RM0385 Datasheet, PDF (1080/1644 Pages) STMicroelectronics – This reference manual targets application developers

Serial peripheral interface / inter-IC sound (SPI/I2S)
RM0385
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In slave mode, the way the frame synchronization is detected, depends on the value of
ASTRTEN bit.
If ASTRTEN = 0, when the audio interface is enabled (I2SE = 1), then the hardware waits for
the appropriate transition on the incoming WS signal, using the CK signal.
The appropriate transition is a falling edge on WS signal when I2S Philips Standard is used,
or a rising edge for other standards. The falling edge is detected by sampling first WS to 1
and then to 0, and vice-versa for the rising edge detection.
If ASTRTEN = 1, the user has to enable the audio interface before the WS becomes active.
This means that the I2SE bit must be set to 1 when WS = 1 for I2S Philips standard, or when
WS = 0 for other standards.
1080/1644
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