Output debug via printf on a Cortex-M3 CPU, stalls at BKPT instruction + confusion about JTAG and sw ports

Question

I have a Keil ULINK2 USB emulator box attached to the JTAG connector on my board, which is working fine with the Cortex-M3 CPU onboard (TI/Stellaris/LuminaryMicro LM3S seri

Answer 1

SW is a two wire interface that provides access to debugging ports on the device.

Arm have a .pdf about it here:

http://www.arm.com/files/pdf/Low_Pin-Count_Debug_Interfaces_for_Multi-device_Systems.pdf

Answer 2

I've done this with the IAR EWW ARM toolchain, but the term semihosting leads me to believe that the Keil approach is similar. There should be an option when specifying the standard library to link in to use semihosting. That will compile/link in a different library which redirects printf / putc through the JTAG port to the debugger.

So look at the options for the project in the Uvision IDE or in the make scripts. In the IAR linker command line this is "--semihosting" but is probably different for the Keil tools.

BKPT is the instruction the tools insert in the source to trigger the debugger. It's how the IDE enables you to add breakpoints to the code when the debugger doesn't support HW breakpoints (or you have used your full complement of them already).

Answer 3

This one was a failure on my part to understand that stdio is not implemented, but rather you must provide your own implementation, usually done inside a file called "retarget.c". The filename is pure convention, but is well documented (as it turns out) inside Keil's uVision/RTLIB documentation

Answer 4

To deal with this problem in Keil uVision just go to project options. In Target tab/Code Generation check the Use MicroLIB checkbox.