LISA-U1 series - System Integration Manual
3G.G2-HW-10002-3
Preliminary
System description
Page 34 of 125
1.6.1.3
Rising edge on RESET_N
The module can be switched on by means of the
RESET_N
input pin: the
RESET_N
signal must be forced to the
low level for at least 50 ms and then released from the low level to generate a rising edge that starts the module
power-on sequence.
The
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.
The electrical characteristics of
RESET_N
are different from the other digital I/O interfaces. The detailed electrical
characteristics are described in the
LISA-U1 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 should not be 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 value of the
V_BCKP
operating range. Make sure to fix the proper level on
RESET_N
in all possible scenarios, to avoid
unwanted switch-on or reset of the module.
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
If a voltage within the operating range is maintained at the
VCC
pin, the module can be switched on by the RTC
alarm when the RTC system reaches a pre-programmed scheduled time. The RTC system will then initiate the
boot sequence by instructing the Power Management Unit to turn on power. Also included in this setup is an
interrupt signal from the RTC block to indicate to the baseband processor that an RTC event has occurred.
1.6.1.5
Additional considerations
The module is switched on when the voltage rises up to the
VCC
operating range: the first time that the module
is used, it is switched on in this way. Then, the proper way to switch off the module is by means of the
AT+CPWROFF command. When the module is in power-off mode, i.e. the AT+CPWROFF command has been
sent and a voltage value within the operating range limits is still provided to the
VCC
pin, the digital input-
output pads of the baseband chipset (i.e. all the digital pins of the module) are locked in tri-state (i.e. floating).
The power down tri-state function isolates the module pins from its environment, when no proper operation of
the outputs can be guaranteed.
The module can be switched on from power-off mode by forcing a proper start-up event (i.e. a falling edge on
the
PWR_ON
pin, or an RTC alarm). After the detection of a start-up event, all the digital pins of the module are
held in tri-state until all the internal LDO voltage regulators are turned on in a defined power-on sequence. Then,
as described in Figure 17, the baseband core is still held in reset state for a time interval: the internal reset signal
(which is not available on a module pin) is still low and any signal from the module digital interfaces is held in
reset state. The reset state of all the digital pins is reported in the pin description table of the
LISA-U1 series
Data
[1]. When the internal signal is released, the configuration of the module interfaces starts: during this
phase any digital pin is set in a proper sequence from the reset state to the default operational configuration.
Finally, the module is fully ready to operate when all interfaces are configured.
