User's Manual

SARA-G3 and SARA-U2 series - System Integration Manual
UBX-13000995 - R08 Objective Specification Design-in
Page 119 of 188
If power saving is enabled the application circuit with the TXD and RXD lines only is not recommended.
During command mode the DTE must send to the module a wake-up character or a dummy “AT” before
each command line (see section 1.9.1.4 for the complete description), but during data mode the wake-up
character or the dummy “AT” would affect the data communication.
Additional considerations
If the USB interface of SARA-U2 modules is connected to the host application processor, the UART can be
left unconnected as not required for AT and data communication, but it is anyway highly recommended
to provide direct access to the TXD, RXD, RTS, CTS pins of SARA-U2 modules for the execution of
firmware upgrade over UART using the u-blox EasyFlash tool and for dignostic purpose: provide a
testpoint on each line to accommodate the access and provide a 0  series resistor on each line to detach
the module pin from any other connected device.
Any external signal connected to the UART interface must be tri-stated or set low when the module is in
power-down mode and during the module power-on sequence (at least until the activation of the V_INT
supply output of the module), to avoid latch-up of circuits and allow a proper boot of the module. If the
external signals connected to the cellular module cannot be tri-stated or set low, insert a multi channel
digital switch (e.g. Texas Instruments SN74CB3Q16244, TS5A3159, or TS5A63157) between the
two-circuit connections and set to high impedance during module power down mode and during the
module power-on sequence.
ESD sensitivity rating of UART interface pins is 1 kV (Human Body Model according to JESD22-A114).
Higher protection level could be required if the lines are externally accessible on the application board.
Higher protection level can be achieved by mounting an ESD protection (e.g. EPCOS CA05P4S14THSG
varistor array) close to accessible points.
2.6.1.2 Guidelines for UART layout design
The UART serial interface requires the same consideration regarding electro-magnetic interference as any other
digital interface. Keep the traces short and avoid coupling with RF line or sensitive analog inputs, since the
signals can cause the radiation of some harmonics of the digital data frequency.
2.6.2 Auxiliary asynchronous serial interface (UART AUX)
Auxiliary UART interface is not supported by SARA-U2 modules.
2.6.2.1 Guidelines for UART AUX circuit design
The auxiliary UART interface can be connected to an application processor if it can be set in pass-through mode
so that the auxiliary UART interface can be accessed for SARA-G3 modules’ firmware upgrade by means of the
u-blox EasyFlash tool and for diagnostic purpose.
It is highly recommended to provide direct access to the TXD_AUX, RXD_AUX pins of SARA-G3 modules
for execution of firmware upgrade over auxiliary UART using the u-blox EasyFlash tool and for diagnostic
purpose: provide a testpoint on each line to accommodate the access and provide a 0  series resistor on
each line to detach the module pin from any other connected device.