LISA-U1 series - System Integration Manual
3G.G2-HW-10002-3
Preliminary
System description
Page 64 of 125
For more details regarding the handling of the DDC (I
2
C) interface and the GPS aiding features please
refer to
u-blox AT Commands Manual
[2] (AT+UGPS, AT+UGPRF, AT+UGPIOC commands) and
GPS
Implementation Application Note
1.10.2
DDC application circuit
The DDC (I
2
C) interface of the LISA-U1 series modules is used only to connect the wireless module to a u-blox
GPS receiver: the DDC (I
2
C) interface is enabled by the AT+UGPS command only (for more details refer to
u-blox
AT Commands Manual
[2]). The
SDA
and
SCL
lines must be connected to the DDC (I
2
C) interface pins of the u-
blox GPS receiver (i.e. the SDA2 and SCL2 pins of the u-blox GPS receiver) on the application board.
To be compliant to the I
2
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
I
2
C bus specifications
[8]. If a LISA-U1 series module is connected
by the DDC (I
2
C) bus to a u-blox GPS receiver (only one device can be connected on the DDC bus), use a pull-up
resistor of 4.7 k . Pull-ups must be connected to a supply voltage of 1.8 V (typical), since this is the voltage
domain of the DDC pins.
V_INT
digital interfaces supply output can be used to provide 1.8 V for the pull-ups
(for detailed electrical characteristics see the
LISA-U1 series
Data Sheet
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 , 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 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.
LISA-U1 series modules support 3 different types of GPS aiding:
Local aiding
Assist
Now
Online
Assist
Now
Offline
The embedded GPS aiding features can be used only if the DDC (I
2
C) interface of the wireless module is
connected to u-blox GPS receivers.
GPIO pins can handle the power on/off of the GPS receiver (“GPS supply enable” function provided by
GPIO2
).
LISA-U1 series modules don’t enter idle-mode when the DDC (I
2
C) interface is enabled by the AT+UGPS
command, even if power saving is enabled by the AT+UPSV command.
The
GPIO2
is by default configured to provide the “GPS 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 GPS receiver connected
to the wireless module by the AT+UGPS command. The pin is set as
Output / High, to switch on the u-blox GPS receiver, if the parameter <mode> of AT+UGPS command is set
to 1
Output / Low, to switch off the u-blox GPS receiver, if the parameter <mode> of AT+UGPS command is set
to 0 (default setting)
