MTX1
Manual
MicroSD Card
The MicroSD card slot has a springloaded mechanism that locks the card in place when
inserted (pushin, pushout). The contacts are goldplated. It is connected to SPI E using four pins. All
pins have external 47Kohm pullups installed, except for MISO, which
requires the internal pullup to
be enabled
. All four pins can be disconnected from the MicroSD card slot using the solder jumpers.
Note that when in the deepest sleep modes and a card is installed, it will likely consume the most
current. Since the minimum load required by the regulator is 100uA, and the rest of the onboard
components may consume less than 75uA, having a card installed may allow disconnection of the
minimum load resistor (solder jumper), which itself consumes 100uA.
32KB SPI SRAM
The 32KB SPI SRAM is the 23K256I/SN from Microchip. It has a very simple protocol, and
can be quite fast operating at 16MHz with sequential access (ie: data capture). It is less suitable for
storage that requires random access. It is connected to SPI D using four pins, all of which have
47Kohm external pull resistors. All four pins can be disconnected using the solder jumpers. A simple
driver is provided in the ASF template. More more information, consult the datasheet.
Audio Amplifier
The audio amplifier is the LM4889MM/NOPB from National Semiconductor. It is a single
channel, class AB, 400mW @ 3.3V amplifier with depop and thermal protection. It is connected to the
XMEGA DAC A0 on pin A2, which can be disconnected using a solder jumper. The shutdown pin is
routed to pin Q3 and has a 47 Kohm pull resistor to keep the IC in shutdown when Q3 is not driven.
The differential gain is set to 2, so the internal 1V reference or the external 1.25V reference can be
used. An 8ohm or 16ohm speaker can be connected to the output which is routed to a two pin header.
A 4ohm load can also be connected, but the amplifier may enter thermal shutdown if using a higher
voltage reference and the signal magnitude remains large for a long enough period of time. For more
information on this IC, please consult the datasheet.
1.25V Precision Reference
The 1.25V precision reference is the ISL60002DIH312ZTK from Intersil. It is a lowpower FGA
reference with a low 20ppm/C temperature coefficient. The initial accuracy is +/5mV. Because each
MTX1 board is intended to be calibrated individually, the initial accuracy was deemed less important
than the temperature coefficient. The reference is connected to both reference inputs, pins A0 and B0.
It can be disconnected using the solder jumpers. The reference voltage of 1.25V was chosen as a
workaround to the ADC errata of the ATxmega128a1. It is intended to be used with the ADC in
differential mode and with signed conversions. The voltage to be measured is connected to the
positive input, and the reference to the negative. This results in conversions in the ~0 to 2.5V range.
See the source code for example setup and usage. For more information on this IC, please consult
the datasheet.
June 2, 2015
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