Integration Manual

LISA-U2 series - System Integration Manual
UBX-13001118 - R25 System description Page 39 of 182
1.6.1.3 Rising edge on RESET_N
When the module is in power-off mode (i.e. switched off with VCC maintained), the module can be
switched on by means of the RESET_N input pin alternatively to the PWR_ON input pin: the RESET_N
signal must be forced low for at least 50 ms and then released to generate a rising edge that starts
the module power-on sequence.
RESET_N input pin can also be used to perform an “external” or “hardware” reset of the module, as
described in section 1.6.3.
Electrical characteristics of the LISA-U2 series RESET_N input are slightly different from the other
digital I/O interfaces: the pin provides different input voltage thresholds. Detailed electrical
characteristics are described in the LISA-U2 series Data Sheet [1].
RESET_N is pulled high to V_BCKP by an integrated pull-up resistor also when the module is in power-
off mode. Therefore an external pull-up is not required on the application board.
The simplest way to switch on the module by means of the RESET_N input pin is to use a push button
that shorts the RESET_N pin to ground: the module will be switched on at the release of the push
button, since the RESET_N will be forced to the high level by the integrated pull-up resistor,
generating a rising edge.
If RESET_N is connected to an external device (e.g. an application processor on an application board)
an open drain output can be directly connected without any external pull-up. A push-pull output can
be used too: in this case, make sure that the high level voltage of the push-pull circuit is below the
maximum voltage operating range of the RESET_N pin (specified in the RESET_N pin characteristics
table in the LISA-U2 series Data Sheet [1]). To avoid an unwanted power-on or reset of the module,
make sure to fix the proper level at the RESET_N input pin in all possible scenarios.
Some typical examples of application circuits using the RESET_N input pin are described in section
1.6.3.
1.6.1.4 Real Time Clock (RTC) alarm
When the module is in power-off mode (i.e. switched off with VCC maintained), it can be switched on
by means of a previously programmed RTC alarm (see the u-blox AT Commands Manual [2], AT+CALA
command) alternatively to the PWR_ON and RESET_N pins: the RTC system will initiate the power-
on sequence.
1.6.1.5 Additional considerations
The module is switched on when the VCC voltage rises up to the normal operating range (i.e. applying
supply properly, as described in section 1.6.1.1): the module is commonly switched on in this way for
the first time. Then, the module must be properly switched off as described in section 1.6.2, e.g. by the
AT+CPWROFF command.
When the module is in power-off mode, i.e. it has been properly switched off as described in the section
1.6.2 (e.g. by the AT+CPWROFF command) and a voltage within the operating range is maintained at
the VCC pins, the module can be switched on by a proper start-up event (i.e. by PWR_ON as described
in Figure 18 and section 1.6.1.2, or by RESET_N as described in section 1.6.1.3, or by RTC alarm as
described in section 1.6.1.4).