SIM900D Hardware Design
3.10 Audio Interfaces
Table12: Pin definition of the Audio interface
Name
Pin
Function
MIC1P
21
Microphone1 input +
MIC1N
20
Microphone1 input -
SPK1P
23
Audio1
(AIN1/AOUT1)
SPK1N
24
Audio1 output-
MIC2P
18
Microphone2 input +
MIC2N
19
Microphone2 input -
SPK2P
26
Audio2
(AIN2/AOUT2)
SPK2N
25
Audio2 output-
The module provides two analog input channels, AIN1 and AIN2, which may be used for both microphone and
line inputs. The AIN1 and AIN2 channels are identical. One of the two channels is typically used with a
microphone built into a handset. The other channel is typically used with an external microphone or external line
input. The module analog input configuration is determined by control register settings and established using
analog multiplexers.
For each channels, the “AT+CMIC” can be used to adjust the input gain level of microphone, use AT+SIDET to
set the side-tone level. In addition, the “AT+CLVL” can also be used to adjust the output gain level of both
receiver and speaker at the same time, use AT+CHFA to activate one of the two audio channels and deactivate the
other one.. For more details, please refer to
document [1].
It is suggested that you adopt one of the following two matching circuits in order to improve audio performance.
The difference audio signals have to be layout according to difference signal layout rules. As show in following
figures
(Note: all components package are 0603.)
There can be recommended National Company’s LM4890 to
adopt an amplifier circuit for audio. Of course it can be selected according to your requirement.
SIM900D_HD_V1.02
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09.04.2010
