User's Manual

LISA-U1/LISA-H1 series - System Integration Manual
3G.G2-HW-10002-2 Advance Information System description
Page 61 of 116
physical link (UART or SPI). Each session consists of a stream of bytes transferring various kinds of data such as
SMS, CBS, GPRS, GPS, AT commands in general. This permits, for example, SMS to be transferred to the DTE
when a data connection is in progress.
The following virtual channels are defined:
 Channel 0: control channel
 Channel 1 – 5: AT commands /data connection
 Channel 6: GPS tunneling
For more details please refer to GSM Mux implementation Application Note [15].
1.10 DDC (I
2
C) interface
1.10.1 Overview
An I
2
C compatible Display Data Channel (DDC) interface for serial communication is implemented. This interface
is dedicated exclusively to access u-blox GPS receivers.
Name
Description
Remarks
SCL
I
2
C bus clock line
Open drain. External pull-up required.
SDA
I
2
C bus data line
Open drain. External pull-up required.
Table 23: DDC pins
The DDC (I
2
C) interface pins ESD rating is 1 kV (contact discharge). A higher protection level could be
required if the lines are externally accessible on the application board. A higher protection level can be
achieved mounting an ESD protection (e.g. EPCOS CA05P4S14THSG varistor array) on the lines
connected to these pins.
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/LISA-H1 series module is
connected through the DDC bus to a single u-blox GPS receiver only (only one device is 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/LISA-H1 series Data Sheet [1]).
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.