LISA-U2 series - System integration manual
UBX-13001118 - R27
System description
Page 87 of 183
C1-Public
to a supply voltage of 1.8 V (typical), since this is the voltage domain of the DDC pins which are not
tolerant to higher voltage values (e.g. 3.0 V).
☞
Connect the DDC (I2C) pull-ups to the
V_INT
1.8 V supply source, or another 1.8 V supply source
enabled after
V_INT
(e.g. as the GNSS 1.8 V supply present in the
any external signal connected to the DDC (I2C) interface must not be set high when the module is
in power-down mode, when the external reset is forced low and during the module power-on
sequence (at least until the switch-on of the
V_INT
supply of DDC pins), to avoid latch-up of
circuits and enable a clean boot of the module. See
description and timings.
DDC local device -mode operation is not supported, the module can only act as a host.
Two lines, the serial data (
SDA
) and serial clock (
SCL
), carry information on the bus.
SCL
is used to
synchronize data transfers, and
SDA
is the data line. Since both lines are open drain outputs, the DDC
devices can only drive them low or leave them open. The pull-up resistor pulls the line up to the supply
rail if no DDC device is pulling it down to GND. If the pull-ups are missing,
SCL
and
SDA
lines are
undefined and the DDC bus will not work.
The signal shape is defined by the values of the pull-up resistors and the bus capacitance. Long wires
on the bus will increase the capacitance. If the bus capacitance is increased, use pull-up resistors with
a nominal resistance value less than 4.7 k
, to match the I2C bus specifications
regarding rise and
fall times of the signals.
☞
Capacitance and series resistance must be limited on the bus to match the I2C specifications
(1.0 µs is the maximum allowed rise time on the
SCL
and
SDA
lines): route the connections as
short as is possible.
☞
If the pins are not used as the DDC bus interface, they can be left unconnected.
1.10.2.2
Connection with u-blox GNSS receivers
General considerations
LISA-U2 modules support these GPS aiding types:
•
Local aiding
•
AssistNow Online
•
AssistNow Offline
•
AssistNow Autonomous
The embedded GPS aiding features can be used only if the DDC (I2C) interface of the cellular module
is connected to the u-blox GNSS receivers.
The GPIO pins of the LISA-U2 series modules can handle:
•
GNSS receiver power-
on/off (“GNSS supply enable” function provided by
GPIO2
)
•
The wake-up from i
dle mode when the GNSS receiver is ready to send data (“GNSS data ready”
function provided by
GPIO3
)
•
The RTC synchronization signal to the GNSS receiver (“GNSS RTC sharing” function provided by
GPIO4
)
The
GPIO2
is configured by default to provide the “GNSS
supply enable” function (parameter
<gpio_mode> of AT+UGPIOC command set to 3 by default), to enable or disable the supply of the u-
blox GNSS receiver connected to the cellular module by the AT+UGPS command. The pin is set as
•
Output / High, to switch on the u-blox GNSS receiver, if the parameter <mode> of AT+UGPS
command is set to 1
