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ARM Cortex-M / SWO trace: A variable is not recorded from the beginning

11-Jun-2021

Errors like missing data or data displayed at the wrong address in the trace recording can be a result of SWO tracing during the PLL initialization. SWO trace during the PLL (Phase-Locked Loop) initialization is not supported by winIDEA.


Possible solutions:

1. Start tracing after PLL is completely initialized.

For example, set a breakpoint after the PLL initialization is concluded and start the trace at that point to capture all the events after the PLL is initialized.




Synchronization packets (SPs) is trace data - an output from the ITM. These types of trace packet formats are unique patterns in the bit-stream that enable capture hardware to identify bit-to-byte alignment. SPs are sent out periodically in the course of a trace with the first one send on the very beginning of the trace. They need to be captured by the BlueBox, under the condition that a correct Clock is configured (SWO Clock e.g. 200 Mhz), to accurately decode information. To get the correct decoding information and capture the first SP the initialization, which changes clock, has to be finished before the recording (New Analyzer Session) can begin. If the BlueBox is not able to catch the initial Syncronization packet it won't be able to decode trace stream until next SP is captured. 



2. If possible use Parallel trace capture method instead of SWO.

winIDEA supports Parallel Trace during the PLL initialization. Note that additional trace pins are required.

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