Transcript Slide 1

Lecture 13: GPIO Inputs
Lecturers:
Professor John Devlin
Mr Robert Ross
Overview
• Introduction to GPIO
• Interfacing inputs as GPIO
• Input ringing
Further Reading
• “A guide to Debouncing”
http://www.ganssle.com/debouncing.pdf
• MSP430x2xx Family User’s Guide
Introduction to GPIO
• GPIO stands for General Purpose Input/Output,
and relates to the pins which can be configured
as digital input or output pins
• TI refer to GPIO as Digital I/O (Chapter 6 of the
MSP430x2xx Family Users Guide)
• The MSP430-2013 has 2 GPIO ports (P1 and
P2)
• P1 has 8 GPIO Pins
• P2 has 2 GPIO Pins
Introduction to GPIO
• All the pins can be individually configured as
either Inputs or Outputs
• All the pins can be configured as triggers for
interrupts
• All P1 pins share one interrupt vector (Flag:
P1IFG), both P2 pins share a second interrupt
vector (Flag: P2IFG)
• Each pin can be individually configured with
internal pull-up or pull-down resistors
• Separate input and output registers (PxIN and
PxOUT)
Configuring GPIO
• This lecture focuses on using GPIO pins
for input, next lecture will discuss using
GPIO pins for output
• Prior to use, pins must be configured as
input or output – this is referred to as
giving the pins a direction
Setting Direction
• To set direction:
–
–
–
–
Write into PxDIR register the direction of all the pins
x will be 1 for Port1, and 2 for Port2 (ie. P1DIR and P2DIR)
‘0’ = Input
‘1’ = Output
• Examples:
– BIS.b #001h, &P1DIR ; Sets Pin 0 of P1 as an output
– BIC.b #002h, &P2DIR ; Sets Pin 1 of P2 as an input
– BIS.b #00101101b, &P1DIR ; Sets Pins 0, 2, 3 and 5 of P1 as
outputs
– BIC.b #11010010b, &P1DIR ; Sets Pins 1, 4, 6 and 7 of P1 as
inputs
– MOV.b #10000100b, &P1DIR; Sets Pins 7 and 2 as outputs, the
rest as inputs
Pull-up / pull-down resistors
Pull-up or pull-down resistors can be activated by setting bits in the PxREN registers,
provided that the pin is con.gured as an input. The MCU behaves randomly if you forget
this step because the inputs floats!!!!!;
Input reading
polling/interrupts
A program can respond to inputs in two ways. For a simple
analogy, suppose I am waiting for my daughter to come
home while writing this book. She has forgotten her key so
I need to unlock the door for her.
• I could go regularly to the door and look out to see
whether she has returned, after each paragraph for
instance. This is polling.
• I could carry on writing continuously until I receive an
interrupt from the doorbell. I then fininish the sentence to
leave the work in a well-defined state and go to the door.
Listing 4.5: Program butled1.c in C to light LED1 when button B1 is pressed.
This version has a single loop containing a decision statement.
// butled1.c - press button to light LED
// Single loop with "if"
// Olimex 1121 STK board , LED1 active low on P2.3,
// button B1 active low on P2.1
// J H Davies , 2006 -06 -01; IAR Kickstart version 3.41A
// ---------------------------------------------------------------------#include <msp430x11x1.h> // Specific device
// Pins for LED and button on port 2
#define LED1 BIT3
#define B1 BIT1
void main (void)
{
WDTCTL = WDTPW | WDTHOLD; // Stop watchdog timer
P2OUT |= LED1; // Preload LED1 off (active low!)
P2DIR = LED1; // Set pin with LED1 to output
for (;;) { // Loop forever
if ((P2IN & B1) == 0) { // Is button down? (active low)
P2OUT &= ˜LED1; // Yes: Turn LED1 on (active low!)
} else {
P2OUT |= LED1; // No: Turn LED1 off (active low!)
}
}
}
Internal Pull Up/Down Resistor
• To enable internal resistor
– Write to PxREN register
– ‘0’ = Disabled
– ‘1’ = Enabled
• Examples:
– BIS.b #00101101b, &P1REN ; Enables
internal resistor on pins 0, 2, 3 and 5 of P1
– BIC.b #11010010b, &P1REN ; Disables
internal resistor on pins 1, 4, 6 and 7 of P1
Input function select
• I/O pins are often multiplexed with special
function peripherals (eg. ADC, DAC, PWM
generator)
• The user needs to specify if the pins are
simply digital I/O or if the special function
is required:
– Write to PxSEL registers
– ‘0’ = Digital I/O
– ‘1’ = Special Function
Unused PINs
• Unused pins should not be left floating –
this can waste power and cause strange
behaviour
• TI recommends two different things can be
done with unused pins:
– Configure as Digital I/O, Output and don’t
connect to anything
– Configure as Digital I/O, Input, and enable the
internal pull-up/down resistor
Σύνδεση με πληκτρολόγιο 3x4
Σύνδεση με πληκτρολόγιο 3x4
Σύνδεση με πληκτρολόγιο 3x4
Reading Input Pins
• The current state of input pins (0 or 1) can be read from
the PxIN Registers
• This is a read-only register, TI notes that attempting to
write to this register will result in increased power
consumption for no useful result
• Remember ‘BIT’ Command is used to perform ‘AND’
updating the flags without writing to the dst register
• Examples:
– Check if Pin 0 on P1 is ‘1’:
BIT #00000001b, &P1IN
JNZ pin_1_high
; Jump if P1.0 was 1
– Check if Pin 4 on P1 is ‘0’:
BIT #00010000b, &P1IN
JZ pin_4_low
; Jump if P1.4 was 0
Register Table
Connecting inputs (Hardware)
• Switches - most
common digital input
to a microcontroller
• Switches should be
connected to a pull-up
resistor (so the input
doesn’t float)
• The MSP430 has
internal pull-up
resistors which can
be used instead
Vcc
MCU
Vcc
MSP
430
10k
Switch Problems - Ringing
Switch Bounce
6
5
4
Voltage (V)
• Switches are
mechanical devices –
a piece of metal is
used to close a circuit
• Unfortunately
switches rarely close
a circuit cleanly - the
metal bounces,
causing “ringing”
• Can anyone see a
problem with this?
3
2
1
0
0
100
200
300
400
500
Time (us)
600
700
800
900
1000
Solutions to ringing
• Hardware: Add additional hardware to ensure
that the microprocessor only receives clean
edges
• Software: Develop an algorithm which detects
transitions but discards ringing
Debouncing - Hardware
Vcc
10k
MCU
10nF
Capacitor Switch Curve
6
5
4
Voltage (V)
• A capacitor can be
used to remove
bouncing
• Output is now more
ramp-like (RC e-t/RC
Charge/Discharge
curves)
• Digital devices prefer
sharp transitions – not
a slow curve
3
2
1
0
0
100
200
300
400
Time (us)
500
600
700
Debouncing - Hardware
Vcc
10k
MCU
10nF
Debounce with Schmitt Trigger
6
5
4
Voltage (V)
• A Schmitt trigger can be
used to convert the curve
to have sharper edges
• A Schmitt trigger will
output a high when input
is over a high threshold
and a low when under a
low threshold – in
between they keep the
same value
• Schmitt triggers therefore
exhibit hysteresis
Input Voltage
3
Schmitt Trigger
2
1
0
0
100
200
300
400
Time (us)
500
600
700
Debouncing - Software
• Software needs to detect discrete switch
presses, without detecting bounces
• When switch is pressed, it should
discriminate between a bounce and a valid
button press.
Debouncing - Software
• Algorithm:
– When switched pressed start counting once
per millisecond
– If switch value = 0, reset counter
– When counter = 20 (or so) the switch has
been debounced
Do we always need to debounce?
• Not necessarily
• What does the system need to know – the
time when a switch is first pressed, or the
number of times a switch is pressed
• Switches that might not need debouncing:
– Game show buzzer
– Ejector seat
– TV remote control (sends same code until
switch is not pressed)
Summary
• GPIO Pins are General Purpose Input and
Output pins
• Before they are used they should be configured:
– Direction (Input or Output) (PxDIR)
– GPIO or Special Function (PxSEL)
– Resistor enable (PxREN)
• If pins are used to drive interrupts, several more
registers need to be setup
• Switches need to be debounced if the number of
switch presses is important