Integration Manual

LISA-U2 series - System Integration Manual
UBX-13001118 - R25 System description Page 87 of 182
specifications [8]: for example, 4.7 k resistors can be commonly used. Pull-ups must be connected
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 (I
2
C) 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 Figure 47 application circuit), as
any external signal connected to the DDC (I
2
C) 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 Figure 18 for the power-on sequence
description and timings.
DDC Slave-mode operation is not supported, the module can only act as a master.
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 I
2
C bus specifications [8] regarding rise and
fall times of the signals.
☞ Capacitance and series resistance must be limited on the bus to match the I
2
C specifications
(1.0 µ s 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 (I
2
C) 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 idle 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)