CP3UB17G38 NSC [National Semiconductor], CP3UB17G38 Datasheet - Page 89

• Manufacturer Part Number: CP3UB17G38
• Description: CP3UB17 Reprogrammable Connectivity Processor with USB Interface
• Manufacturer: NSC [National Semiconductor]

Figure 18. Accessing Three Devices in Network Mode
16.3
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 CP3UB17, the two optional input clock sources are
the 12-MHz Aux1 clock and the 48-MHz PLL output clock
(also used by the USB node). 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 tristate their data transmit pins after the PCM data
word has been transferred. The required bit clock rate f
can be calculated by the following equation:
kHz
frame sync)
frame sync)
STD/SRD
(auxiliary
(auxiliary
f
n = Number of Slots per Frame
f
Data Length = Length of data word in multiples of 8 bits
P
f
f
SRCLK
bit
Sample
bit_error
bit
SRFS
ideal
SFS
= n × f
= n × f
BIT CLOCK GENERATION
= f
= Sample Frequency in Hz
[%] = (f
Audio In
Sample
Data from/to
Sample
Codec 1
Slot0
bit
/ f
bit
× Data Length
× Data Length = 2 × 8 kHz × 16 = 256
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
bit
89
The ideal required prescaler value P
as follows:
Therefore, the real prescaler value is 47. This results in a bit
clock error equal to:
16.4
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. Be aware, however, that a
frame may be longer than the valid data word.
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.
16.5
16.5.1
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
16.5.2
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:
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.
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
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
www.national.com