User's Manual

SARA-G3 and SARA-U2 series - System Integration Manual
2.7.2 Digital audio interface
SARA-G300 and SARA-G310 modules do not provide digital audio interface.
2.7.2.1 Guidelines for digital audio circuit design
The I
2
S digital audio interface of SARA-G3 and SARA-U2 series modules can be connected to an external digital
audio device for voice applications.
Any external digital audio device compliant to the configuration of the digital audio interface of the cellular
module can be used, given that the external digital audio device has to provide:
• The opposite role: slave for SARA-G3 modules that act as master only; slave or master for SARA-U2 modules
that may act as master or slave
• The same mode and frame format: PCM / short alignment or Normal I
2
S mode / long alignment mode with
o data in 2’s complement notation
o MSB transmitted first
o word length = 16-bit
o frame length = 17-bit or 18-bit in PCM / short alignment mode (16 + 1 or 16 + 2 clock cycles, with
Word Aligment / Synchronization signal set high for 1 clock cycle or 2 clock cycles), or
o frame length = 32-bit in Normal I
2
S mode / long alignment mode (16 x 2 clock cycles)
• The same sample rate and serial clock frequency: as the clock frequency depends on the frame length and
the sample rate, the clock frequency can be
o 17 x <I2S_sample_rate> or 18 x <I2S_sample_rate> in PCM / short alignment mode, or
o 16 x 2 x <I2S_sample_rate> in Normal I
2
S mode / long alignment mode
• Compatible voltage levels (1.80 V typ.), otherwise it is recommended to connect the 1.8 V digital audio
interface of the module to the external 3.0 V (or similar) digital audio device by means of appropriate
unidirectional voltage translators (e.g. Texas Instruments SN74AVC4T774 or SN74AVC2T245), using the
module V_INT output as 1.8 V supply for the voltage translators on the module side
For the appropriate selection of a compliant external digital audio device, see the section 1.10.2 including
subsections 1.10.2.1 and 1.10.2.2. See the +UI2S AT command description in the u-blox AT Commands
Manual [3] for further details regarding the capabilities and the possible settings of I
2
S digital audio interface of
SARA-G3 and SARA-U2 series modules.
Figure 79 and Table 53 describe an application circuit example for the I
2
S digital audio interface of SARA-U2
modules providing voice capability using an external audio voice codec. DAC and ADC integrated in the external
audio codec respectively converts an incoming digital data stream to analog audio output through a mono
amplifier and converts the microphone input signal to the digital bit stream over the digital audio interface.
• The module’s I
2
S interface (I
2
S master) is connected to the related pins of the external audio codec (I
2
S slave).
• The V_INT output supplies the external audio codec, defining proper digital interfaces voltage level.
• The external audio codec is controlled by the SARA-U2 module using the DDC (I
2
C) interface: this interface
can be concurrently used to communicate with u-blox GNSS receivers and with an external audio codec.
• The CODEC_CLK pin of the SARA-U2 module (that provides a suitable digital output clock) is connected to
the clock input of the external audio codec to provide clock reference.
• Additional components are provided for EMC and ESD immunity conformity: a 10 nF bypass capacitor and a
series chip ferrite bead noise/EMI suppression filter provided on each microphone line input and speaker line
output of the external codec as described in Figure 79 and Table 53. The necessity of these or other
additional parts for EMC improvement may depend on the specific application board design.
UBX-13000995 - R12 Early Production Information Design-in
Page 141 of 196