Texas Instruments | Audio Serial Interface Configurations for Audio Codecs | Application notes | Texas Instruments Audio Serial Interface Configurations for Audio Codecs (Rev. A) Application notes

Texas Instruments Audio Serial Interface Configurations for Audio Codecs (Rev. A) Application notes
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Application Report
SLAA469A – September 2010 – Revised June 2019
Audio Serial Interface Configurations for Audio Devices
Jorge Arbona, Uttam Agarwal .......................................................................... Audio Converter Products
ABSTRACT
An audio serial interface (ASI) provides a means to transfer non-buffered audio data between processors
and audio converters. This data is typically encoded in PCM twos complement format, although other
format variations may be possible to achieve companding for lower data rate transfers. Audio converters
based on the delta-sigma (ΔΣ) architecture require an internal master clock that operates at a much faster
rate than the target sample rate. Although there are several means to obtain this master clock, take care
to ensure that this clock does not drift with respect to the ASI. This application report discusses several
configurations that prevent such situations.
1
2
Contents
Introduction ................................................................................................................... 1
ASI Configurations ........................................................................................................... 2
List of Figures
1
...................................................................
1
Repeated Sample (Master Clock Slower than Ideal)
2
ASI Slave Mode .............................................................................................................. 3
3
ASI Slave Mode (Independent Master Clock)
4
ASI Slave Mode (Generating Master Clock from BCLK) ............................................................... 4
5
ASI Master Mode ............................................................................................................ 5
...........................................................................
2
4
Introduction
Each system has different requirements when it comes to interfacing to an audio device ASI. The most
common configurations are the master and slave modes. When the audio device ASI is configured in
master mode, its bit clock (BCLK) and word clock (WCLK) pins are output. In slave mode, BCLK and
WCLK are inputs to the device ASI. This relationship might seem straightforward. However, take care
when the ASI is configured in slave mode to ensure that the oversampled data that is decimated always
fall within the correct target rate time slot.
If a master clock is a free-running clock and it is fed to a converter, it is not frequency-locked to the frame
clock (WCLK) of an independent ASI. Any deviation from the ideal eventually results in a skipped or
repeated sample (assuming that the architecture repeats samples). For example, if a host processor
provides an ideal 48-kHz WCLK with respect to absolute time, its respective ideal master clock could be
exactly (128 ● WCLK) = 6.144 MHz. If a master clock from a non-ideal crystal is provided directly to the
converter modulator with a 0.001% error, this clock could result in 6.14393856 MHz. Eventually this slower
clock results in a repeated sample out of the ASI bus. Of course, there is no such thing as an ideal master
or ASI clocks.
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ASI Configurations
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Figure 1 illustrates a simplified case in which a hypothetical analog-to-digital converter (ADC) operating at
Nyquist frequency (for simplification purposes) results in a duplicated sample on the ASI bus. In this
example, the hypothetical converter ideally hands over its data on the middle of a frame on the falling
edge of the master clock. These data should then be ready to be transferred in the beginning of the next
frame. As shown in Figure 1, the master clock is actually slower than the ideal. This configuration
eventually drifts the clock enough (with respect to the ASI frame) such that there is a frame that does not
receive new data (as shown at the end of frame #3).
Master Clock
1
Frame
2
6
5
4
Internal
3
Conversion
2
1
0
6
5
4
ASI Handoff 3
2
1
0
3
4
5
Sample
Repeated
Sample
Sample
Figure 1. Repeated Sample (Master Clock Slower than Ideal)
If the master clock is faster than the ideal, then two ADC samples may be written within a single frame
resulting in a skipped sample through the ASI bus. For the DAC data received from the ASI bus, a faster
modulator clock than the ideal results in duplicate samples and a slower modulator clock results in
skipped samples at the modulator output.
2
ASI Configurations
As a general requirement, the internal master clock of a converter must be frequency-locked with the ASI
frame clock (typically available through WCLK pin). This condition does not mean that the master clock
and the ASI frame need to be phase-locked. What is important is that these clocks do not drift over time
with respect to each other.
In TI’s AIC family of devices, the ASI is composed of a bit clock, word clock, data in, and data out. The bit
clock (typically the BCLK pin) clocks DOUT (ADC) data, and latches DIN (DAC) data for each word clock
(WCLK) frame. The ASI bus has timing requirements itself (which can be found in the respective device
data sheet). However, these are not related to the MCLK, only to the ASI itself.
2
Audio Serial Interface Configurations for Audio Codecs
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ASI Configurations
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2.1
ASI Slave Mode
In slave mode, the host processor generates the bit clock and word clock from a system clock. To obtain
an audio clock, the system clock is often synthesized to a number divisible by 44.1 kHz or 48 kHz.
Figure 2 shows such example. This configuration ensures that the frame clock (accessible at the device
WCLK pin) does not drift with respect to the master clock generated by the frequency synthesizer.
24 MHz
Frequency
Synthesizer
MCLK
12.288 MHz
8x
Divider
Clock
Generator
ASI
DS
BCLK
1.536 MHz
32x
Divider
WCLK
48 kHz
Audio Codec
Host Processor
Figure 2. ASI Slave Mode
For cases where a master clock output is not available, obtaining the audio device MCLK directly from the
frequency synthesizer source (for example, 24-MHz clock shown in Figure 2) may or may not be suitable
in some applications. Some hosts may receive another clock, such as a USB start of frame (SOF) tick, as
a reference which may continually change the phase of the ASI bus relative to the external clock.
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ASI Configurations
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Figure 3 shows a configuration that should be avoided. Because the crystal and the frame clock are
independent of each other, they eventually drift and cause skipped and repeated samples.
24 MHz
12.288 MHz
Crystal +
Oscillator
Frequency
Synthesizer
MCLK
12.288 MHz
8x
Divider
Clock
Generator
ASI
DS
BCLK
1.536 MHz
32x
Divider
WCLK
48 kHz
Host Processor
Audio Codec
Figure 3. ASI Slave Mode (Independent Master Clock)
Some audio devices, such as the TAS2505 and TAS2557, are capable of deriving the internal master
clock from an external BCLK, as shown in Figure 4. However, BCLK must be fast enough to be within the
PLL input frequency specification. For low WCLK frequencies, this condition can be solved by increasing
the number of BCLK cycles per WCLK frame enough to satisfy these requirements.
12.288 MHz
MCLK
Clock
Generator
8x
Divider
DS
ASI
BCLK
1.536 MHz
32x
Divider
WCLK
48 kHz
Host Processor
Audio Codec
Figure 4. ASI Slave Mode (Generating Master Clock from BCLK)
4
Audio Serial Interface Configurations for Audio Codecs
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2.2
ASI Master Mode
In master mode, the BCLK and WCLK are outputs from the audio device. These clocks are derived from
an external master clock, such as an oscillator, as shown in Figure 5. The ASI bus is derived from the
device master clock input which prevents drift between both.
24 MHz
Crystal +
Oscillator
MCLK
Clock
Generator
ASI
DS
BCLK
1.536 MHz
2
IS
Module
WCLK
48 kHz
Host Processor
Audio Codec
Figure 5. ASI Master Mode
2.3
ASI Hybrid Modes
Two additional modes are possible in most AIC family devices:
• BCLK is the output and WCLK is the input.
In this case, BCLK is internally derived from MCLK and sent to the host processor, which in turn
should generate a WCLK that conforms to the ASI specification of the device. BCLK does not drift with
respect to MCLK; thus, the generated WCLK does also not drift.
Additional information about this configuration can be found in the Configuring I2S to Generate BCLK
from Codec Devices & WCLK from McBSP Port Application Report.
• BCLK is the input and WCLK is the output.
This mode has similar constraints to the ones mentioned in Section 2.1. In this mode, the audio device
monitors the BCLK pin to keep the WCLK output timing within the ASI bus specification. BCLK must
not drift with respect to the master clock to ensure that the generated WCLK does not drift with respect
to MCLK.
The internal master clock can also be generated from the BCLK (similar to Figure 4, but with WCLK as
an output).
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Revision History
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Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Original (September 2010) to A Revision ............................................................................................... Page
•
•
•
6
Changed instances of "codec" to "device". ............................................................................................ 1
Edited application report for clarity. ..................................................................................................... 1
Changed TLV320AICxxx to TAS25xx. ................................................................................................. 4
Revision History
SLAA469A – September 2010 – Revised June 2019
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