Hardware layout and configuration
UM1855
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DocID027351 Rev 3
2.7.1 Digital
microphones
U35 and U36 on board of STM32L476G-EVAL are MP34DT01TR MEMS digital omni-
directional microphones providing PDM (pulse density modulation) outputs. To share the
same data line, their outputs are interlaced. The combined data output of the microphones
is directly routed to STM32L476ZGT6 terminals, thanks to the integrated input digital filters.
The microphones are supplied with programmable clock generated directly by
STM32L476ZGT6.
As an option, the microphones can be connected to U29, Wolfson audio codec device,
WM8994. In that configuration, U29 also supplies the PDM clock to the microphones.
Regardless of where the microphones are routed to, STM32L476ZGT6 or WM8994, they
can be power-supplied from either VDD or MICBIAS1 output of the WM8994 codec device.
Table 7
shows settings of all jumpers associated with the digital microphones on the board.
2.7.2 Headphones
outputs
The STM32L476G-EVAL evaluation board can drive two sets of stereo headphones.
Identical or different stereo audio content can be played back in each set of headphones.
The STM32L476ZGT6 sends up to two independent stereo audio channels, via its SAI1
TDM port, to the WM8994 codec device. The codec device converts the digital audio stream
to stereo analog signals. It then boosts them for direct drive of headphones connecting to
3.5 mm stereo jack receptacles on the board, CN20 for Audio-output1 and CN21 for
Audio_output2.
Figure 6
shows a top view of the CN20 and CN21 headphones jack
receptacles.
The CN21 jack takes its signal from the WM8994 codec device’s output intended for driving
an amplifier for loudspeakers. A hardware adaptation is incorporated on the board to make it
compatible with a direct headphone drive. The adaptation consists of coupling capacitors
blocking the DC component of the signal, attenuator and anti-pop resistors. The WM8994
codec device’s loudspeaker output must be configured by software in linear mode called
“class AB” and not in switching mode called “class D”.
The I²C-bus address of WM8994 is 0b0011010.
Table 7. Digital microphone-related jumper settings
Jumper
Setting
Configuration
JP14
JP14
Default setting
.
PDM clock for digital microphones comes from STM32L476ZGT6
JP14
PDM clock for digital microphones comes from WM8994 codec.
JP16
JP16
Default setting
.
Power supply of digital microphones is VDD.
JP16
Power supply of digital microphones is generated by WM8994 codec.
