Programming Microcontrollers
B. Furman
19MAR2011
Learning Objectives




Explain and apply best practices in writing a
program for a microcontroller
Explain the structure of a program for the
Arduino
Explain the concept of a ‘bit mask’ and use it to
determine if a bit is clear or set
Use bit-wise logical operators to create bit
masks and extract bits
Mechatronics Concept Map
Power
Source
User
Interface
ME 106
ME 120
Controller
(Hardware & Software)
ME 106
Power
Interface
INTEGRATION
ME 106
ME 154
ME 157
ME 195
Signal
Conditioning
ME 106
ME 190
ME 187
ME 106
ME 120
Actuator
Sensor
ME 120
ME 297A
System to
Control ME 110 ME 182
ME 136 ME 189
ME 154 ME 195
ME 157
BJ Furman 22JAN11
Recap Last Lecture

Binary and hex numbers


Digital pins can be inputs or outputs


Why use hex?
What is the difference?
Pins are bidirectional for digital I/O


Which Arduino function do you use?
DDRx (x = B, C, or D for ATmega328) register
determines direction

8-bit register
7


6
5
a ‘1’ in DDRx means…?
a ‘0’ in DDRx means…?
4
3
2
1
0
Test Your Comprehension

Write code to make all pins of PORTD to be
outputs (Arduino and alternate)

DDRD = 0xFF;
 DDRD = 0b11111111;
 DDRD = 255;

Arduino style
pinMode(0, OUTPUT);
pinMode(7, OUTPUT);
Write code to make pins 5, 3, and 1 of
PORTD to be outputs, and the rest inputs


DDRD = 0b00101010;
DDRD = 0x2A;

DDRD | = (1<<5) | (1<<3) | (1<<1);
Arduino style
pinMode(1, OUTPUT);
pinMode(3, OUTPUT);
pinMode(5, OUTPUT);
Structure of an Arduino Program

An arduino program == ‘sketch’
 Must have:



setup()


setup()
loop()
configures pin modes and
registers
loop()

runs the main body of the
program forever


like while(1) {…}
Where is main() ?


Arduino simplifies things
Does things for you
/* Blink - turns on an LED for DELAY_ON msec,
then off for DELAY_OFF msec, and repeats
BJ Furman rev. 1.1 Last rev: 22JAN2011
*/
#define LED_PIN= 13; // LED on digital pin 13
#define DELAY_ON = 1000;
#define DELAY_OFF = 1000;
void setup()
{
// initialize the digital pin as an output:
pinMode(LED_PIN, OUTPUT);
}
// loop() method runs forever,
// as long as the Arduino has power
void loop()
{
digitalWrite(LED_PIN, HIGH); // set the LED on
delay(DELAY_ON); // wait for DELAY_ON msec
digitalWrite(LED_PIN, LOW); // set the LED off
delay(DELAY_OFF); // wait for DELAY_OFF msec
}
Best Practices and Patterns -1

Programmer’s block

At a minimum:






Program name
Description of what
the program does
Author
Revision number
Revision date/time
Even better:




Creation date
Inputs
Outputs
Method/algorithm
/* Blink - turns on an LED for DELAY_ON msec, then
off for DELAY_OFF msec, and repeats
BJ Furman rev. 1.1 Last rev: 22JAN2011
*/
Best Practices and Patterns -2

Avoid ‘hard coding’
constants

Use #define and
symbolic names
instead



Why?
Symbolic names are
usually put in all
caps to differentiate
from variables
See me106.h
#define LED_PIN = 13; // LED on digital pin 13
#define DELAY_ON = 1000;
#define DELAY_OFF = 1000;
How to Twiddle Bits

Recall the example of the seat belt
indicator system

ATmega328
D3
C code snippet (not full program)
#define LATCHED 0
#define ENGAGED 0
pinMode(0, INPUT); // key switch
pinMode(1, INPUT); // belt latch switch
pinMode(2, OUTPUT); // lamp
pinMode(3, OUTPUT); // buzzer
key_state=digitalRead(0);
belt_state=digitalRead1);
if(key_state==ENGAGED)
if(belt_state==LATCHED)
digitalWrite(3, LOW);
digitalWrite(2, LOW);
else
digitalWrite(2, HIGH);
digitalWrite(3, HIGH);
else
;
D2
VTG= +5V
1
D0, D1
0
Bit Manipulation Practice

See the handout on Bit Manipulation

Setting bits
 Clearing bits
 Toggling bits
Challenge:
Make bits 5 and 3 of PORTB high and the rest low
Summary of Bit Manipulation

Setting a bit (making it a ‘1’)

Bitwise OR the PORTx register with the
corresponding bit mask


Clearing a bit (making it a ‘0’)

Bitwise AND the PORTx register with the
corresponding complemented bit mask


Ex. PORTB | = _BV(3);
Ex. PORTB & = ~( _BV(3) );
Toggling a bit (making it flip)

Bitwise XOR the PORTx register with the
corresponding bit mask

Ex. PORTB ^ = _BV(3);
Bit Twiddling Practice

Make Arduino pins 11 – 13 to be outputs
and pins 8 – 10 to be inputs
Use the Arduino method
2. Use the ‘all-at-once’ (general) method
1.

Check if pin 9 is high

If pin 9 is high, make pin 13 high and pin 11
low

Else both pins 13 should be low
Use the Arduino method
2. Use the general port-style method
1.
Pull-up Resistors

Pins configured as INPUTS can be ‘pulled
up’ to VTG

Why is this useful?
Puts an input pin in a known state (logic high) if no
external influence has pulled it down (to logic low)
 Example of a switch connected between a pin and
ground


How is it done?
When the pin is configured as an input, SET the
corresponding bit in PORTxn
 Undone by clearing the bit

Redo Seat Belt Sensor System

Use port-style programming
#define LATCHED 0
#define ENGAGED 0
DDRD | = _BV(2) | _BV(3); // D2 and D3 are OUTPUTs
PORTD | = _BV(0) | _BV(1); // turn on pull-ups for D0 and D1
current_state = ~PIND; // invert for active-low switches
key_state=current_state & ( _BV(0) )
belt_state=current_state & ( _BV(1) )
if(key_state==ENGAGED)
if(belt_state==LATCHED)
PORTD & = ~( _BV(2) | _BV(3) ); // buzzer and lamp off
else
PORTD | = ( _BV(2) | _BV(3) ); // buzzer and lamp on
else
PORTD & = ~( _BV(2) | _BV(3) ); // buzzer and lamp off
ATmega328
D3
D2
VTG= +5V
1
D0, D1
0
Key on D0
Belt on D1
Recap ATmega Digital I/O

Pins are bi-directional. Can configure as:



Inputs – _______ determines the pin voltage
Outputs – ______ determines the pin voltage
Direction determined by bits in DDRx register


Where x is B, C, D for the ATmega328 (and DDRx
corresponds to all 8 pins associated with the port)
If configured as output:

Program can specify a pin to be high (VTG) or low (GND) by
writing a corresponding 1 or 0 (respectively) to PORTx
register


Ex. To make Port D pins 7, 3, and 4 low, and the rest high
PORTD=___________; (write in binary, then in hex)
Recap ATmega Digital I/O, cont.

If pins configured as input, this means:

External device can pull pin voltage high or low


i.e. take up to VTG or take down to GND
You can determine the state of the port
pins by reading the PINx register
Grabs all eight logic levels at the same time
PD7
 Ex. PORTD configured as inputs

uint8_t a_pins;
a_pins=PIND;
What is the content of a_pins:
binary:__________
hex:_____
PD6
PD5
PD4
PD3
PD2
PD1
PD0
VTG
Recap ATmega Digital I/O, cont.

If pins configured as input, cont.:

Can turn pull-up resistors on or off by
writing a 1 or 0 to corresponding pins
in PORTx


A pull-up resistor internally connects a
pin to VTG to give it a defined state
(logic high, i.e., 1)
Ex. Write the code that will:
Make Port D pins inputs
 Turn on pull-up resistors
 Read the voltages on the pins and store
them in a variable, testD
 What is the value of testD in binary
and hex?

PD7
PD6
PD5
PD4
PD3
PD2
PD1
PD0
VTG
Reading PORTD Pins Example
unsigned char testD;
PD7
DDRD=0;
PD6
testD=PIND;
PD5
What is the content of testD?
PD3
binary: 11111001
PD4
PD2
PD1
hex: F9
PD0
VTG
ATmega328 Features
ATmega328 data sheet p. 1
http://www.atmel.com/dyn/resources/prod_documents/doc8271.pdf
ATmega328 Internal Architecture
ATmega328 data sheet pp. 2, 5
PORT Pin
Schematics
ATmega328 datasheet, pp. 76-77
ATmega328 Port Pin Details
See the ATmega328 data sheet, pp. 76-94
 Port pin functionality is controlled by three
register (special memory location) bits:


DDRx


PORTxn


Data Direction bit in DDRx register (read/write)
PORTxn bit in PORTx data register (read/write)
PINxn

PINxn bit in PINx register (read only)