MT-DB-U2
Manual
D+, D
USB data pins. Can be used for panelmount connectors.
PWM Filters
There are three PWM filters, which can be used to smooth out a PWM square wave
into an analog voltage (with some ripple). The outputs of these filters can be connected to
header pins C6, C5, and B7 using solder jumpers J4, J5, and J3 respectively (factory default).
The filters consist of a 1K resistor and 100nF capacitor. The cutoff frequency is:
f
3db
=
1
/
2
RC
=
1
/
2
1K
∗
100nF
=
1
/
2
0.0001
=
1600
Hz
This is a firstorder lowpass filter that can output levels from 0V to Vcc. All three filter
inputs are connected to the 16bit timer 1 output compare pins (
OC.1A, OC.1B, and OC.1C).
The filter inputs are always connected to the microcontroller. Therefore, if the solder jumpers
are configured to connect the microcontroller pins directly to the header pins, there will be
some loading on the pin (1K resistor in series with a 100nF capacitor to ground).
Clock Source
By default, a 16 MHz crystal is installed and connected to the XTAL pins of the
Atmega32U2. This 16 Mhz clock must be divided by 2 in software if the Vcc voltage is less
than 4.5V. If an external clock is is used, solder jumper J6 can be switched to connect the
microcontroller pin directly to header pin rather than the onboard crystal. An external clock
signal can then be applied to pin X1 (XTAL1). This will also free up microcontroller pin C0
(XTAL2), which can be configured in the microcontroller as a GPIO pin and routed to header
pin X2 by switching solder jumper J7.
HWB Jumper / RESET button / LED
The HWB Jumper is used to select either the bootloader or user application. The pin is
sampled after reset or powerup. Note that the hardware HWB function of the ATmega32U2 is
disabled. That is, the HWBE fuse is disabled. The bootloader startup code is always run after reset or
powerup (BOOTRST fuse is set). The code samples the state of the HWB pin. If the pin is low, the
bootloader continues to run. If the pin is high, the user application is run. The green LED will pulse on
and off using a continuously changing PWM period when the DFU bootloader is running. If the
preinstalled demo program is running, it will be lit when USB is connected. Otherwise, the state of the
LED is controlled by the user application. The bootloader always runs at 8 MHz, which is compatible
with lower voltages. The user may set the cpu speed to 16MHz in software, if running at 5V.
October 29, 2015
9
