Introduction - Segger J-Link User Manual

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9.1

Introduction

As power consumption is an important factor for embedded systems, CPUs provide
different kinds of low power modes to reduce power consumption of the target sys-
tem. The useful this is for the application, the problematic it is during debug. In gen-
eral, how far debugging target applications that make use of low power modes is
possible, heavily depends on the device being used as several behavior is implemen-
tation defined and differs from device to device. The following cases are the most
common ones:
1. The device provides specific special function registers for debugging to keep
some clocks running necessary for debugging, while the device is in a low power
mode.
2. The device wakes up automatically, as soon as there is a request by the debug
probe on the debug interface
3. The device powers off the debug interface partially, allowing the debug probe to
read-access certain parts but does not allow to control the CPU.
4. The device powers off the debug interface completely and the debug probe loses
the connection to the device (temporarily)
While cases 1-3 are the most convenient ones from the debug perspective because
the low power mode is transparent to the end user, they do not provide a real-world
scenario because certain things cannot be really tested if certain clocks are still
active which would not be in the release configuration with no debug probe attached.
In addition to that, the power consumption is significantly higher than in the release
config which may cause problems on some hardware designs which are specifically
designed for very low power consumption.
The last case (debug probes temporarily loses connection) usually causes the end of
a debug session because the debugger would get errors on accesses like "check if
CPU is halted/hit a BP". To avoid this, there is a special setting for J-Link that can be
activated, to handle such cases in a better way, which is explained in the following.
J-Link / J-Trace (UM08001)
CHAPTER 9
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Low Power Debugging