WRITING CODE Introductory Comments /* Disgruntled Avians Lab 6
ENGR355 by Tim Kyle Rev 01.45 1/1/2014 LM-3551 Tilt Sensor attached
at SPI port 1. 16x2 LCD parallel interface p5-p12 Reset switch p2
*/
Slide 4
WRITING CODE Include, Global Definitions, Interface Assignments
and Constants #include mbed.h const float pi=3.1415; //constant
declaration, does not ever change DigitalOut BlueLED p6; //defines
pin 6 as BlueLED int program_cycles=0; //global variable
declaration A list of the official libraries can be found at:
http://developer.mbed.org/handbook/Homepage
http://developer.mbed.org/handbook/Homepage
Slide 5
WRITING CODE Functions and/or Function Prototypes void
porcine_splash() {} //Displays lots of advertisements so my program
will make lots of money. void display_lots_of_advertisements{
insert_lots_of_code_here(); collect_the_money_here(); }
Slide 6
WRITING CODE Main function Your code should never end. Often
this means it will end with a while(1) or similar unending command.
int main() { porcine_splash(); while(1) {
display_lots_of_advertisements(); } //if program execution
continues down here, uninitialized program memory is executed and
BAD things might happen.
Slide 7
GPIO GENERAL PURPOSE INPUT/OUTPUT Pins that can be used as
input, output or a combination of both. Most processor pins
function as GPIO some have additional functions. On the mbed, the
pins capable of GPIO are labelled p5-p30 and p33-p36. An
alternative labelling is the processor labelling that includes the
port number (ie. P0_8, for Port 0 Pin 8)
Slide 8
GPIO GENERAL PURPOSE INPUT Measures the voltage of a given pin
to determine its state. Pin 2.0V is a logic high. DigitalIn enables
an automatic pull down resistor Without a pull up or pull down
resistor, the logic input can float randomly between states when
the switch is open. HIGH IMPEDIENCE LOGIC INPUT ON MICROCONTROLLER
3.3V
Slide 9
GPIO GENERAL PURPOSE INPUT The pull down resistor will pull the
logic input low when the switch is open. When the switch is closed
the logic input transitions high. HIGH IMPEDIENCE LOGIC INPUT ON
MICROCONTROLLER 3.3V
Slide 10
GPIO GENERAL PURPOSE OUTPUT 0 = Logic Low (V OL 0.4V)pin will
sink current 1 = Logic High (V OH V DD -0.4V) pin will source
current The guaranteed minimum source/sink current is 4mA/pin. This
is low compared to other microprocessors. More typical is 20mA/pin
Not all pins are created equally. P21 is the high current output
pin and is guaranteed to drive at least 20mA P27 and P28 are the
open drain pins and are guaranteed to sink at least 20mA It is safe
to source or sink more current (20mA/pin) in the prototype
phase.
Slide 11
GPIO GENERAL PURPOSE INPUT & OUTPUT A pin or group of pins,
called a bus, can switch roles between inputs and outputs. This is
useful when the pins are driving a bidirectional bus. In the
diagram below, data flows back and forth between the LCD and the
Microprocessor on this 4 bit parallel data bus. mbed d1 d2d3d4
d5d6d7d8 p1 p2 p3 p4 p5 p6 p7 p8 LCD
Slide 12
GPIO mbed d1 d2d3d4 d5d6d7d8 p1 p2 p3p4 p5p6p7p8 LCD GENERAL
PURPOSE INPUT & OUTPUT When the mbed is sending commands to the
LCD the mbed bus on p1-p8 they are acting as outputs.
Slide 13
GPIO mbed d1 d2d3d4 d5d6d7d8 p1 p2 p3 p4 p5 p6 p7 p8 LCD
GENERAL PURPOSE INPUT & OUTPUT When the LCD is sending a
response the mbed bus must act as inputs to receive the reply.
Slide 14
GPIO Switches A common device connected to a GPIO input is a
switch. Switches come in various styles with various features.
Tactile Switch Knife Switch Missile Switch Light Switch Easy
Button
Slide 15
GPIO Switches Bounce When a switch is moved from one state to
another, after the contacts smash together or apart, they bounce
before making a solid logic level.
Slide 16
GPIO Debouncing Switch Methods Off-Processor Methods RC Filter
Flip Flops Dedicated IC Software Methods State Machine Software Low
Pass Filter Edge detect followed by a time delay DebounceIn
interface (mbed specific)
Slide 17
GPIO LEDs driven by a GPIO output. LEDs are handy but without
proper current limiting, they can runaway and over heat. Runaway
Forward voltage decreases with an increase in temperature,
increasing the drive current, increasing the temperature until they
overheat and fail. Prevent Runaway with a current limiting
resistor.
Slide 18
GPIO Indication LED Specifications Significant Specifications
for Indication Grade LEDs Forward Voltage (between 1V and 5V)
Generally varies by color Forward Current (between 1mA and 20mA)
They are related and typically shown as a graph on the
datasheet.
Slide 19
LED Resistor Sizing
Slide 20
LED Resistor Sizing
Slide 21
LED Resistor Sizing
Slide 22
LED Resistor Sizing 3.3V GPIO OUTPUT R Current Source GPIO
OUTPUT ALMOST 0V R Current Sink 3.3V Inverted Logic
Slide 23
LED Resistor Sizing 3.3V GPIO OUTPUT R Current Source
i=10mA
Slide 24
LED Resistor Sizing R Current Source i=10mA V 1.8V V 3.3V + + _
_
Slide 25
LED Resistor Sizing R Current Source i=10mA V 1.8V V 3.3V + + _
_
Slide 26
LED Resistor Sizing R Current Source i=10mA V 1.8V V 3.3V + + _
_ 3.3V - 1.8V - (0.010A*R)=0 1.5V/0.010A = R R=150
Slide 27
LED An LED only works in one direction. Current normally flows
from the longer leg (anode) to the shorter leg (cathode). Cathode
is usually marked by a flat spot on the plastic housing.
