cp3bt13 National Semiconductor Corporation, cp3bt13 Datasheet - Page 112

• Manufacturer Part Number: cp3bt13
• Description: Cp3bt13 Reprogrammable Connectivity Processor With Bluetooth-r And Can Interfaces
• Manufacturer: National Semiconductor Corporation

www.national.com
Figure 57. Accessing Three Devices in Network Mode
17.3
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 CP3BT13, 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. The input clock is divided
by the value of the prescaler BCPRS[7:0] + 1 to generate
the bit clock.
The bit clock rate f
equation:
The ideal required prescaler value P
as follows:
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
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
f
bit
frame sync)
frame sync)
STD/SRD
= n × f
(auxiliary
(auxiliary
f
n = Number of Slots per Frame
f
Data Length = Length of data word in multiples of 8 bits
P
f
SRCLK
bit
Sample
bit_error
SRFS
ideal
SFS
= n × f
BIT CLOCK GENERATION
Sample
= f
= Sample Frequency in Hz
[%] = (f
Audio In
Sample
bit
Data from/to
Codec 1
can be calculated by the following equation:
Slot0
× Data Length = 2 × 8 kHz × 16 = 256 kHz
bit
/ f
bit
× Data Length
bit
- f
Data from/to
can be calculated by the following
Audio In
Codec 2
Slot1
Frame
/P
bit_error
Data from/to
real
Codec 3
Slot2
) / f
ideal
.
bit
× 100
can be calculated
Slot2
DS057
112
The ideal required prescaler value P
as follows:
Therefore, the real prescaler value is 47. This results in a bit
clock error equal to:
17.4
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.
17.5
17.5.1
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
17.5.2
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:
T RX FIFO Overrun - ASCR.RXEIP = 1
T RX FIFO Almost Full (Warning Level) - ASCR.RXIP = 1
T TX FIFO Under run - ASCR.TXEIP = 1
T 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.
P
f
= (256 kHz - 12 MHz/47) / 256 kHz × 100 = 0.27%
bit_error
ideal
FRAME CLOCK GENERATION
AUDIO INTERFACE OPERATION
Clock Configuration
Interrupts
= f
= (f
Audio In
bit
- f
/ f
Audio In
bit
= 12 MHz / 256 kHz = 46.875
/P
real
) / f
bit
ideal
× 100
can be calculated