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Introduction to microcontrollers
A microcontroller is a small computer on a single integrated circuit
containing:
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Processor core (CPU)
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Memory
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Programmable input/output peripherals.
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Program memory
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More . . .
Microcontrollers are designed for embedded applications, in contrast to
the microprocessors used in personal computers or other general purpose
applications.
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Applications
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Car industry
Telephones
Appliances
Cameras
Displays
Gadgets
Remote controls
Washing machine
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Remote schematic
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Typical Input and output
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Switches
LEDs
LCD
Sensors(light,sound, humidity)
ADC (often in the chip)
DAC
Control Power Electronics
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PIC microcontroller
Made by
Microchip Technology
Peripheral Interface Controller
We will be using the PIC16F84A.
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Simple ( 35 instructions RISC )
Cheap ( ~ £3 )
Robust (I haven't broken one yet!!!)
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Background reading
• The Quintessential PIC (quite advanced)
• Many simpler online articles on PIC
programing.
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VOltmeter demo
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PIC 16F84A pin layout
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Summary of pins
OSC1/CLKIN
: Oscillator crystal input.
OSC2/CLKOUT : Oscillator crystal output.
MCLR(inv)
: Master clear(reset)input. Programming voltage input.
RA0 - RA3
RA4/T0CKI
RB0/INT
RB1 - RB7
: Bi-directional I/O port.
: Bi-directional I/O port. Clock input to the TMR0 timer.
: Bi-directional I/O port. External interrupt pin.
: Bi-directional I/O port.
VSS
VDD
: Ground
: Positive supply(+2.0V to +5.5V)
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PIC development.
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LAB INTRO DEMO
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END OF LECT 1
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Understanding the architecture
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To program a PIC you need to know about the device.
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Not like high level programming (Matlab JAVA C?).
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The internal structure of the device is called the architecture.
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The following slide shows the internal structure of the PIC16F84A
• Registers
• ALU
• Buses
• Memory (Program / User)
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FOCUS program data IO
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Registers
Registers store binary numbers.
In the PIC16F84A registers are 8 bit.
Programming a PIC is mostly about moving information from one register
to another and simple operation on the data.
The WORKING register is a special register that is used a lot!
The ARITHMETIC LOGIC UNIT (ALU) is used to operate on data.
The STATUS register --- bits tell us about result of an operation.
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ALU and working register
IN
STATUS
REGISTER
ALU
WORKING REGISTER
OUT
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File Registers.
The PIC has 2 banks of 8 bit registers.
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Some of these are special purpose
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IO
Status
interrupts
68 general purpose storage
● Store variables
● counters
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Registers and addresses
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Each register has a unique address.
Instructions use this address to read/write a particular register
7 bits from opcode
Selects the bank
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Important registers
STATUS :
W:
PC:
result of an operation (e.g. 0)
also used to select register bank
special register used to store result of operation
(not got an address)
program counter. Address in program memory of next
instruction. Usually incremented by 1 after instruction
has been executed.
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PIC16F84A program memory
Program memory is separate from user data (Harvard architecture)
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1024 words of program memory (instructions)
Each instruction has 14 bits
You can not read or write to program memory
Execution starts at address 0
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Opcode and Operands
Instructions have
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opcode
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what it does (e.g. move something)
operand(s)
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What it does it too (e.g. what we move)
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Machine Instructions (PIC16F84A)
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Each instruction is 14 bits
For example 000010 0 0100000
000010
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--- subtract the contents of a register from W
0
--- were we put result (W or file register)
010000 --- which register
Definition:
opcode (instruction)
000010
operand (data or address of data)
0 010000
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Assembly code
We would go mad trying to program in machine code so we use
ASSEMBLY CODE
For example 000010 0 0100000
is written as
subwf 20h, w
Subtract W from register number 20h and put the result in W.
A program called an assembler converters assembly code into machine
code (MPLABX uses mpasm).
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Instruction Descriptions (page 37 ...)
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mpasm reference (Assembly language).
If you want to know more about the assembler please look at the E_Book
mpasm_reference.pdf
You can find this in the Resources folder.
http://people.bath.ac.uk/eespjl/courses/Embedded/Resources/E-books/
Note that it might be easier to learn from the examples (depends on how
your brain works).
There are many features of mpasm that are not in the examples.
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Simple program (REDO)
We want to do:
Assembly code:
movf
addlw
movwf
15,w
D'101'
NUM_2
NUM_2 = NUM_1 + 101;
; Copy the variable NUM_1 to W
; Add the literal constant 101 decimal to it
; Copy NUM_1+101 into NUM_2
Address
Column one is blank
(unless it's a label)
0
1
2
Machine code:
001000 0 0001111
111110 0 1100101
000000 1 0010000
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Simulator doing prev slide
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TALK ABOUT LAB1 PROGRAM.
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Template program
NUM EQU 0
ORG
MOV ..