02 General Purpose Input - Output on the Arduino

Signals
0 Information is transferred in the digital/analogue world by
signals.

0 Electrical Signals are given a logic equivalency to enable
control and sensing from the physical universe.

0 1 == High == True == 3-5V

0 0 == Low == False == 0-2V

0 And thus the binary number system was born.

Digital and Analogue
information
0 Signals can be defined as any time varying quantity
that can be said to represent information.

0 Digital signals, at any given point of time take up only
one of a few discrete predefined states.

0 Analogue signals are not restricted to any particular
state but continuously changes itself w.r.t. time.

Hardware communication
0 Every electronic system will have two logic levels called
VCC and GND.

0 Analogue signals in this can take any value between VCC
and GND.

0 Digital Signals can only take the states of VCC and GND
states.

0 Digital devices either control or respond to either VCC or
GND.

Ports
0 A microcontroller can digitally communicate with the
outside world through Ports.

0 Every microcontroller has its own PORTs controlled
by specific port registers.

0 Ports are a collection of PINS on the microcontroller.

0 Each pin can be controlled digitally by the
microcontroller.

PORTS on the arduino
0 The atmega328 has three ports
0 B (digital pin 8 to 13)
0 C (analog input pins)
0 D (digital pins 0 to 7)

0 Each Port has a
0 PORTx register that controls the value of the port.
0 DDRx register that controls the port from output/input
mode.

Digital Pins
0 Numbered 1 – 13 on the ardiono.

0 Can be set to Input/Output by pinMode(pin) function.

0 Can be set to output by digitalwrite(pin, state)
function.

0 digitalread(pin) returns the state of the pin.

Digital PIN as output
0 The pin is in a state of low impedance.

0 This means the pin can supply current to other
circuits. (40mA)

0 Shorts or attempting to draw more current can
damage the transistors on that pin (or fry the board).

0 Use resistors to limit current from the pins. (1-
10kohms)

LEDs
0 Light Emitting diode.

0 Use R = (VS - VL) / I to calculate the
resistor value to put in series.
0 Vs = 5V; VL = 2V (depends on color)
0 I = 40mA (max current required)

0 Do not connect LEDs in parallel!!

0 The longer end is VCC and the shorter end
is GND.

PIN13 on the Arduino
0 The arduino has an LED + resistor mounted on the
board.

0 Open the program in folder 1. Blink in the arduino IDE.
0 Upload that program onto the board.
0 Watch blinky!

0 Can you make it blink faster?
0 Then slower?

Connecting External LED
0 Open folder 2.LED Breadboard

0 Connect LEDs as shown in the circuit diagram.

0 Avoid connecting multiple LEDs to a single PIN.

0 If the PIN is HIGH the LED will light up.

Digital Input
0 Digital Input can be sensed by all digital pins of the
arduino.

0 Pins when set up as inputs are said to be in a High
impedance state.
0 Which means they tend to draw very little current from the
circuit they are sensing and that means that small changes
in current are enough to change the state of the PIN.

0 If an input pin is left unconnected:
0 State may change randomly
0 Pin may capacitively couple & report the state of a nearby pin.

Switches
0 Switches are mechanical devices that “switch”
between VCC and GND states.

Switching states
0 We need to switch between VCC and GND to use with
digitalread().

0 But, tactile switches can only open and close circuits.

0 In each of the cases, if switch is pressed, the PIN goes into a
state, when released the PIN is disconnected or in a random
state.

Enter Pull-down resistors
0 Using these resistors (10kohms) provide both
states with a single push button switch.

0 When unpressed, the PIN state will read the GND
state through the pull-down resistor. (the
Resistor pulls the pin to GND when no i/p is
present)

0 When pressed, the switch will read the VCC
because it is the path of least resistance.

0 If VCC and GND are swapped in the diagram, the
resistor is called a Pull-up resistor with reversal
in logic levels.

Reading a switch
0 Open 3. LED_Switch folder.

0 Connect switch + resistor and LED + resistor circuits
from the breadboard as shown.

0 Run Switch 1.pde on the arduino IDE and upload it
onto the arduino.

Code
0 Switch is read using digital read. This will return either 1
or 0. (HIGH / LOW).

0 This is stored in a variable.

0 LED is set to that variable.

0 So if switch is pressed, 1 is sensed and LED is set to 1.
0 When switch is released, 0 is sensed and LED turns off.

0 Voila!! LED is controlled by a switch.

Interrupts
0 Interrupts are signals that indicate to the
microcontroller that a particular event has occurred.

0 Internal
0 Divide by 0 interrupts
0 Timer interrupts
0 ADC interrupts

0 External
0 Pin 2 & 3

External Interrupt
0 Pin 2 and 3 are set up for external interrupts.

0 Allow the arduino to continue doing tasks intstead of
continuosly checking for external signal changes.

0 INT0 is on pin2 and INT1 is on pin3

0 Use attachinterrupt() to associate interrupts to a
function.

attachInterrupt(interrupt, function, mode)

0 Interrupt: can be either 0 or 1 signifing which interrupt is
being set up.

0 Function : is the name of the function to be called when
the interrupt fires.

0 Mode:
0 LOW to trigger the interrupt whenever the pin is low,
0 CHANGE to trigger the interrupt whenever the pin changes
value
0 RISING to trigger when the pin goes from low to high,
0 FALLING for when the pin goes from high to low.

The ISR
0 The interrupt service routine is the function that is
called every time the interrupt fires.

0 Delay() and millis() are not supported inside an ISR.

0 Serial data received might be lost when the ISR
returns.

0 All variables that get modified within the ISR must be
volatile.

#include <avr/interrupt.h>

int ledPin = 12;
volatile int state = LOW;
void setup() {
Serial.begin(9600);
pinMode(12, OUTPUT);
attachInterrupt(0, blink, FALLING);
}

void loop() {
digitalWrite(ledPin,state);
}

void blink(){
state = !state;
}

Remember
0 Make sure there are no loose connections on the
breadboard.

0 Make sure you’ve gotten the upload successful
message on the IDE.

0 Make sure switch GND, LED’s GND and arduino’s GND
are one.

0 Don’t dismantle your circuits after completion.

Communication
0 Information is also transferred from arduino to
computer by setting a pin HIGH or LOW in sucession
to create a data waveform.

0 Each of this ‘1’s and ‘0’s are called as “bits”.

0 Because the data is transferred one after another on
the same line, it is called serial transfer of data.

0 Kinda simillarish to morse code.

TX and RX pins
0 These pins light up when serial
data is transmitted or received.

0 In the Arduino, the serial port
goes through the FTDI chip and is
converted to USB.

0 Pins 0 & 1 are the serial pins and
serial data can be tapped from
these pins.

0 Don’t use these pins for GPIO
during serial transfers.

Baud rate
0 Baud rate is the unit of symbol rate.

0 A baud rate of 1 kBd = 1,000 Bd is synonymous to a
symbol rate of 1,000 symbols per second.

0 In binary, a symbol is either a ‘1’ or ‘0’.

0 In serial communication systems, both the receiving
machine and the transmitting system should be set to
the same baud rate. (9600/11500 usually)

usage

0 In setup(){ } put serial.begin(9600);

0 In loop, use serial.println(data)

0 Open serial monitor to see data.

Serial Program
0 Open the folder 4. Serial and upload the arduino with
the code.

0 The program prints numbers from 1-100 sequentially.

0 Use the serial monitor on the Arduino to see the
stream.

0 Any program can open the COMx port associated with
your arduino and read data transmitted by the
arduino.

02 General Purpose Input - Output on the Arduino

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