Integration Manual

TOBY-L2 and MPCI-L2 series - System Integration Manual
UBX-13004618 - R04 Advance Information Design-in
Page 100 of 141
Additional considerations
Do not apply voltage to any UART interface pin before the switch-on of the UART supply source (V_INT),
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. TI
SN74CB3Q16244, TS5A3159, or TS5A63157) between the two-circuit connections and set to high
impedance before V_INT switch-on.
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 and it can be achieved by
mounting an ESD protection (e.g. EPCOS CA05P4S14THSG varistor array) close to accessible points.
If the UART interface pins are not used, they can be left unconnected on the application board, but it is
recommended providing accessible test points directly connected to all the UART pins (TXD, RXD, RTS,
CTS, DTR, DSR, DCD, RI) for diagnostic purpose, in particular providing a 0  series jumper on each line
to detach each UART pin of the module from the DTE application processor.
2.6.2.2 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.