u-blox LISA-C2 Series System Integration Manual Download Page 35

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LISA-C2 series and FW75-C200 - System Integration Manual

UBX-13000620 - R21

Early Production Information

System description

Page 35 of 103

interface. A 2

interface connected to the GNSS receiver is not necessary: AT commands via the UART serial

interface of the cellular module allows a fully control of the GNSS receiver from any host processor.
LISA-C modules feature embedded GNSS aiding that is a set of specific features developed by u-blox to enhance
GNSS performance, decreasing Time-To-First-Fix (TTFF), thus allowing it to calculate the position in a shorter time
with higher accuracy.
The DDC (I

C) interface of all LISA-C series can be used to communicate with u-blox GNSS receivers. The cellular

module acts as an I

C master, which can communicate to two I

C slaves as allowed by the I

C bus specifications.

For more details regarding the handling of the DDC (I

C) interface and the GNSS aiding features, refer to

blox AT Commands Manual

[3] (AT+UGPS, AT+UGPRF, AT+UGPIOC AT commands) and

GPS

Implementation Application Note

[12].

1.10.2

DDC application circuit

The DDC (I

C) interface of LISA-C modules is used to connect the cellular module to a u-blox GNSS receiver: the

communication with the u-blox GNSS receiver by DDC (I

C) interface is enabled by the AT+UGPS command (for

more details refer to

u-blox AT Commands Manual

[3]). The

SDA

and

SCL

lines must be connected to the DDC

C) interface pins of the u-blox GNSS receiver (i.e. the SDA2 and SCL2 pins of the u-blox GNSS receiver) on the

application board to allow the communication between the cellular module and the u-blox GNSS receiver.
To be compliant to the I

C bus specifications, the module bus interface pads are open drain output and pull up

resistors must be used. Since the pull-up resistors are not mounted on the module, they must be mounted
externally. Resistor values must conform to the

C bus specifications

[11]. If a LISA-C module is connected by the

DDC (I

C) bus to a u-blox GNSS receiver (only one device can be connected on the DDC bus), use a pull-up

resistor of 4.7 k

Ω

. Pull-ups must be connected to a supply voltage of 1.8 V (typical), since this is the voltage

domain of the DDC pins.

Connect the DDC (I

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 1.8 V supply present in Figure 19 application circuit).

DDC Slave-mode operation is not supported, the module can act as master only.
Two lines, 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 nominal resistance
value lower than 4.7 k

Ω

, to match the

C bus specifications

[11] with respect to rise and fall times of the signals.

Capacitance and series resistance must be limited on the bus to match the I

C specifications (1.0 µs is the

maximum allowed rise time on the

SCL

and

SDA

lines): route connections as short as possible.

If the pins are not used as DDC bus interface, they can be left unconnected.