STM32F4 discovery (Cortex-M4) has ETB, a buffer storing instruction traces. How can I use OpenOCD and on-chip st-link debugger to pull out the traces from ETB?
I am little confused between SWO/SWD. What should I be using? Also, do I need any additional hardware for extracting traces?
Thank you
I'm afraid I never heard of STM32F4 including an Embedded Trace Buffer (ETB) in the implemented subset of the ARM core and its CoreSight features. I think this is because ETB is an optional feature, and ST has decided not to configure/implement this ETB option in its STM32F4 controllers and the ARM core they embed.
I looked up the programming/reference manuals and datasheet of an upper-level representative of STM32F4xx family, and I didn't find anything about ETB, which seems to confirm this assumption.
Now, ETB is not the only option if one wants to stream trace data out of one's MCU:
STM32F4 controllers all have an Instruction Trace Macrocell (ITM), which can alternatively deliver a software-defined char output stream or snapshots of data values or the program counter that are collected either at your breakpoints or just periodically, with assistance by the Data Watchpoint/Trace (DWT) unit. You can use the ITM
printf())The ITM is usable through the SWO pin, using any adapter hardware like any version of ST-Link, j-Link, uLink-* etc. It is a proper trace interface since it works without stopping the CPU at breakpoints, so examination won't break your system's real-time properties.
Many STM32F4 controllers (AFAIK, those with >= 100 pins) include an Embedded Trace macrocell (ETM), which is able to trace program counter (PC) and data of every CPU cycle, so you can use this one to trace the entire control flow (and data flow) of your controller, also without stopping it at any breakpoint.
The humongous amount of data to be traced (make sure you have a free USB3 port...) can only be delivered in a useful way through the synchronous port interface around the GPIOE group (alternate functions TRACECLK+TRACED0/1/2/3 => 5 pins in total), which is connected to the Trace Port Interface Unit (TPIU) next to the ETM.
In order to use this technology, you need the more expensive variants of debug adapters like j-Trace, uLink-Pro or Lauterbach. The cheapest ETM-capable adapter I'm aware of (haven't used it yet, though) is QTrace by PDQlogic starting around £379. The others are available for about 1-4 k£/k€/k$.
The way your question sounds tells me that you probably just started programming STM32's. Therefore I recommend you to get a development board with an embedded ST-Link inside. This is the cheapest solution to get (SWD debug running first, and then) SWO trace running. Atollic blog has a nice intro how to do that quickly.