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RM0090: Reference manual - STMicroelectronics

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<strong>RM0090</strong> Serial peripheral interface (SPI)<br />

Figure 303. PCM standard waveforms (16-bit)<br />

CK<br />

WS<br />

short<br />

frame<br />

WS<br />

long<br />

frame<br />

SD<br />

For long frame synchronization, the WS signal assertion time is fixed 13 bits in master<br />

mode.<br />

For short frame synchronization, the WS synchronization signal is only one cycle long.<br />

Figure 304. PCM standard waveforms (16-bit extended to 32-bit packet frame)<br />

CK<br />

WS<br />

short<br />

frame<br />

WS<br />

long<br />

frame<br />

SD<br />

Note: For both modes (master and slave) and for both synchronizations (short and long), the<br />

number of bits between two consecutive pieces of data (and so two synchronization signals)<br />

needs to be specified (DATLEN and CHLEN bits in the SPI_I2SCFGR register) even in slave<br />

mode.<br />

27.4.4 Clock generator<br />

fixed to 13-bit<br />

16-bit<br />

MSB LSB MSB<br />

fixed to 13-bit<br />

MSB<br />

16-bit<br />

LSB<br />

The I 2 S bitrate determines the dataflow on the I 2 S data line and the I 2 S clock signal<br />

frequency.<br />

I 2 S bitrate = number of bits per channel × number of channels × sampling audio frequency<br />

For a 16-bit audio, left and right channel, the I 2 S bitrate is calculated as follows:<br />

I 2 S bitrate = 16 × 2 × FS Doc ID 018909 Rev 3 824/1416

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