National CP3BT26 - Manual (Page 146)

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CP3BT26
Figure 69. Accessing Three Devices in Network Mode
20.3 BIT CLOCK GENERATION
An 8-bit prescaler is provided to divide the audio interface
input clock down to the required bit clock rate. Software can
choose between two input clock sources, a primary and a
secondary clock source.
On the CP3BT26, the two optional input clock sources are
the 12-MHz Aux1 clock (also used for the Bluetooth LLC)
and the 48-MHz PLL output clock (also used by the USB
node). The input clock is divided by the value of the prescal-
er BCPRS[7:0] + 1 to generate the bit clock.
The bit clock rate f
bit
can be calculated by the following
equation:
f
bit
= n × f
Sample
× Data Length
n = Number of Slots per Frame
f
Sample
= Sample Frequency in Hz
Data Length = Length of data word in multiples of 8 bits
The ideal required prescaler value P
ideal
can be calculated
as follows:
P
ideal
= f
Audio In
/ f
bit
The real prescaler must be set to an integer value, which
should be as close as possible to the ideal prescaler value,
to minimize the bit clock error, f
bit_error
.
f
bit_error
[%] = (f
bit
- f
Audio In
/P
real
) / f
bit
× 100
Example:
The audio interface is used to transfer 13-bit linear PCM
data for one audio channel at a sample rate of 8k samples
per second. The input clock of the audio interface is 12 MHz.
Furthermore, the codec requires a minimum bit clock of 256
kHz to operate properly. Therefore, the number of slots per
frame must be set to 2 (network mode) although actually
only one slot (slot 0) is used. The codec and the audio inter-
face will put their data transmit pins in TRI-STATE mode af-
ter the PCM data word has been transferred. The required
bit clock rate f
bit
can be calculated by the following equation:
f
bit
= n × f
Sample
× Data Length = 2 × 8 kHz × 16 = 256 kHz
The ideal required prescaler value P
ideal
can be calculated
as follows:
P
ideal
= f
Audio In
/ f
bit
= 12 MHz / 256 kHz = 46.875
Therefore, the real prescaler value is 47. This results in a bit
clock error equal to:
f
bit_error
= (f
bit
- f
Audio In
/P
real
) / f
bit
× 100
= (256 kHz - 12 MHz/47) / 256 kHz × 100 = 0.27%
20.4 FRAME CLOCK GENERATION
The clock for the frame synchronization signals is derived
from the bit clock of the audio interface. A 7-bit prescaler is
used to divide the bit clock to generate the frame sync clock
for the receive and transmit operations. The bit clock is di-
vided by FCPRS + 1. In other words, the value software
must write into the ACCR.FCPRS field is equal to the bit
number per frame minus one. The frame may be longer than
the valid data word but it must be equal to or larger than the
8- or 16-bit word. Even if 13-, 14-, or 15-bit data is being
used, the frame width must always be at least 16 bits wide.
In addition, software can specify the length of a long frame
sync signal. A long frame sync signal can be either 6, 13,
14, 15, or 16 bits long, depending on the external codec be-
ing used. The frame sync length can be configured by the
Frame Sync Length field (FSL) in the AGCR register.
20.5 AUDIO INTERFACE OPERATION
20.5.1 Clock Configuration
The Aux1 clock (generated by the Clock module described
in Section 11.9) must be configured, because it is the time
base for the AAI module. Software must write an appropri-
ate divisor to the ACDIV1 field of the PRSAC register to pro-
vide a 12 MHz input clock. Software also must enable the
Aux1 clock by setting the ACE1 bit in the CRCTRL register.
For example:
PRSAC &= 0xF0;
// Set Aux1 prescaler to 1 (F = 12 MHz)
CRCTRL |= ACE1; // Enable Aux1 clk
20.5.2 Interrupts
The interrupt logic of the AAI combines up to four interrupt
sources and generates one interrupt request signal to the
Interrupt Control Unit (ICU).
The four interrupt sources are:
„ RX FIFO Overrun - ASCR.RXEIP = 1
„ RX FIFO Almost Full (Warning Level) - ASCR.RXIP = 1
„ TX FIFO Under run - ASCR.TXEIP = 1
„ TX FIFO Almost Empty (Warning Level) - ASCR.TXIP=1
In addition to the dedicated input to the ICU for handling
these interrupt sources, the Serial Frame Sync (SFS) signal
is an input to the MIWU (see Section 13.0), which can be
programmed to generate edge-triggered interrupts.
Slot1
Frame
Slot0
SFS
SRCLK
(auxiliary
frame sync)
SRFS
(auxiliary
frame sync)
STD/SRD
Data from/to
Codec 1
Data from/to
Codec 3
Data from/to
Codec 2
Slot2 Slot2
DS057