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<body>
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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) &&
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(ITM->TCR & ITM_TCR_ITMENA) &&
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(ITM->TER & (1UL << 0)) )
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{
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while (ITM->PORT[0].u32 == 0);
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ITM->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 -> 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 -> 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"> </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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