Instrumentation Trace Microcell

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The ITM can be used to redirect `printf()` function calls. This allows a `printf()` style debugging capabilities on an embedded platform, which typically doesn't have a standard terminal window for output.

The ITM is part of the Coresight debugging and trace system, shown in the right of the below diagram.

printf() ultimately relies on the function fputc() to do the low-level character outputting. So this is the function we are going to replace. The following code shows the re-definition of fputc().

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int fputc(int ch, FILE *f) {
    return ITM_SendChar(ch);
}

Note that the passed in file argument f is ignored altogether.

ITM_SendChar() is a Cortex-M3 function provided by ARM and is defined in core_cm3.h. The declaration of it is shown below (for reference purposes).

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static __INLINE uint32_t ITM_SendChar (uint32_t ch);