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Simple two state (state machine?)

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 * A simple example of a two state "state machine".
 * Using the watchdog timer, as an interval timer
 * to set a flag every ~1 second. Said flag is then
 * acted on from within main().
 * Source written by Yyrkoon of forum.43oh.com
 * Written for the v1.5 (msp430G2553) Launchpad
 * Compiled, uploaded, and tested  using Energia, with 
 * the Energia framework bypassed. effectively gcc-msp430.
 * the following source is fairly basic, mostly common 
 * knowlege, and as such is open to public domain.
#include <msp430.h>
#include <stdint.h>
#include "uart.h"

#define FLAG_second_elapsed BIT0            // Flag bit position to determine if a second has elapsed.

volatile uint16_t ticks = 0;                // Counter used to store WDT "ticks"
volatile uint8_t flags = 0;                 // "bit field" used to store global state flags.

void printf(char *, ...);                   // printf() output pointed to UART through putc()

int main(void) 
  WDTCTL = WDT_MDLY_32;                     // Configure Watchdog as 32ms interval timer( @ 1Mhz CPU frequency )
  BCSCTL1 = CALBC1_1MHZ;                    // Load calibrated constants for 1Mhz operation.
  DCOCTL = CALDCO_1MHZ;                     // DCO == 1Mhz

  uart_initialize();                        // Initialize UART. TX only for debugging purposes.

  IE1 |= WDTIE;                             // Enable WDT interrupt
    if(flags & FLAG_second_elapsed)         // Test if a second has elapsed.
      flags &= ~(FLAG_second_elapsed);      // Clear flag
      printf("tick \n\r");                  // Do whatever needs to done every interval here
    _BIS_SR(LPM0_bits + GIE);               // Put CPU into sleep, enable global interrupts.

  return 0;

__attribute__( (interrupt (WDT_VECTOR)) )

void WDT_INTERVAL_ISR(void) 
  if ((++ticks % 32) == 0)                   // Test if ~1 second has elapsed. Change if CPU is not 1MHz
  {                                          // Using power of 2 as Suggested by Rickta59 on these forums
    flags |= FLAG_second_elapsed;            // If we're here, a second has elapsed, so set corresponding flag.

  _BIC_SR_IRQ(LPM0_bits);                    // leave the ISR and exit sleep mode




An example of a simple two state "state machine".


All comments are welcome. This code is meant to provoke discussion. In hopes that we may learn from one another.


For example, one thing I am unclear on myself is re-enabling GIE every time the main loop is cycled. Since in effect the WDT ISR only brings the CPU out of sleep, and does not disable global interrupts. I am assuming GIE does not need to set every loop cycle but again . . . unclear.

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