29.6.2.1.5 Relation Between MCKn, SCKn, and Sampling Frequency fs

Based on sampling frequency fs, the SCKn frequency requirement can be calculated:

SCKn frequency: $\begin{matrix} f_{SCKn} & = & f s \times total_number_of_bits_per_frame \end{matrix}$ ,
Where $\begin{matrix} total_number_of_bits_per_frame & = & number_of_slots \times number_of_bits_per_slots \end{matrix}$ .

The number of slots is selected by writing to the Number of Slots in Frame bit field in the Clock Unit n Control (CLKCTRLn) register: $\begin{matrix} number_of_slots & = & NBSLOTS + 1 \end{matrix}$ .
The number of bits per slot (8, 16, 24, or 32 bit) is selected by writing to the Slot Size bit field in CLKCTRLn: $number_of_bits_per_slot = 8 \times (SLOTSIZE + 1)$ .
Consequently, $f_{SCKn} = 8 \times f s \times (NBSLOTS + 1) \times (SLOTSIZE + 1)$ .

The clock frequencies

f_{SCKn}

and

f_{MCKn}

are derived from the generic clock frequency

f_{GCLK_I2S_n}

$\begin{matrix} f_{GCLK_I2S_n} & = & f_{SCKn} \times (CLKCTRLn.MCKDIV + 1) \\ = & 8 \times f s \times (NBSLOTS + 1) \times (SLOTSIZE + 1) \times (MCKDIV + 1) \end{matrix}$
, and
$\begin{matrix} f_{GCLK_I2S_n} & = & f_{MCKn} \times (MCKOUTDIV + 1) \end{matrix}$ .

Substituting the right hand sides of the two last equations yields:

f_{MCKn} = \frac{f_{GCLK_I2S_n}}{MCKOUTDIV + 1}

f_{MCKn} = \frac{8 \cdot (SLOTSIZE + 1) \cdot (NBSLOTS + 1) \cdot (MCKDIV + 1)}{MCKOUTDIV + 1}

If a Host Clock output is not required, the GCLK_I2S generic clock can be configured as SCKn by writing a '0'to CLKCTRLn.MCKDIV. Alternatively, if the frequency of the generic clock is a multiple of the required SCKn frequency, the MCKn-to-SCKn divider can be used with the ratio defined by writing the CLKCTRLn.MCKDIV field.

The FSn pin is used as Word Select in I²S format and as Frame Synchronization in TDM format, as described in I²S Format - Reception and Transmission Sequence with Word Select and TDM Format - Reception and Transmission Sequence, respectively.

SAM D21/DA1 Family

29.6.2.1.5 Relation Between MCKn, SCKn, and Sampling Frequency fs