MTX1
Manual
prescalers can be used to obtain the various clocks. Be aware that the ATxmega128a1 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 XMEGA TX pin (USB AVR RX/D pin)
is connected to the XMEGA PDI_DATA pin, the debugger cannot be used when using TX as this
would cause contention. However, solder jumper J19 can be disconnected to avoid contention, but TX
will then be disconnected from the USB module. TX will still available on the header pin, so you can
connect to an external serial bridge if debugging requires it. 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
routed to a header, to be used for other purposes. External pull resistors are installed to keep the
peripheral pins at a defined state during boot or when the peripheral is disconnected. They pull chip
select lines to the deselected state to minimize power consumption. Most solder jumpers are
connected by default. To disconnect for the first time, a small trace connecting the two jumper pads
must be cut. To reconnect, create a solder bridge across the pads. Jumper J1 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, it may be desired to ground this on the device side. An 0603
SMT component may be soldered on the solder jumper pads as well.
USB Serial Bridge
The USB Serial bridge allows the XMEGA to communicate with a computer over USB by
simply using a USART. There is no need to learn the USB protocol or utilize a USB library. All USB
functionality is handled by the USB AVR (AT90USB162). It simply relays bytes between the XMEGA
and the host. The MTX1 uses two pins on USART F (RX and TX) in asynchronous mode and three
pins (adding XCK) in synchronous mode. Optionally, a USB ready signal is available on the XCK pin.
To minimize power consumption. TX should be tristated before entering sleep. This is due to the
sharing of PDI_DATA and TX. PDI_DATA has a pulldown active, which will consume current when TX
is set to output high. All three pins can be disconnected from the USB AVR using the solder jumpers.
June 2, 2015
10
