National CP3BT26 - Manual (Page 158)

www.national.com 158
CP3BT26
21.0 CVSD/PCM Conversion Module
The CVSD/PCM module performs conversion between
CVSD data and PCM data, in which the CVSD encoding is
as defined in the Bluetooth specification and the PCM en-
coding may be 8-bit µ-Law, 8-bit A-Law, or 13-bit to 16-bit
Linear.
The CVSD conversion module operates at a fixed rate of
125 µs (8 kHz) per PCM sample. On the CVSD side, there
is a read and a write FIFO allowing up to 8 words of data to
be read or written at the same time. On the PCM side, there
is a double-buffered register requiring data to be read and
written every 125 µs. The intended use is to move CVSD
data into the module with a CVSD interrupt handler, and to
move PCM data with DMA. Figure 75 shows a block dia-
gram of the CVSD to PCM module.
Figure 75. CVSD/PCM Converter Block Diagram
21.1 OPERATION
The Aux2 clock (generated by the Clock module described
in Section 11.9) must be configured, because it drives the
CVSD module. Software must set its prescaler to provide a
2 MHz input clock based upon the System Clock (usually
12 MHz). This is done by writing an appropriate divisor to
the ACDIV2 field of the PRSAC register. Software must also
enable the Aux2 clock by setting the ACE2 bit within the
CRCTRL register. For example:
PRSAC &= 0x0f;
// Set Aux2 prescaler to generate
// 2 MHz (Fsys = 12 MHz)
PRSAC |= 0x50;
CRCTRL |= ACE2; // Enable Aux2 clk
The module converts between PCM data and CVSD data at
a fixed rate of 8 kHz per PCM sample. Due to compression,
the data rate on the CVSD side is only 4 kHz per CVSD
sample.
If PCM interrupts are enabled (PCMINT is set) every 125 µs
(8 kHz) an interrupt will occur and the interrupt handler can
operate on some or all of the four audio streams CVSD in,
CVSD out, PCM in, and PCM out. Alternatively, a DMA re-
quest is issued every 125 µs and the DMA controller is used
to move the PCM data between the CVSD/PCM module
and the audio interface.
If CVSD interrupts are enabled, an interrupt is issued when
either one of the CVSD FIFOs is almost empty or almost full.
On the PCM data side there is double buffering, and on the
CVSD side there is an eight word (8 × 16-bit) FIFO for the
read and write paths.
Inside the module, a filter engine receives the 8 kHz stream
of 16-bit samples and interpolates to generate a 64 kHz
stream of 16-bit samples. This goes into a CVSD encoder
which converts the data into a single-bit delta stream using
the CVSD parameters as defined by the Bluetooth specifi-
cation. There is a similar path that reverses this process
converting the CVSD 64 kHz bit stream into a 64 kHz 16-bit
data stream. The filter engine then decimates this stream
into an 8 kHz, 16-bit data stream.
21.2 PCM CONVERSIONS
During conversion between CVSD and PCM, any PCM for-
mat changes are done automatically depending on whether
the PCM data is µ-Law, A-Law, or Linear. In addition to this,
a separate function can be used to convert between the var-
ious PCM formats as required. Conversion is performed by
setting up the control bit CVCTL1.PCMCONV to define the
conversion and then writing to the LOGIN and LINEARIN
registers and reading from the LOGOUT and LINEAROUT
registers. There is no delay in the conversion operation and
it does not have to operate at a fixed rate. It will only convert
between µ-Law/A-Law and linear, not directly between µ-
Law and A-Law. (This could easily be achieved by convert-
ing between µ-Law and linear and between linear and A-
Law.)
If a conversion is performed between linear and µ-Law log
PCM data, the linear PCM data are treated in the left-
aligned 14-bit linear data format with the two LSBs unused.
If a conversion is performed between linear and A-Law log
PCM data, the linear PCM data are treated in the left-
aligned 13-bit linear data format with the three LSBs un-
used.
Peripheral Bus
Filter
Engine
16-Bit Shift Reg
1-Bit 64 kHz
1-Bit 64 kHz
16-Bit
16-Bit
64 kHz
64 kHz
16-Bit Shift Reg
16-Bit 8 kHz
CVSD
Encoder
DMA Interrupt
2 MHz
Clock Input
CVSD
Decoder
u/A-Law
16-Bit 8 kHz
u/A-Law
DS058