title: TI MSP430 Interrupt Problems After UART Code Port
body: <p>I am using the MSP430F2013 processor for an application, which doesn't have a UART. I need a UART, and so I used the TI's sample code "msp430x20x3_ta_uart2400.c" to emulate one using the Timer module. This all worked fine (compiled with IAR Embedded Workbench), having tested it using PuTTY to transmit characters to a development board and a loopback to echo them to the terminal.</p>
  
  <p>That was a de-risking exercise, and now I've come to port that code into my application's state machine. Having done this, I'm having issues surrounding the timer interrupts and low power sleep modes. Here's the snippet of my code around the entry into the low power (sleep) mode:</p>
  
  <pre class="lang-c prettyprint-override"><code>// Prepare the UART to receive one byte.
  prepare_receiver();
  
  // Enter low power mode 1.
  __bis_SR_register(LPM1_bits + GIE);
  
  // Check whether the full message has been received.
  if(true == get_message_complete())
  {
      process_event(e_euart_message_received, NULL);
  }
  </code></pre>
  
  <p>What I'm seeing on the debugger (C-Spy) is that sometimes it will execute the <code>bis_SR_register()</code> line on first entry and then go to the <code>if</code> statement, i.e., ignoring the fact that I've asked it to go to sleep. On other occasions, when it does go to sleep when it should, the ISR triggers correctly and eventually brings me back to the <code>if</code> statement to continue program execution (as I'm expecting). However, if I try to step to the next statement, the application freezes on that first line, i.e., I can't advance.</p>
  
  <p>I can't think of anything functionally different from TI's example that I'm doing, so I figure my problem must be something to do with how I've ported it. For example, my Timer ISR and the code I've posted here are in different compilation units - would this sort of decision have any bearing on things? I'm aware my question might be a little vague but unfortunately I can't post all of my code, so instead I'm looking for someone with MSP experience who might be able to suggest some things to look at or some potential pitfalls that I may have fallen into.</p>