JTAG¶
JTAG is an interface for low-level debugging of digital devices. It gives instruction by instruction control over the microprocessor and allows data to be read and written to arbitrary memory and register locations. It is typically used with a debugging tool like gdb when hacking low level routines and hardware peripherals (we use it when working on libmaple) or to flash a new bootloader.
Note that the STM32 on the Maple has a built-in low level serial debugger which could also be used to flash bootloaders, and that the ASSERT framework allows basic debugging over a USART serial channel. We expect only fairly advanced users to use this feature.
Wiring Diagram¶
JTAG wiring diagram (large version) to connect a standard 20-pin ARM JTAG device to the 8-pin JTAG port on the Maple.
The Maple has holes for a 8-pin JTAG header but that header is not soldered on by default. If you know ahead of time that you’ll be needing it, and you order straight from LeafLabs, add a comment to your order and we can probably solder one on for no charge. Otherwise, you can simply attach standard 0.1” pitch male header pins (either the exact 4x2 block or two 4-pin pieces of breakaway straight header). For a one-off usage hack, the header can be jammed in the holes and twisted to ensure electrical contact.
Compatible Devices¶
We have had good experience with the Olimex ARM-USB-OCD device, which costs about 55 euro plus shipping (as of November 2010).
Recommended Reading¶
- Wikipedia Article on Joint Test Action Group (JTAG)
- STM32/gdb/OpenOCD HOWTO
- STMicro documentation for STM32F103RB microcontroller:
- Datasheet (pdf)
- Reference Manual (pdf)
- There’s a thread on JTAG in our forum which you may find useful.
