LISA-U series - System Integration Manual
UBX-13001118 - R17
Advance information
System description
Page 41 of 190
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-U 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-U1 series Data Sheet
[1] and
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-U1 series Data Sheet
and
LISA-U2 series Data Sheet
[2]). 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
[3], 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 19 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).
Figure 19 shows the modules power-on sequence from the power-off mode, describing the following phases:
The external supply is still applied to the VCC inputs as it is assumed that the module has been previously
switched off by means of the AT+CPWROFF command: the V_BCKP output is internally enabled as proper
VCC is present, the RESET_N of LISA-U series is set to high logic level due to internal pull-up to V_BCKP, the
PWR_ON is set to high logic level due to external pull-up connected to V_BCKP or VCC.
The PWR_ON input pin is set low for a valid time period, representing the start-up event.
All the generic digital pins of the modules are tri-stated until the switch-on of their supply source (V_INT):
any external signal connected to the generic digital pins must be tri-stated or set low at least until the
activation of the V_INT supply output to avoid latch-up of circuits and allow a proper boot of the module.
The V_INT generic digital interfaces supply output is enabled by the integrated power management unit.
The internal reset signal is held low by the integrated power management unit: the baseband processor core
and all the digital pins of the modules are held in reset state, which is reported for each pin of the module in
the pin description table of LISA-U1 series Data Sheet [1] and LISA-U2 series Data Sheet [2].
