SARA-G3 and SARA-U2 series - System Integration Manual
UBX-13000995 - R26 Design-in
Page 116 of 217
2.3.1.2 Guidelines for PWR_ON layout design
The power-on circuit (PWR_ON) requires careful layout since it is the sensitive input available to switch on the
SARA-G3 and SARA-U2 series modules until a valid VCC supply is provided after that the module has been
switched off by the AT+CPWROFF command: ensure that the voltage level is well defined during operation and
no transient noise is coupled on this line, otherwise the module might detect a spurious power-on request.
2.3.2 Module reset (RESET_N)
2.3.2.1 Guidelines for RESET_N circuit design
As described in Figure 24, the module has an internal pull-up resistor on the reset input line: an external pull-up
is not required on the application board.
Connecting the RESET_N input to a push button that shorts the RESET_N pin to ground, the pin will be
externally accessible on the application device: according to EMC/ESD requirements of the application, provide
an additional ESD protection (e.g. EPCOS CA05P4S14THSG varistor array) on the line connected to this pin, close
to accessible point, as described in Figure 53 and Table 34.
The ESD sensitivity rating of the RESET_N pin is 1 kV (Human Body Model according to JESD22-A114). A
higher protection level can be required if the line is externally accessible on the application board, e.g. if
an accessible push button is directly connected to RESET_N pin. A higher protection level can be achieved
by mounting an ESD protection (e.g. EPCOS CA05P4S14THSG varistor array) close to accessible points.
Connecting the RESET_N input to an external device (e.g. application processor), an open drain output can be
directly connected without any external pull-up, as described in Figure 53 and Table 34: the internal pull-up
resistor provided by the module pulls the line to the high logic level when the RESET_N pin is not forced low by
the application processor. A compatible push-pull output of an application processor can be used too.
SARA-G3 series
SARA-U2 series
18
RESET_N
Reset
push button
ESD
Open
Drain
Output
Application
Processor
SARA-G3 series
SARA-U2 series
18
RESET_N
TP
TP
Figure 53: RESET_N application circuits using a push button and an open drain output of an application processor
Varistor for ESD protection
Table 34: Example of ESD protection component for the RESET_N application circuit
If the external reset function is not required by the customer application, the RESET_N input pin can be
left unconnected to external components, but it is recommended providing direct access on the
application board by means of an accessible test-point directly connected to the RESET_N pin.
2.3.2.2 Guidelines for RESET_N layout design
The reset circuit (RESET_N) requires careful layout due to the pin function: ensure that the voltage level is well
defined during operation and no transient noise is coupled on this line, otherwise the module might detect a
spurious reset request. It is recommended to keep the connection line to RESET_N as short as possible.
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