National CP3BT26 manual CVSD/PCM Conversion Module, Operation, PCM Conversions

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.

2 MHz
Clock Input Interrupt	DMA

16-Bit 8 kHz			1-Bit 64 kHz
	16-Bit
		CVSD
			16-Bit Shift Reg
		Encoder
u/A-Law	64 kHz
16-Bit 8 kHz	Filter
	Engine
	16-Bit		1-Bit 64 kHz
		CVSD
u/A-Law			16-Bit Shift Reg
		Decoder
	64 kHz

Peripheral Bus

DS058

Figure 75. CVSD/PCM Converter Block Diagram

21.1OPERATION

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.

21.2PCM 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.

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