MTX1
Manual
Power / Status LEDs
There are two green LEDs that are used to indicate USB status, the mode of operation,
communication activity, programmer status, and more. The following table lists LED functionality in
each mode. Both LEDs are turned off in sleep mode.
LED Functionality
Mode
STS LED
PWR LED
AVRISP mkII Programmer Programmer Activity
PWM pulsing
Configuration Mode
On
On
USB Serial Bridge
RX Activity
TX Activity
DFU Bootloader
On
Off
Power Supply
The MTX1 can be powered via USB or via an external header. Both sources are connected to
the input of a 1A, 3.3V LDO linear regulator through Schottky diodes rated at 2A each (2A was chosen
also to keep the dropout voltage low throughout the range of current). The diodes provide reverse
polarity protection as well as ensuring that current will not flow from one source to the other. For
example, if the external header has a greater voltage than the USB VBUS voltage, the diode prevents
VBUS from rising to the level of the external voltage.
Note that there is a minimum load of 100uA
for this regulator.
The MTX1 can consume less than 75uA in the deepest sleep modes. Use an
external load resistor between 3.3V and Gnd to ensure that this requirement is met. A MicroSD card
inserted may consume enough to meet the specification without the resistor.
The 3.3V regulator has thermal protection and foldback current limiting. There is a 10uF
capacitor on both the input and output. Note that 10uF is the maximum allowed by the USB
specification. When using the external header, additional capacitance may be needed with higher
impedance voltage sources (ie: batteries, long cable runs). The regulator input can also be routed
through J2 to the header pin labeled 5V (near the relay driver).
Voltages greater than 5.5V on the
external power input header require J2 to be disconnected, which will disable the relay driver kickback
protection.
Clock Sources / RTC
By default, a 32.768KHz crystal is installed and connected to the TOSC pins of the XMEGA
(R0 and R1). An HC49 crystal landing is available as well, with 22pF load capacitors preinstalled. An
external clock can also be connected to pin R1. There are several internal clock options as well. The
demo program makes use of the 32MHz internal RC oscillator. This oscillator is configured to be auto
calibrated by a DFLL, which uses the 32.768KHz crystal as input. The crystal is also the source for the
RTC. A 2MHz RC oscillator and two different 32KHz oscillators are also available. A PLL and
June 2, 2015
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