MT-X1S
Manual
prevents VBUS from rising to the level of the external voltage.
Note that there is a minimum load of
100uA for this regulator.
The MTX1S can consume less than 75uA in the deepest sleep modes. An
onboard load resistor between 3.3V and Gnd is provided to ensure that this requirement is met. A
MicroSD card inserted may consume enough to meet the specification without the resistor, thus it can
be disconnected by using solder jumper J32.
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
prescalers can be used to obtain the various clocks. Be aware that the ATxmega128a1u requires both
the 2MHz and 32MHz oscillators to be running and both DFLLs to be enabled for either DFLL to
operate due to errata. Atmel ASF (Atmel Software Framework) does not support this arrangement, but
the example code shows how to set this up. Also note that the DFLL calibrated oscillators will still not
be as accurate as an external high speed crystal. If using an external crystal, it must be 0.4MHz to
16MHz. The PLL can be used to obtain higher clock speeds.
Programming Headers
The PDI header has the standard 6pin layout. Because an onboard programmer is provided,
an external programmer is not necessary. However, debugging requires use of an external debugger
connected to the PDI header or the JTAG header. Because the RX/D pin is shared with the XMEGA
PDI_DATA pin, an external programmer/debugger cannot be used when using the serial bridge as this
would cause contention. Jumper J19 can be disconnected to avoid this contention, but the onboard
serial bridge will no longer be usable (an external bridge can be used if needed for debugging).
Alternatively, the JTAG header can be used for debugging. When using an external debugger or
programmer on the PDI header, the USB AVR should be in any mode other than the PDI programmer.
An ISP header is available for programming the USB AVR. It can also be programmed over USB (see
Firmware Updates).
Solder Jumpers / USB Shield
There are many solder jumpers on the PCB connecting XMEGA pins to the onboard peripheral
devices. This allows unused devices to be disconnected, freeing up the XMEGA pin, which is also
March 2, 2017
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