Embedded system: MSP430g2553 IAR programming ports/pins BASIC Input / Output syntax

Question

To put it simply : How do I define and use ports / pins correctly in IAR EW with MSP430g2553?

Ill use example to clarify what I do not understand.

I have a simple state machine with 3 states. I want to program 3 pins for input and 2 pins for output. Then, depending on inputs I manage state.

First, is this correct way of defining inputs / outputs ?

 P1DIR |= BIT0 + BIT1; //pins 1.0 and 1.1 output
 P1DIR &= ~BIT2 + BIT3 + BIT4; // pins 1.2 , 1.3, 1.4 input

The above seems to me fairly straightforward to use, however, bigger question is how do I reference input pins in code ? And how do I set output based on input?

To further my problem, here is my starting code for this state machine and I've put in pseudocode where I dont understand how to write syntax. Would be of great help if anyone could fill in the pseudocode and commentate a bit. I've looked many tutorials but I dont seem to get this simple thing from them.

# include "msp430g2553.h"
# include "stdio.h"
# include "math.h"

#define START 1
#define LEFT_ON 2
#define RIGHT_ON 3

char STATE;

main ()
{

 P1DIR |= BIT0 + BIT1; //port 1.0 and 1.1 output
 P1DIR &= ~BIT2 + BIT3 + BIT4; // port 1.2 , 1.3, 1.4 input
 
 WDTCTL = WDTPW + WDTHOLD;
 STATE =START;
 
 
 while(1)
 {
  //STATE = START;
   
   
   switch (STATE)
   {
   case START:
     {
     // Starting state I want both outputs to be set 1, I dont know how
       set p1.0 to 1
       set p1.1 to 1
       puts("START");

     //check inputs to switch state

      if (1.2 == 1 & 1.3==0 & 1.4==0) {
        STATE = RIGHT_ON;
      } else if (1.2 == 0 & 1.3==0 & 1.4==1)) {
        STATE = LEFT_ON;
      }  
     
     break;
     }
     
   case LEFT_ON:
     {
       // Here I wish to to put 1.0 output to 1 and 1.1 output to 0
          p1.0 set to 1
          p1.1 set to 0
       // now check if 1.3 is 1
          if (1.3 == 1) { 
             STATE = START;
             }     
       break;
     }
             
   case RIGHT_ON:
     {
      // Here I wish to to put 1.0 output to 0 and 1.1 output to 1
          p1.0 set to 0
          p1.1 set to 1
       // now check if 1.3 is 1
          if (1.3 == 1) { 
             STATE = START;
             }     
       break;
      }
   }//end of Switch  
 }// end of while
 
 
}// end of main

Answer 1

First, is this correct way of defining inputs / outputs ?

I assume P1DIR is the correct data direction register (I don't know this particular MCU in detail), but apart from that: no, it isn't correct. First of all, use bitwise OR | not addition. They give the same result but + makes the code look strange and a bit harder to read. The average C programming book will tell you to do:

P1DIR |= BIT0 | BIT1;

Note that P1DIR |= ... will leave all pins currently set as output as they are. That may or may not be what you want.

To set a port pin active then simply do the same, SOME_PORT_REGISTER |= PIN_MASK;. Similarly, you can toggle a single pin with ^= which is bitwise XOR. Or set it to zero with &= ~(mask).

P1DIR &= ~BIT2 + BIT3 + BIT4; is wrong, ~ is a unary operator that only applies to one operand. Corrected code:

P1DIR &= ~(BIT2 | BIT3 | BIT4);

---

I've looked many tutorials

Most tutorials on the web are unfortunately quite bad. Start by reading a decent C programming book before anything else. Once you've learnt the basics of C, you can go look for tutorials.

For example this article about register access I wrote here, it assumed that the reader already knows C: How to access a hardware register from firmware?

As for full beginner tutorials, I think this one has better quality than most: Embedded Software in C for an ARM Cortex M, Jonathan W. Valvano and Ramesh Yerraballi. (It's Cortex M not MSP430 but the principles are very similar no matter MCU. The same author also has an older tutorial that used NXP HSC12 examples, which is another 16 bitter even more similar to MSP430.)