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Audio Interface Module (AIM)
MCF5253 Reference Manual, Rev. 1
Freescale Semiconductor
17-43
Upon request Freescale can supply example s/w code that implements the Frequency measurement and
controls the XTRIM output to allow CRIN to be locked to the EBU clock input.
The PLL maintaining phase relation between the incoming source signal and internal signal is mainly
digital. It is necessary, however, to measure the phase/frequency of the incoming signal in relationship with
the CRIN clock in order to be able to steer the sample rate convertor clock. The circuit shown in
is used for maintaining this phase relationship.
Figure 17-24. Frequency Measurement Circuit
Associated with the Frequency measurement block are two registers. (See
). The circuit will
measure the frequency of the incoming clock by comparison with the audio CRIN clock (which is typically
16.93 MHz or 11.2896MHz). Multiplexer
(1)
selects the incoming clock source. Registers
(2)
,
(3)
, and xor
(4)
are an edge detector. Multiplexer
(5)
is a by-pass for the IEC958 input. The rest of the circuit is a
second-order filter with a bandwidth of approximately 80 Hz. The output FreqMeas(31:0) is an unsigned
number, giving the frequency of the selected source as a function of the CRIN clock. The filter is calculated
internally in 48 bit precision. The 16 LSBs are not sent out.
The value read from the FreqMeas Register is calculated as follows:
•
For measurement of the IIS inputs: FreqMeas = (IIS SCLK Freq
×
2)/Faudio
×
(2 ** 15)
×
Gain
•
For measurement of the EBU input: FreqMeas = (EBU Freq) / Faudio
×
(2 ** 15)
×
Gain
Table 17-24. PhaseConfig and Frequency Measure Register Addresses
Address
Name
Width
Description
Access
MBAR2 + 0xA3
PhaseConfig
8
Phase Configuration (gain and source select)
R/W
MBAR2 + 0xA8
FreqMeas
32
Frequency measurement
R
xor
1
D
D
D
D
2
3
4
5
6
+
7
8
9
10
11
+
12
13
14
15
16
sat
sat
FreqMeas[31:0]
CRIN
SCLK1
SCLK2
SCLK3
EBU-In
Select
Содержание MCF5253
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