User's Manual

TOBY-R2 series - System Integration Manual
UBX-16010572 - R04 Design-in
Page 108 of 147
2.6.3 DDC (I
2
C) interface
2.6.3.1 Guidelines for DDC (I
2
C) circuit design
Communication with u-blox GNSS receivers over DDC (I
2
C) is not supported by “02” product versions.
The DDC I
2
C-bus master interface can be used to communicate with u-blox GNSS receivers and other external
I
2
C-bus slaves as an audio codec. Beside the general considerations reported below, see:
 the following parts of this section for specific guidelines for the connection to u-blox GNSS receivers
 the section 2.7.1 for an application circuit example with an external audio codec I
2
C-bus slave.
To be compliant with the I
2
C bus specifications, the module bus interface pads are open drain output and pull up
resistors must be mounted externally. Resistor values must conform to I
2
C bus specifications [12]: 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 Figure 61 application circuit), as any external signal
connected to the DDC (I
2
C) interface must not be set high before the switch-on of the V_INT supply of
DDC (I
2
C) pins, to avoid latch-up of circuits and let a proper boot of the module.
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 [12] 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.
ESD sensitivity rating of the DDC (I
2
C) pins is 1 kV (Human Body Model according to JESD22-A114).
Higher protection level could be required if the lines are externally accessible and it can be achieved by
mounting an ESD protection (e.g. EPCOS CA05P4S14THSG varistor array) close to accessible points.