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UM10503
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© NXP B.V. 2015. All rights reserved.
User manual
Rev. 2.1 — 10 December 2015
1211 of 1441
NXP Semiconductors
UM10503
Chapter 44: LPC43xx/LPC43Sxx I2S interface
44.6.9.1 Notes on fractional rate generators
A fractional rate generator can introduce output jitter with some divide settings. This is
because the fractional rate generator is a fully digital function, so the output clock
transitions are synchronous with the source clock, whereas a theoretical perfect fractional
rate may have edges that are not related to the source clock. Therefore the output jitter
will not be greater than plus or minus one source clock between consecutive clock edges.
For example, if X = 0x07 and Y = 0x11, the fractional rate generator will output 7 clocks for
every 17 (11 hex) input clocks, distributed as evenly as it can. In this example, there is no
way to distribute the output clocks in a perfectly even fashion, so some clocks will be
longer than others. The output is divided by 2 in order to square it up, which also helps
with the jitter. The frequency averages out to exactly (7/17) / 2, but some clocks will be a
slightly different length than their neighbors. It is possible to avoid jitter entirely by
choosing fractions such that X divides evenly into Y, such as 2/4, 2/6, 3/9, 1/N, etc.
44.6.10 I2S Receive Clock Rate register
The MCLK rate for the I2S receiver is determined by the values in the RXRATE register.
The required RXRATE setting depends on the peripheral clock rate (PCLK_I2S =
CLK_APB1_I2S
) and the desired MCLK rate (such as 256 fs).
The receiver MCLK rate is generated using a fractional rate generator, dividing down the
frequency of PCLK_I2S. Values of the numerator (X) and the denominator (Y) must be
chosen to produce a frequency twice that desired for the receiver MCLK, which must be
an integer multiple of the receiver bit clock rate. Fractional rate generators have some
aspects that the user should be aware of when choosing settings. These are discussed in
. The equation for the fractional rate generator is:
I2S_RX_MCLK = PCLK_I2S * (X/Y) /2
Note: If the value of X or Y is 0, then no clock is generated. Also, the value of Y must be
greater than or equal to X.
Table 1009.I2S Transmit Clock Rate register (TXRATE, address 0x400A 2020 (I2S0) and
0x400A 3020 (I2S1)) bit description
Bit
Symbol
Description
Reset
value
7:0
Y_DIVIDER
I2S transmit MCLK rate denominator. This value is used to divide
PCLK to produce the transmit MCLK. Eight bits of fractional divide
supports a wide range of possibilities. A value of 0 stops the clock.
0
15:8
X_DIVIDER
I2S transmit MCLK rate numerator. This value is used to multiply
PCLK by to produce the transmit MCLK. A value of 0 stops the
clock. Eight bits of fractional divide supports a wide range of
possibilities. Note: the resulting ratio X/Y is divided by 2.
0
31:16
-
Reserved, user software should not write ones to reserved bits.
The value read from a reserved bit is not defined.
-