Integration Manual

LISA-U2 series - System Integration Manual
UBX-13001118 - R25 System description Page 38 of 182
1.6.1.2 Low pulse on PWR_ON
When the module is in power-off mode, i.e. it has been cleanly 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 means of the PWR_ON input pin: a falling edge must
be provided on the PWR_ON pin, which must be then held low for an appropriate time period as
specified in the LISA-U2 series Data Sheet [1].
The electrical characteristics of the PWR_ON input pin are different from the other digital I/O
interfaces; the detailed electrical characteristics are described in the LISA-U2 series Data Sheet [1].
☞ The PWR_ON pin has high input impedance and is weakly pulled to the high level on the module.
Avoid keeping it floating in a noisy environment. To hold the high logic level stable, the PWR_ON
pin must be connected to a pull-up resistor (e.g. 100 kΩ) biased by the V_BCKP supply pin of the
module.
Following are some typical examples of application circuits to turn the module on using the PWR_ON
input pin.
Connecting the PWR_ON input to an external device (e.g. application processor), use an open drain
output on the external device with an external pull-up resistor (e.g. 100 kΩ) biased by V_BCKP supply
pin of the module.
A push-pull output of an application processor can also be used: in this case the pull-up can be used
to pull the PWR_ON level high when the application processor is switched off. If the high-level voltage
of the push-pull output pin of the application processor is greater than the maximum input voltage
operating range of the V_BCKP pin (see the V_BCKP input characteristics of the supply/power pins
table in the LISA-U2 series Data Sheet [1]), the V_BCKP supply cannot be used to bias the pull-up
resistor: the supply rail of the application processor or the VCC supply could be used, but this will
increase the V_BCKP (RTC supply) current consumption when the module is in not-powered mode
(VCC supply not present). Using a push-pull output of the external device, take care to fix the proper
level in all the possible scenarios to avoid an inappropriate switch-on of the module.
1:1 scaling
Open
Drain
Output
Application
processor
LISA-U2 series
Rext
2
V_BCKP
19
PWR_ON
Figure 17: PWR_ON application circuits using an open drain output of an application processor
Reference
Description
Remarks
Rext
100 kΩ Resistor 0402 5% 0.1 W
External pull-up resistor
Table 17: Example of pull-up resistor for the PWR_ON application circuit