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<h1>CMSIS Debug Support</h1>
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<hr>
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<h2>Cortex-M3 ITM Debug Access</h2>
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<p>
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  The Cortex-M3 incorporates the Instrumented Trace Macrocell (ITM) that provides together with 
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  the Serial Viewer Output trace capabilities for the microcontroller system. The ITM has 
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  32 communication channels which are able to transmit 32 / 16 / 8 bit values; two ITM 
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  communication channels are used by CMSIS to output the following information:
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</p>
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<ul>
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        <li>ITM Channel 0: used for printf-style output via the debug interface.</li>
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        <li>ITM Channel 31: is reserved for RTOS kernel awareness debugging.</li>
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</ul>
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<h2>Debug IN / OUT functions</h2>
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<p>CMSIS provides following debug functions:</p>
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<ul>
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        <li>ITM_SendChar (uses ITM channel 0)</li>
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        <li>ITM_ReceiveChar (uses global variable)</li>
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        <li>ITM_CheckChar (uses global variable)</li>
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</ul>
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<h3>ITM_SendChar</h3>
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<p>
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  <strong>ITM_SendChar</strong> is used to transmit a character over ITM channel 0 from 
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  the microcontroller system to the debug system. <br>
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  Only a 8 bit value is transmitted.
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</p>
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<pre>
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static __INLINE uint32_t ITM_SendChar (uint32_t ch)
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{
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  /* check if debugger connected and ITM channel enabled for tracing */
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  if ((CoreDebug->DEMCR & CoreDebug_DEMCR_TRCENA)  &amp;&amp;
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      (ITM-&gt;TCR & ITM_TCR_ITMENA)                  &amp;&amp;
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      (ITM-&gt;TER & (1UL &lt;&lt; 0))  ) 
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  {
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    while (ITM-&gt;PORT[0].u32 == 0);
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    ITM-&gt;PORT[0].u8 = (uint8_t)ch;
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  }  
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  return (ch);
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}</pre>
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<h3>ITM_ReceiveChar</h3>
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<p>
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  ITM communication channel is only capable for OUT direction. For IN direction
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  a globel variable is used. A simple mechansim detects if a character is received.
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  The project to test need to be build with debug information.
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</p>
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<p>
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  The globale variable <strong>ITM_RxBuffer</strong> is used to transmit a 8 bit value from debug system
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  to microcontroller system. <strong>ITM_RxBuffer</strong> is 32 bit wide to enshure a proper handshake.
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</p>
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<pre>
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extern volatile int ITM_RxBuffer;                    /* variable to receive characters                             */
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</pre>
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<p>
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  A dedicated bit pattern is used to determin if <strong>ITM_RxBuffer</strong> is empty
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  or contains a valid value.
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</p>
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<pre>
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#define             ITM_RXBUFFER_EMPTY    0x5AA55AA5 /* value identifying ITM_RxBuffer is ready for next character */
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</pre>
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<p>
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  <strong>ITM_ReceiveChar</strong> is used to receive a 8 bit value from the debug system. The function is nonblocking.
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  It returns the received character or '-1' if no character was available.
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</p>
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<pre>
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static __INLINE int ITM_ReceiveChar (void) {
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  int ch = -1;                               /* no character available */
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  if (ITM_RxBuffer != ITM_RXBUFFER_EMPTY) {
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    ch = ITM_RxBuffer;
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    ITM_RxBuffer = ITM_RXBUFFER_EMPTY;       /* ready for next character */
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  }
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  return (ch); 
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}
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</pre>
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<h3>ITM_CheckChar</h3>
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<p>
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  <strong>ITM_CheckChar</strong> is used to check if a character is received.
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</p>
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<pre>
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static __INLINE int ITM_CheckChar (void) {
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  if (ITM_RxBuffer == ITM_RXBUFFER_EMPTY) {
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    return (0);                                 /* no character available */
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  } else {
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    return (1);                                 /*    character available */
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  }
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}</pre>
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<h2>ITM Debug Support in uVision</h2>
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<p>
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  uVision uses in a debug session the <strong>Debug (printf) Viewer</strong> window to 
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  display the debug data.
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</p>
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<p>Direction microcontroller system -&gt; uVision:</p>
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<ul>
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  <li>
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    Characters received via ITM communication channel 0 are written in a printf style
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    to <strong>Debug (printf) Viewer</strong> window.
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  </li>
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</ul>
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<p>Direction uVision -&gt; microcontroller system:</p>
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<ul>
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  <li>Check if <strong>ITM_RxBuffer</strong> variable is available (only performed once).</li>
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  <li>Read character from <strong>Debug (printf) Viewer</strong> window.</li>
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  <li>If <strong>ITM_RxBuffer</strong> empty write character to <strong>ITM_RxBuffer</strong>.</li>
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</ul>
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<p class="Note">Note</p>
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<ul>
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  <li><p>Current solution does not use a buffer machanism for trasmitting the characters.</p>
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  </li>
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</ul>
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<h2>RTX Kernel awareness in uVision</h2>
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<p>
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  uVision / RTX are using a simple and efficient solution for RTX Kernel awareness.
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  No format overhead is necessary.<br>
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  uVsion debugger decodes the RTX events via the 32 / 16 / 8 bit ITM write access
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  to ITM communication channel 31.
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</p>
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<p>Following RTX events are traced:</p>
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<ul>
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  <li>Task Create / Delete event
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    <ol>
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      <li>32 bit access. Task start address is transmitted</li>
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      <li>16 bit access. Task ID and Create/Delete flag are transmitted<br>
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          High byte holds Create/Delete flag, Low byte holds TASK ID.
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      </li>
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    </ol>
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  </li>
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  <li>Task switch event
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    <ol>
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      <li>8 bit access. Task ID of current task is transmitted</li>
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    </ol>
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  </li>
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</ul>
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<p class="Note">Note</p>
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<ul>
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  <li><p>Other RTOS information could be retrieved via memory read access in a polling mode manner.</p>
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  </li>
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</ul>
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<p class="MsoNormal"><span lang="EN-GB">&nbsp;</span></p>
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<hr>
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<p class="TinyT">Copyright ? KEIL - An ARM Company.<br>
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All rights reserved.<br>
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Visit our web site at <a href="http://www.keil.com">www.keil.com</a>.
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