient Smart HouseLecture 1: Introduction to Microcontrollers

Outline

Introduction to microcontrollers

Microcontroller architectures

Programming languages

Introduction to C Programming Language

What is a Microcontroller?A microcontroller is a “system-on-a-chip” typically intended for embedded applications such as telephones, automobile engine control systems, remote controls, office machines, appliances, toys, etc.

• Low cost (few dollars to $10s)• Low power• Usually low speed• High degree of integration

– Onboard memory for storing program and variables– Analog-to-Digital, Digital-to-Analog Converters (ADCs,

DACs)– clock(s), timer(s)– lots of Inputs and Outputs (I/O)

• Often emphasize interrupt response time over throughput (instructions per second)

• Often programmed using a low-level language (e.g., Assembly)

A typical home in the US is likely to have between one and two dozen microcontrollers, compared to just a few microprocessors (desktop, laptop computers, etc.)

A typical mid-range car can have over 50 microcontrollers.

Microcontrollers vs. Microprocessors

Generic Microcontroller Architecture

The components of a microcontroller (and microprocessors, too) are connected via three data buses (sets of parallel wires) - data, address, and control.

The central processing unit fetches instructions and data, performs arithmetic or logical operations on the data, and stores the results in special local register or general purpose memory (ROM or RAM).

The I/O interfaces allow the microcontroller to communicate with and sense signals from the outside world.

*Cady, Fredrick M. Assembly and C Programming for the Freescale HCS12 Microcontroller. New York: Oxford University Press, 2008

Generic Microcontroller Architecture

If the instructions and data are stored in separate memory spaces, the architecture is said to be a “Harvard” architecture.

If they are stored in a common memory space the architecture is said to be a “Von Neumann” architecture.


Microcontroller Components: CPU

Three main parts: –ALU (contains adders, multipliers, logic functions, etc.)–Special registers (Accumulator, Index register, Program counter)–Control unit (Instruction Decoder, Sequence Controller)


How it All Works

1. The program counter places the address containing the next instruction to be executed on the address bus

2. The instruction is read from memory and decoded by the Instruction Decoder (some examples are ADD, logical AND, shift contents of a register or memory right or left, etc. - there are lots of them)


How it All Works

3. If data are required, the address of the data is read and the data fetched

4. The instruction is executed5. Any results are placed in the appropriate register or memory6. The PC is advanced to the location of the next instruction


What You Need to Know

In order to program a microcontroller you need to know three things:

– the programming model (describes special-purpose registers)

– the memory map– the instruction set (next lecture)

The Programming ModelAccumulator A, 8 bits wide (used for general-purpose calculations, logical functions, etc.)

Index Register, 16 bits wide, formed by concatenating the 8-bit H register and the 8-bit X register (used for things like counting and addressing) - denoted H:X

Program Counter (PC), 16 bits wide, points to the location of the next instruction to be executed

Condition Code Register (CCR), 8 bits wide, keeps track of whether the last operation resulted in a zero, negative number, carry/borrow, two’s complement overflow, etc.

Stack Pointer (SP), 16 bits wide, the stack is used to store the status of the system in case of an interrupt

MemoryThe HCS08 has a 16-bit-wide address bus, which means it can address 216 = 65,536 individual memory locations (not all of these are implemented in the device)

There are two types of memory present:– RAM (Random Access Memory) - this type can be written

to or read from as the program runs, however the information in it is lost when the microcontroller is powered down

– Flash ROM (Read Only Memory) - this is a type of memory that retains its contents when the chip is shut off. It can be erased and reprogrammed in large blocks but only when the program is downloaded; once the program is running, it can’t be written to by the microcontroller

Typically the program code and any data constants are stored in ROM, variables are stored in RAM.

MC9S08QG8 Memory MapI/O registers, control registers

Usually used for variable data storage and the Stack

Usually used for program code and storage of constants and Interrupt vectors

C (and other higher-level languages)

The Programmer’s Model of the HCS08 and other microcontrollers is abstracted quite a bit when programming in C or other high-level programming languages Typically, instead of assigning variables to memory locations manually as in assembly, you instead declare the variable names and types at the beginning of the program (or subroutine), and the C compiler takes care of allocating memory for these variablesLikewise, the accumulator (A) and Index Registers are generally not directly accessed in C. Instead, the programmer declares variables and works with those variables; the compiler then determines the best way to implement these higher-level instructions using HCS08 assembly-code instructions

Energy Efficient Smart HouseProgramming the Freescale HCS08 Microcontroller in C

Numerical notation in C

Hexadecimal numbers are indicated by a prepended “0x”Binary numbers are indicated by a prepended “0b”Decimal numbers have no precedent

Example:65,535 = 0xFFFF = 0b1111111111111111

Note that this is different from, e.g., Intel machines

65,535 = 0FFFFHor Freescale assembly language, where65,535 = $FFFF

Programming languagesAll processors are programmed in “machine code” using instructions and data represented by binary numbers (i.e., bits)

This is a cumbersome process. To speed the process assembly languages were developed.

As an example, the machine code sequence to load the Accumulator with the number 2210 is

0b10000110 ($A6) Load Accumulator0b00010110 ($16) 2210

The equivalent Assembly code would beLDA #22

The Assembler (CodeWarrior for your microcontroller) converts this into the machine code above

Programming languages

The trend is toward programming using higher-level languages such as C or C++The higher the level of programming language, the less control you have over the resulting machine code – but the easier the code is to write. This makes it feasible to write programs of considerable complexity in reasonable amounts of time As a small example, to add two numbers (A, at 1000 and B, at 1001) together in assembly, you would write:LDA $1000 ;Loads the first numberADDA $1001 ;Adds it to the second numberSTA $1000 ;Stores the result back in memoryIn C, this would be written simply as:A=A+B; //Add B to A

A Short Introduction to C

The “C” programming language• Originally developed by Dennis M. Ritchie at Bell Labs• Later greatly expanded as C++• One of the most widely used programming languages• Almost as fast as assembly language, but much easier• “The C Programming Language” by Kernighan & Ritchie

(ISBN 0131103628)

A Short Introduction to C

C code comprises four kinds of statements:• Instructions - these are the things the microcontroller will

perform while executing your program • Compiler Directives - these are directions to the C

compiler that give it additional information on how to compile your program (for instance, to “include” an additional file containing definitions or subroutines)

• Variable declarations - these are symbols representing locations or variable names in your program

• Comments - these are statements that you include in your program to document what you are doing

A (very) Short Introduction to C(This introduction will provide enough of a background in C to complete the exercises in this module, as well as to give a general flavor of the language. For a more in-depth look at C, we recommend a good C textbook such as The C Programming Language by Kernighan & Ritchie)

C programming essentially resembles algebra, with a few additions to allow various programming operations. Variables are given names (such as: a, temp, indoorTemperature, maxWidth, quotient, runningAverage, etc), and various operations can be performed with these variables

Variables must be declared at the beginning of a program or subroutine; this instructs the compiler to set aside memory space for these variables. In addition, the specific machine-code operations performed will depend on the type of the variable (floating point number, signed 8-bit integer, unsigned 16-bit integer, etc.)

Some examples:a = a + 1; //Add one to the value in “a”a = a * 2; //Multiply “a” by twoa = a & 0x0F; //Perform a bit-by-bit AND function,

//keeping only the lowest four bits//of “a”. (All other bits in “a” are //reset back to zero.)

a = a | 0x04; //Sets the third bit of “a” (the fours’

// place) to 1. (It may have already// been either zero or one; it will

be// a 1 after this instruction.)

msleep(10); //Call the “msleep” function to delay

//for ten milliseconds. (This function

//is not a built-in part of C, and must

//be defined. We provide it for you.a = b + 5; //Set “a” equal to “b” plus 5. (The value

// of “b” is not changed.)

Some More ExamplesPTADD = 0xFF; //Set Port A direction to all-outputs

PTAD = 0x00; //Set all Port A outputs to zero

b=0; //while (b<10){ // PTAD = 0xFF; // (A short routine to turn the outputs msleep(10); // of Port A on and off every 10ms, PTAD = 0x00; // for ten times.) msleep(10); // b++;

}

if(a==7){ //An “if” statement, to turn PTAD PTAD = 0x00;} // off if “a” is equal to 7, and on else{ // otherwise. PTAD = 0x01;}

Note that if you are assigning a value, you use a single equals sign. If you are comparing or testing a value, use a double equals sign!

// Here is an example program in C. Anything on a line that appears after// two forward slashes is a comment. (These lines are a comment, for example.)// Comments are not processed at all by the compiler; they are there for the// benefit of the programmers writing (and later, maintaining) the code.

#include <stdio.h> //An “include” instruction, which tells the compiler to insert// the contents of the requested file here. This file can contain// additional needed subroutines, functions, and definitions.// It is included here because it provides the “printf()” – // formatted print – functionality, to be used in our program.

void main(){ //The declaration of the main program body. // The “void” part means that it does not return a value when done.

int x; // Declares “x” to be an integer. This sets aside memory for this// value. (One big difference from Assembly language is that the// specific memory location of “x” is not necessarily specified.)

printf(“Hello, World!\n”); //Prints “Hello, World!” to the display, and then//skips to the next line (the “\n” part stands for New Line.)

for(x=1;x<=10;x++){ // A “for” loop. The first part (“x=1”) defines the conditions at the start// of the loop; the second condition (“x<=10”) is the “continuation”// condition – the loop will run while this is true – and the “x++” part// is code executed each time through the loop. In this case, “x++” is// C shorthand for “Add one to the value stored in x.”

printf(“X is %d\n”,x); // Prints out a diagnostic message showing what x is. This will// be executed ten times (once for each value of x.)

} // A closing brace, to show the end of the “for” loop

} //Another closing brace, to show the end of the “main()” program.

An example program in C

Energy Efficient Smart House

In the following series of laboratory exercises, you will learn to program a microcontroller to measure and control temperatures in a “smart” house and to devise strategies for intelligent control to reduce energy consumption

Energy Efficient Smart HouseI/O Ports and Timing

Background

Microcontrollers communicate with the outside world via “Ports,” which are internal 8-bit memory locations (addresses) that are connected to physical external pins on the chipThese ports are multifunctional - they can be, e.g.,

• digital signals• analog signals• analog-to-digital converters• timing signals• serial communications• lots more

You can tell the microcontroller what function each bit of a port should be by writing to its “control” registers

MC9S08QG8 16-pin DIP Pinouts

Notice that each pin can serve multiple functionsFor example, pin 6 can be PORT B, Pin 6 (PTBD bit 6), a serial data line (SDA), or an input from an external crystal clock (XTAL) The default is “Parallel I/O Port,” which is what we’ll be using in the lab, so you don’t have to do anything to tell it what function the ports should be

The HSC08 has two parallel bidirectional I/O ports, Port A (PTAD) and Port B (PTBD)Logically, the ports look to the processor like ordinary memory locations that may be read or written to:

• Port A is referenced as variable “PTAD”• Port B is referenced as variable “PTBD”

Bits of either port can be made to be an input or an output by writing a 0 (input) or 1 (output) to the corresponding bit of the port’s Data Direction Register (DDR) Port A has 4 bidirectional I/O bits, PTAD bit 0 through PTAD bit 3.Port B has 8 I/O bits, PTBD bit 0 through PTBD bit 7

Parallel I/O Ports

The ports are configured via “Data Direction Registers” (DDRs) on a bit-by-bit basis.Logical “1” configures the bit to be an output, “0” to be an inputThe DDR variable for Port A is PTADDThe DDR variable for Port B is PTBDD

Parallel I/O Ports

For example, in the codePTBDD = 0x07;PTBD = 0x08;

the first line makes bits 0 through 3 of Port B all outputs and bits 4 through 7 all inputsThe second line makes bit 3 of Port B = 1.The result will be that pin 9 of the microcontroller (PTBD bit 3) will be high (about 3.3 volts)The instruction

PTBD = 0x00;will result in 0 volts appearing at that pin

Parallel I/O Ports

If, at the same time an external device (a switch, external logic, sensor, etc.) is attached to pin 8 (PTB4) and produces a 3.3 volt signal while bits 5-7 are grounded, the instruction

a = PTBD;will result in a 1 being written to bit 4 of the variable “a”

Parallel I/O Ports

Parallel I/O PortsSometimes it’s necessary to test if one or more bits in a register or memory address are equal to 1You do this by using a Boolean “AND” function, then testing for zero (A single ampersand represents the bitwise AND function)For instance, to test if bits 1 or 2 of Port A (PTAD) are set to 1, you could use the following code fragment:

a = PTAD & 0b00000110;if (a==0){ /* Put code to run when bits are zero here */ } else { /* Put code to run when one or more bits are one here */ }

Timing Delay LoopsThere are many ways, including using built-in timer functionality or writing custom assembly code, to get a microcontroller to delay for a certain amount of time. These techniques will be covered in depth if you take any course in beginning microcontroller programming, and so are not covered here A delay function – msleep() – is provided for you; simply call it with the (integer) number of milliseconds that you want to wait For instance, to wait 50 milliseconds, you could use:

msleep(50);

This subroutine is available only if the corresponding code is copied into your program body (or if you use the provided C programming stationery)

Putting it all togetherSuppose you want to turn an LED attached to PTB0 (pin 12) on for 1 second then off for 1 second, and repeat the cycle continuously

Here’s the code:

void main(){

PTBDD = 0xFF; // make PTB0 an output while(1){

PTBD = 0x01; // turn on bit PTB0

msleep(1000); //Delay 1000ms=1sec

PTBD = 0x00 // turn off bit PTB0

msleep(1000); //Delay 1000ms=1sec

}}

Some Basic Concepts in Thermodynamics

In a typical house heat can be lost via three mechanisms:

Conduction - heat transfer between adjacent particles in a solid due to a temperature difference

Convection - heat transfer due to the motion of molecules in a fluid both by random motion at the molecular level and by bulk motion of the fluid

Radiation - heat transfer by electromagnetic radiation

Suppose you have a room at temperature Tin surrounded by walls in contact with the outside environment at temperature Tout

The room loses heat to the walls through convection and radiation, the conduction in the walls transfer the heat to the outside surface, and the heat is transferred to the outside air by convection and radiation



We can characterize the temperature behavior of the room using Newton’s law of cooling:The rate at which the temperature inside the room will change is proportional to the difference in temperature inside and outside

dTin(t)/dt = -[Tin (t) - Tout]

where t is the time and is a characteristic time constantThis may be integrated to give the inside temperature as a function of time

Tin (t) = Tout + Tin (0) - Tout) e-t/

Controlling the TemperatureTo control the temperature of the Smart House you need to understand two components:

The first is a thermoelectric heat pump

• Based on the Peltier effect – when a current passes through a junction of dissimilar metals, heat is pumped from one metal to the other

• Can be used as a thermoelectric cooler (TEC) or as a heater if the direction of the current is reversed

• Typical efficiencies are in the range of 5-20% (compared with resistive heating, which is essentially 100% efficient)

• Most efficient when the difference between the inside and outside temperatures is small

To use the TE heat pump, just sandwich it between two heatsinks and connect to your circuit, as described In the lab exercise – if the wrong side becomes cold, just reverse the wires

Controlling the TemperatureTo control the temperature of the Smart House you need to understand two components:

The second is the Maxim DS18B20 Programmable Resolution Digital Thermometer

• Digital thermometer-on-a-chip

• Includes all electronics to sense, digitize, and transmit the temperature to the microcontroller

• Output is degrees Celsius with ½ degree accuracy and selectable resolution ranging from 1/2 to 1/16 degree

To use the DS18B20, connect it to the circuit as shown in the instructions in the lab exercise and schematic

To get the temperature, use the subroutine GetTemp, as described in the lab exercise

Now you’re ready to write your own code to control the house climate.

Background Lecture

Documents

Transcript of Background Lecture