JTAG support — Usage

Use of JTAG for debugging

JTAG can be used to single-step and debug loaded RAM applications, or even applications resident in ROM.

Debugging of ROM applications is only possible if using hardware breakpoints. The ARM7TDMI core of the AT91SAM7A2 only supports two such hardware breakpoints, and so they should be used sparingly. If using a GDB front-end such as Eclipse, check it has not set unnecessary extra breakpoints. Some JTAG devices give the option of whether to set hardware or software breakpoints by default. Be sure to configure your device appropriately.

Abatron BDI2000 notes

On the Abatron BDI2000, the bdi2000.at91sam7a2ek.cfg file should be used to setup and configure the hardware to an appropriate state to load programs. This includes setting up the PLL and flash memory controller.

The bdi2000.at91sam7a2ek.cfg file also contains an option to define whether hardware or software breakpoints are used by default, using the BREAKMODE directive in the [TARGET] section. Edit this file if you wish to use software break points, and remember to use the boot command on the BDI2000 command line interface to make the changes take effect.

On the BDI2000, debugging can be performed either via the telnet interface or using arm-elf-gdb and the bdiGDB interface. In the case of the latter, arm-elf-gdb needs to connect to TCP port 2001 on the BDI2000's IP address. For example:

(gdb) target remote 111.222.333.444:2001

By default when the BDI2000 is powered up, the target will always run the initialization section of the bdi2000.at91sam7a2ek.cfg file (which configures the SDRAM among other things), and halts the target. This behaviour is repeated with the reset halt command.

If the board is reset when in 'reset halt' mode (either with the 'reset halt' or 'reset' commands, or by pressing the reset button) and the 'go' command is then given, then the board will boot from ROM as normal.

It is also possible for the target to always run, without initialization, after the reset button has been pressed. This mode is selected with the reset run command. This conveniently allows the target to be connected to the JTAG debugger, and be able to reset it with the reset button, without being required to always type 'go' every time. Thereafter, invoking the reset command will repeat the previous reset style. Also in this mode, exceptions will be handled by board software, rather than causing the JTAG debugger to halt the CPU.

Suitably configured RAM applications can be loaded either via GDB, or directly via the telnet CLI. For example:

SAM7A2>load 0x00201000 /test.bin bin
Loading /test.bin , please wait ....
Loading program file passed
SAM7A2>go 0x00201000

Consult the BDI2000 documentation for information on other formats.

Configuration of RAM applications

If the JTAG device has initialized the processor, such as by using the bdi2000.at91sam7a2ek.cfg configuration on the BDI2000, applications can be loaded directly into RAM without requiring a ROM monitor. This loading can be done directly through the JTAG device, or where supported by the JTAG device, through GDB.

In order to configure the application to support this mode, some configuration settings are required. Firstly CYGSEM_HAL_USE_ROM_MONITOR must be disabled. Secondly the CYGDBG_HAL_DIAG_TO_DEBUG_CHAN option should be enabled in order to prevent HAL diagnostic output being encoded into GDB ($O) packets. Selecting the JTAG startup type in the configuration tool sets these options automatically.

Running RAM applications

Once loaded and running via JTAG, HAL diagnostic output will appear by default on the serial port.