CC1110Fx / CC1111Fx
13.15.8.1 Clock Generation
Please note that to stay within the timing
requirements of the I2S specification [7], a
minimum value of 3.35 should be used for the
(NUM/ DENOM) fraction.
When the I2S is configured as master, the
frequency of the SCK clock signal must be set
to match the sample rate. The clock frequency
must be set before master mode is enabled.
The fractional divider is made such that most
normal sample rates should be supported for
most normal word sizes with a 24 MHz system
clock frequency (CC1111Fx). Examples of
supported configurations for a 24 MHz system
clock are given in Table 57. Table 58 shows the
configuration values for a 26 MHz system clock
frequency. Notice that the generated I2S
frequency is not exact for the 44.1 kHz, 16 bits
word size configuration at 26 MHz. The
numbers are calculated using the following
formulas, where Fs is the sample rate and W is
the word size:
SCK is generated by dividing the system clock
using a fractional clock divider. The amount of
division is given by the 15 bit numerator, NUM ,
and 9-bit denominator, DENOM, as shown in the
following formula:
Fclk
NUM
Fsck
=
2(
)
DENOM
NUM
DENOM
where
> 3.35
Fclk is the system clock frequency and Fsck is the
Fsck
Fs =
I2S SCK sample clock frequency.
2*W
The value of the numerator is set in the
I2SCLKF2.NUM[14:8]:I2SCLKF1.NUM[7:0]regi
-sters and the denominator value is set in
I2SCLKF2.DENOM[8]:I2SCLKF0.DENOM[7:0].
Fclk
DENOM 4*W * Fs
NUM
CLKDIV=
=
I2SCLKF2 I2SCLKF1 I2SCLKF0
Fsck (kHz)
Word Size (W)
CLKDIV
93.75
Exact
Yes
8
8
0x01
0x01
0x04
0x00
0x77
0x77
0xE2
0x7D
0x04
0x08
0x93
0x10
8
16
16
16
46.875
8.503401
7.8125
Yes
44.1
48
Yes
Yes
Table 57: Example I2S Clock Configurations (CC1111Fx, 24 MHz)
I2SCLKF2 I2SCLKF1 I2SCLKF0
Fsck (kHz)
Word Size (W)
CLKDIV
101.5625
50.78125
9.21201
8.46354
Exact
Yes
Yes
No
8
8
0x06
0x06
0x8A
0x06
0x59
0x59
0x2F
0x59
0x10
0x20
0x1B
0xC0
8
16
16
16
44.1
48
Yes
Table 58: Example I2S Clock Configurations (CC1110Fx, 26 MHz)
13.15.8.2 Word Size
in the data registers and the remaining low
order bits will be discarded.
The word size must be set before master
mode is enabled. The word size is the number
of bits used for each sample and can be set to
a value between 1 and 33. To set the word
13.15.9 Slave Mode
The I2S is configured as a slave device by
setting I2SCFG0.MASTERto 0. When in slave
mode the SCK and WS signals are generated
by an external I2S master and are inputs to the
I2S interface.
size, write word size
–
1
to the
I2SCFG1.WORDS[4:0] bits. Setting the word
size to a value of 17 or more causes the I2S to
pad each word with 0’s in the least significant
bits since the data registers provide maximum
16 bits. This feature allows samples to be sent
to an I2S device that takes a higher resolution
than 16 bits.
13.15.9.1 Word Size
When the I2S operates in slave mode, the word
size is determined by the master that
generates the WS signal.
If the size of the received samples exceeds 16
bits, only the 16 most significant bits will be put
SWRS033E
Page 164 of 239
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