MT-DB-U4
User Guide
Boot Jumper / RESET button / LED
This section does not apply to the Atmel DFU bootloader option, in which case, the fuses will be left
at their factory default settings and the factory installed Atmel DFU bootloader will not be overwritten.
The boot jumper (labeled HWB) selects between the bootloader and user application. The pin is
sampled after reset or powerup. Note that the hardware HWB function of the ATmega32U4 is disabled
(HWBE fuse is disabled) and the bootloader startup code is always run after reset or powerup (BOOTRST
fuse is set). This startup code then samples the state of the HWB pin. If it is high, the user application runs.
Otherwise, the bootloader continues to run, waiting for programming instructions while pulsing the LED.
The LED remains on when jumping to the user application. The bootloader always runs at 8 MHz, which is
compatible with lower voltages. It remains at 8MHz when jumping to the user application. The user may
then set the cpu speed to 16MHz in software, if running at 5V.
It is not necessary to remove and replace the jumper when switching between the bootloader and
the user application. The jumper can be left on. After FLASH programming, the CDC bootloader will
automatically jump to the application. If using the DFU bootloader, then you can command FLIP or dfu
programmer to jump to the application. Then, when running the application, the reset button can be
pressed to reenter the bootloader. This is useful when writing and debugging firmware. When the firmware
is complete, the jumper can be removed so that future resets will always run the application.
ISP Header
The ISP header is configured by default to allow ISP programming using an external programmer.
That is, RESET is routed to pin 5. Pin 1 is marked on the board (it is the pin closest to the chip). The ISP
header can be reconfigured so that pin PB0 (SS) is connected to pin 5 rather than RESET. This can be
done by switching solder jumper J2, next to the ISP header, to the position opposite to the “ISP” label. This
allows the MTDBU4 to be used as an AVRISPmkII programmer itself, using Dean Camera's AVRISPmkII
software available at
http://www.fourwalledcubicle.com/
. A precompiled hex file will be made available at
http://www.mattairtech.com/
on the MTDBU2 product page. Note that when using the ISP header in this
way, Vcc and ground are output to the target board. Therefore, the target board should not be powered
itself. You should also verify that it is safe to power the target board through the ISP connector. Another
use for the ISP header configured with SS on pin 5 is to make use of SPI, either as a master or slave. SPI
can also be used on the normal DIL headers.
December 30, 2015
10
http://www.mattairtech.com/
