Butterfly IO Ports

1
Butterfly I/O Ports

• CS-212
• Dick Steflik

2
I/O for our labs

• To get data into and out of our Butterfly its a
  little trickier than using printf and scanf as
  you did in CS211
• Since the Butterfly doesn't have an Operating
  System we need to write and read data directly
  to/from the I/O ports
• Access to the ports is through the Special
  Function Registers (SFRs) that are defined
  symbolically in iom169.h which is included in
  your program by io.h

3
io.h
include ltavr/iom165p.hgt elif defined
(__AVR_ATmega168__)? include
ltavr/iom168.hgt elif defined (__AVR_ATmega168P__)?
include ltavr/iom168p.hgt elif defined
(__AVR_ATmega169__)? include
ltavr/iom169.hgt elif defined (__AVR_ATmega169P__)?
include ltavr/iom169p.hgt elif defined
(__AVR_ATmega8HVA__)? include
ltavr/iom8hva.hgt elif defined (__AVR_ATmega16HVA__
)?
4
iom169.h
/ iom169.h - definitions for ATmega169 / /
This should be up to date with data sheet version
2514J-AVR-12/03. / ifndef _AVR_IOM169_H_ defin
e _AVR_IOM169_H_ 1 / This file should only be
included from ltavr/io.hgt, never directly.
/ ifndef _AVR_IO_H_ error "Include
ltavr/io.hgt instead of this file." endif ifndef
_AVR_IOXXX_H_ define _AVR_IOXXX_H_
"iom169.h" else error "Attempt to include
more than one ltavr/ioXXX.hgt file." endif /
I/O registers / / Port A / define PINA
_SFR_IO8(0x00)? define DDRA _SFR_IO8(0x01)? de
fine PORTA _SFR_IO8(0x02)? / Port B / define
PINB _SFR_IO8(0x03)? define DDRB
_SFR_IO8(0x04)?
5
sfr_defs.h

• included in every compile via io.h
• contains macro definitions for accessing Special
  Function Registers as if they were just c
  language variables
• in iom169.h the statement define
  PINA _SFR_IO8(0x00) the symbol PINA is mapped
  to the SFR at address 0x00 in SFR memory
• _SFR_IO8( ) is a macro (look in sfr_defs.h )?

6
Why this is done

• in your program you include io.h and io.h in
  turn includes iom169.h (because in your project
  definition you picked the ATmega169 as the
  processor)?
• pass 1 of the compiler does all includes and
  macro substitutions ( in our example with PINA
  all occurrences of the symbol PINA in your
  program will be replaced with a reference to SFR
  0x00)?
• This makes all SFR references look like
  references to C variables

7
Memories
8
Getting Data Into and Out Of

• Initialize the ports
• set the Data Direction Registers (DDRs)?
• use PORTB for input
• use the 8 input DIP switch for inputting data
• don't forget the pull-up resistors
• use PORTD for showing output
• use 8 LEDS
• don't forget the current limiting, series
  resistors

9
Design

• This warrants 3 subroutines
• one to do initialization
• one to do input
• one to do output
• These should be written very general purpose as
  we will use them for many of the labs

10
daemons

• a daemon is a program that will sit in memory
  forever and run until the system loses power
• since most embedded system run forever (until
  they lose power) our main() should be written as
  a never ending loop.
• processing to be done can be done either in the
  body of the loop or asynchronously in response to
  an external or internal event (interrupt)?

11
Interrupts

• How Interrupts are handled
• Somewhere in your program you define an Interrupt
  Servicing Subroutine, the compiler will put the
  address of this routine into the proper location
  in the interrupt vector (low memory)?
• When the interrupt occurs the system state is
  saved, a branch to the ISS is made via the
  interrupt vector
• The ISS executes and returns the state of the
  system and you are right where you were before
  the interrupt occurred

12
ATmega169 Interrupt Vector
13
What we want to do

• use 8 LEDs for output
• 8 switches for input
• 1 button to tell when it is OK to read the
  switches
• to do this we should us an interrupt

14
The way it should work
/ Interrupt handler / // disable
interruptes // read the switches //
save the data at the next location in an array
// light the lights // enable
interrupts int main ()? // initialize
ports B D // initial Port E for interrupt
on PE2 // loop forever this is our daemon
15
volatile keyword

• the keyword volatile should be used to define
  any variable that may be modified inside of an
  interrupt handler
• volatile flags a variable to the optimizing
  compiler to not optimize variables with the
  volatile keyword

volatile char switches 0 volatile
uint8_t flags 0
16
char uint8_t

• Remember the char data type can be used for ASCII
  characters or a signed 8 bit integer
• The AVR header files define another data type
  that allows an unsigned 8 bit integer, this is
  convenient for working with gpio ports as they
  are not usually signed data.

17
internal pullups for gpio inputs

• on many MCUs you must add an external pullup
  resistor on gpio input lines
• Atmel MCUs have built-in internal pullup
  resistors
• to use the internal pullups write a one to that
  line after it has been defined as an input line
  in the DDR

// define Port B as all outputs and Port D as all
inputs DDRB 0xFF DDRD 0x00 //turn on the
internal pullup resistors for Port D PORTD 0xFF
18
Setting up an interrupt handler

• Signal or ISR
• the ISR macro has superceded the Signal macro and
  is the preferred way of setting up an interrupt
  handler

ISR( name of the int from processor .h file)?
........ ISR(SIG_PIN_CHANGE1)? ....
19
Pin Change Interrupts

• Input lines on Ports B and E can be set up to
  generate a pin change interrupt
• Each port has an associated mask register
• to enable an interrupt put a 1 into the correct
  position in the appropriate mask register
• Port B PCMSK1
• Port E PCMSK0

20
PCINT Mapping
PORT E PCINT7 7 SIG_PIN_CHANGE0 P
CINT6 6 PCINT5 5 PCINT4 4 PCINT3 3 PCINT2 2 PCINT
1 1 PCINT0 0 PORT B PCINT15 7
SIG_PIN_CHANGE1 PCINT14 6 PCINT13 5 PCINT12 4
PCINT11 3 PCINT10 2 PCINT9 1 PCINT8 0
21
Example
//set bit 2 of Port B to cause an
interrupt PCMSK1 0x04 // set bit 2 of
Port B to cause interrupt EIMSK 0xC0
// Ext Int Mask Reg EIFR 0xC0 //
Ext Int Flag Reg DDRB 0xFB // make
bit 2 an input line PORTB 0x04 //
turn on pullup ISR(SIG_PIN_CHANGE1)?
cli( ) // disable interrupts . .
. sei( ) // enable interrupts