MT-DB-U4
User Guide
Power Configuration
Power Configuration
Bus Powered 5V
By default, the MTDBU4 is configured for 5V from the USB connector (Vbus). In this
configuration, solder jumper J1 is set to the 5V position. This shorts 5V (Vbus) to Vcc. Thus, the 5V
and Vcc pins are both outputs. The 3.3V pin is also an output from the AVR internal regulator, which
must be enabled. This pin can supply about 55mA.
Externally Powered – 3.4V to 5.5V
In this configuration, disconnect solder jumper J1 (not set to 5V or 3.3V). Then supply 3.4V to
5.5V to the Vcc pin, which is now an input. The 5V pin still outputs 5V when the USB cable is plugged
in. The 3.3V pin is also an output from the AVR internal regulator, which must be enabled. This pin
can supply about 55mA. Note that when using a voltage less than 4.5V, the AVR should be set to run
at 8MHz or less. This can be done in software using the prescaler (no need to change the crystal).
Externally Powered – 3.0V to 3.6V
In this configuration, change your code to disable the internal 3.3V regulator. Disconnect solder
jumper J1 (not set to 5V or 3.3V). Then supply 3.0V to 3.6V to both the Vcc pin and the 3.3V pin,
which are now both inputs. Alternatively, the solder jumper J1 can be set to the 3.3V position so that
only the Vcc pin need be connected. Note that unlike the AT90USBXX2 or ATmegaXXU2, the internal
3.3V regulator cannot be used to power Vcc because the regulator is disabled on reset or powerup.
Also note that if the bootloader is set to run (HWB jumper installed) the regulator will be enabled. The
regulator is then disabled before jumping to the user application. The regulator is never enabled if the
application is configured to run (HWB jumper not installed). Therefore, if the bootloader is to be used
in this configuration, only 3.3V should be connected to Vcc. The 5V pin still outputs 5V when the USB
cable is plugged in. In this configuration, the AVR should be set to run at 8MHz or less. This can be
done in software using the prescaler (no need to change the crystal).
USB Shield
Jumper J3 can be soldered to connect the USB shield to ground. The USB specification calls
for the USB shield to be connected to ground on the host side only. However, some prefer to have it
grounded on the device side as well, though a ground loop would be formed. An 0603 SMT
component may be soldered on the solder jumper pads as well.
December 30, 2015
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