LISA-U2 series - System Integration Manual
UBX-13001118 - R19
Early Production Information
System description
Page 38 of 175
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 the 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
LISA-U2 series Data Sheet
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
LISA-U2 series Data Sheet
To avoid 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 the 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 an RTC alarm previously programmed (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 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).