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EMUL-ARM: About the ARM ETM Trace

EMUL-ARM has full support for the ARM ETM (Embedded Trace Macrocell). We have two different interfaces for this:

  • CTM (Compact Trace Module) - traditional trace functionality

Traditionally, JTAG debuggers have not had the capability to support trace. This is not a design choice by the debugger designer, it is determined by the silicon vendor. Trace support capability has only been available with In-Circuit Emulators which rely on the availability of "bond-out" chips, however there are no bond-out chips for ARM at this time.

To facilitate tracing capabilities, ARM has created the ETM which adds a logic block inside the MCU and a number of pins (4, 8 or 16 data bits + 4 status bits) that send the information. The more pins, the more information can be transferred. With fewer pins, and while trying to trace too much information, there will be an overflow internally in the MCU. Regardless of the implementation, the ETM is always capable of tracing program flow if all other options are disabled.

An MCU manufacturer that wants to use the ETM needs to license it from ARM and then put it in their MCU. This results in licensing fees, additional silicon area and pins, which is probably why there have not been any general-purpose MCUs with that technology available. However, this is about to change. Philips has released their LPC210x MCU, Atmel has announced a new ARM9-based device with ETM, and there are more devices that are not yet announced.

A trace is an optional part of an emulator system that supplies advanced debugging capabilities that include:

  • Trace buffer size is 128k frames
  • Storing the execution of instructions so that the history of execution that led up to an error situation can be analyzed
  • Storing only a small portion of the program execution (filtering), and can be applied on address range, among others
  • Setting complex conditions that identify very specific events caused by the microcontroller and which then can stop the trace (triggering)
  • Storing timestamp information for every recorded "frame", allowing you to measure how long it takes to execute certain functions

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