Strategies For Cortex-M Devices - Segger J-Link User Manual

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5.8.2

Strategies for Cortex-M devices

J-Link supports different specific reset strategies for the Cortex-M cores. All of the
following reset strategies are available in JTAG and in SWD mode. All of them halt the
CPU after the reset.
5.8.2.1 Type 0: Normal
This is the default strategy. It works well for most Cortex-M devices. J-Link tries to
reset both, core and peripherals by setting the SYSRESETREQ & VECTRESET bits in
the AIRCR. The VC_CORERESET bit is used to halt the CPU before it executes a single
instruction.
On devices that are known to have a bootloader, this bootloader is started after the
core & peripherals have been reset and stopped before trying to start the application
program, thus ensuring that the bootloader (which may perform important initialisa-
tions) has a chance to do so.
This type of RESET can fail:
One reason is that the CPU is in power down state. In this case, the reset pin is used
to reset the device. If this fails as well, then Connect-under-Reset is executed.
Other reasons why the initial reset may not work are typically shortcomings in the
silicon (sometimes only in Beta silicon). Some of these reasons are:
• Watchdog continues to run when CPU is halted
• SYSRESETREQ also reset debug unit
5.8.2.2 Type 1: Core
Only the core is reset via the VECTRESET bit. The peripherals are not affected. After
setting the VECTRESET bit, J-Link waits for the S_RESET_ST bit in the Debug Halting
Control and Status Register (DHCSR) to first become high and then low afterwards.
The CPU does not start execution of the program because J-Link sets the
VC_CORERESET bit before reset, which causes the CPU to halt before execution of
the first instruction.
5.8.2.3 Type 2: ResetPin
J-Link pulls its RESET pin low to reset the core and the peripherals. This normally
causes the CPU RESET pin of the target device to go low as well, resulting in a reset
of both CPU and peripherals. This reset strategy will fail if the RESET pin of the target
device is not pulled low. The CPU does not start execution of the program because J-
Link sets the VC_CORERESET bit before reset, which causes the CPU to halt before
execution of the first instruction.
5.8.2.4 Type 3: Connect under Reset
J-Link connects to the target while keeping Reset active (reset is pulled low and
remains low while connecting to the target). This is the recommended reset strategy
for STM32 devices. This reset strategy has been designed for the case that communi-
cation with the core is not possible in normal mode so the VC_CORERESET bit can not
be set in order to guarantee that the core is halted immediately after reset.
5.8.2.5 Type 4: Reset core & peripherals, halt after bootloader
Same as type 0, but bootloader is always executed. This reset strategy has been
designed for MCUs/CPUs which have a bootloader located in ROM which needs to run
at first, after reset (since it might initialize some target settings to their reset state).
When using this reset strategy, J-Link will let the bootloader run after reset and halts
the target immediately after the bootloader and before the target application is
started. This is the recommended reset strategy for LPC11xx and LPC13xx devices
where a bootloader should execute after reset to put the chip into the "real" reset
state.
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