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How to use ITM Printf

lpcxpresso-support's picture

Updated: Now also compatable with the newlib nohost library.

ITM Overview

As part of its SWO Trace functionality, LPCXpresso IDE v8.0.0 introduced the ability to make use of the ITM : The Instrumentation Trace Macrocell (ITM) block provides a mechanism for sending data from your target to the debugger via the SWO trade stream. This communication is achieved though a memory-mapped register interface. Data written to any of 32 stimulus registers is forwarded to the SWO stream. Unlike other SWO functionality, using the ITM stimulus ports requires changes to your code and so should not be considered non-intrusive.

Using the ITM to handle printf and scanf

The ITM stimulus registers facilitate printf style debugging. LPCXpresso IDE uses ARM's standard scheme of treating any data written to stimulus port 0 (0xE0000000) is as character data. A minor addition to your project can redirect the output of printf to this port.

Note LPCXpresso IDE v8.0.0 only supports ITM via stimulus port 0. Future version of LPCXpresso IDE plan to extend this functionality.

Scanf functionality is achieved via a special global variable, which allows the debugger to send characters from the console to the target (using the trace interface). The debugger writes data to the global variable named ITM_RxBuffer to be picked up by scanf.

To use this functionality with an LPC Open project you need to:

  1. Include the attached file (retarget_itm.c) in your project.
  2. Ensure you are using the redlib semihost or redlib or newlib nohost library.
  3. Then simply add calls to printf and scanf to your code.

If you just linking against the LPCOpen Chip library, then this is all you need to do. However if you are also linking against an LPCOpen board library then you will likely see build errors of the form:

../src/retarget.h:224: multiple definition of `__sys_write'
../src/retarget.h:240: multiple definition of `__sys_readc'

This problem can be corrected by either:

  • locating the source file board.c within the LPCOpen board library and comment out the line: #include "retarget.h, or
  • locating the file board.h and enable the line: #define DEBUG_SEMIHOSTING

SWO ITM console view

Data written to the ITM stimulus port 0 is presented in this view (as below) . The view shows the ITM console for the active debug session. Text entered into this console is sent to the target if a suitable receiving buffer exists (specifically the global int32_t ITM_RxBuffer).

Note that once the target is terminated the view is cleared.

Within this view, there are the standard start and stop buttons.

  • When ITM trace is started, and data sent from the target will be displayed in this view.
  • When ITM trace is stopped, any data sent from the target will not be captured.

Note displaying or not displaying ITM trace data has no impact on the performance of code running on the target MCU.

In addition to the standalone ITM Console view, the ITM console is also displayed as part of the standard console viewer . It can be displayed by selecting the “Display Selected Console” button and choosing the console named "<your project> ITM Console". This view persists after the target is terminated, unlike the standalone ITM console view.

Note that the standard console viewer switches automatically between consoles to show consoles that are being written to. This switching can be confusing as the ITM console is easily lost among the other consoles displayed there. It is easier to keep track of the standalone ITM console.

ITM vs Semihosting

ITM printf and semihosting printf may seem to offer similar features however there are significant differences between these two schemes.

  • A semihosted event halts the MCU and needs support from the debug tools to handle the semihosted operation.
    • Without debug tools attached, a semihosted event will permanently halt the MCU.
  • Semihosting events will not be lost, since the MCU will be halted until the event is processed.
  • Semihosting is supported on all LPC MCUs.
  • An ITM event sends data to a port and does not cause the MCU to halt.
    • Without debug tools attached, an ITM event will not halt the MCU, it will simply be ignored.
    • ITM operations could be left within production code, thereby allowing logging information to be captured simply by connecting debug tools to a running MCU.
  • ITM is only available on LPC MCUs using Cortex-M3 and Cortex-M4 cores.
  • Within the LPCXpresso IDE version 8.0, ITM operations only support printf and scanf functionality.

For further information please see the following FAQs:

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