LISA-U1 series - System Integration Manual
3G.G2-HW-10002-3
Preliminary
System description
Page 33 of 125
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 k
Ω
) biased by the
V_BCKP
supply pin of the module.
Once the module has been turned on,
PWR_ON
pin has no effect. On the other hand it makes no sense to keep
this pin low once the module has been turned on: if the pin is kept low it will draw unnecessary current.
Following are some typical examples of application circuits to turn the module on using the
PWR_ON
input pin.
The simplest way to turn on the module is to use a push button that shorts the
PWR_ON
pin to ground: in this
case the
V_BCKP
supply pin can be used to bias the pull-up resistor.
If the
PWR_ON
input is connected to an external device (e.g. application processor), it is suggested to use an
open drain output on the external device with an external pull-up resistor (e.g. 100 k
Ω
) biased by the
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 voltage value as
V_BCKP
operating range,
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). To avoid an unwanted switch on of the module be
sure to fix the proper level on
PWR_ON
in all possible scenarios.
LISA-U1 series
Rext
2
V_BCKP
19
PWR_ON
Power-on
push button
ESD
Open
Drain
Output
Application
Processor
LISA-U1 series
Rext
2
V_BCKP
19
PWR_ON
Figure 16: PWR_ON application circuits using a push button and an open drain output of an application processor
Reference
Description
Remarks
Rext
100 k
Ω
Resistor 0402 5% 0.1 W
External pull-up resistor
ESD
CT0402S14AHSG - EPCOS
Varistor array for ESD protection
Table 12: Example of pull-up resistor and ESD protection for the PWR_ON application circuits
