CS4237B Datasheet, PDF (85/114 Pages) Cirrus Logic – CrystalClear Advanced Audio System with 3D Sound

CS4237B
reading until the FIFO is empty, at which time
the requests will stop. When ACF is cleared, the
ADPCM adaptation will continue.
When PEN is cleared (playback disabled), the
ADPCM block’s accumulator and step size are
cleared. When PEN is set, the ADPCM block
will start converting. When pausing the playback
stream is desired, audio data should not be sent
to the codec which will cause a data underrun.
This can be accomplished by disabling the DMA
controller or not sending data in PIO mode. The
underrun will be detected by the WSS Codec
and the adaptation will freeze. When data is sent
to the codec, adaptation will resume. It is critical
that all playback ADPCM samples are sent to the
codec, since dropped samples will cause errors
in adaptation. Whereas toggling PEN resets the
accumulator and step size, the APAR bit (I17)
only resets the accumulator without affecting the
step size.
DMA Registers
The DMA registers allow easy integration of this
part into ISA systems. Peculiarities of the ISA
DMA controller require an external count
mechanism to notify the host CPU of a full
DMA buffer via interrupt. The programmable
DMA Base registers provide this service.
The act of writing a value to the Upper Base
register causes both Base registers to load the
Current Count register. DMA transfers are en-
abled by setting the PEN/CEN bit while
PPIO/CPIO is clear. (PPIO/CPIO can only be
changed while the MCE bit is set.) Once trans-
fers are enabled, each sample that is transferred
by a DMA cycle will decrement the Current
Count register (with the exception of the
ADPCM format) until zero is reached. The next
sample after zero generates an interrupt and re-
loads the Current Count registers with the values
in the Base registers.
For all data formats except ADPCM, the DMA
Base registers must be loaded with the number
of samples, minus one, to be transferred between
"DMA Interrupts". Stereo data contains twice as
many samples as mono data; however, 8-bit data
and 16-bit data contain the same number of sam-
ples. Symbolically:
DMA Base register₁₆ = N_S - 1
Where N_S is the number of samples transferred
between interrupts and the "DMA Base regis-
ter₁₆" consists of the concatenation of the upper
and lower DMA Base registers.
For the ADPCM data format, the contents of the
DMA Base registers is calculated differently
from any other data format. The Base registers
must be loaded with the number of BYTES to be
transferred between "DMA interrupts", divided
by four, minus one. The same equation is used
whether the data format is stereo or mono
ADPCM. Symbolically:
DMA Base register₁₆ = N_b/4 - 1
Where N_b is the number of BYTES transferred
between interrupts and the "DMA Base regis-
ter₁₆" consists of the concatenation of the upper
and lower DMA Base registers.
PLAYBACK DMA REGISTERS
The playback DMA registers (I14/15) are used
for sending playback data to the DACs in
MODE 2 and 3. In MODE 1, these registers
(I14/15) are used for both playback and capture;
therefore, full-duplex DMA operation is not pos-
sible.
When the playback Current Count register rolls
under, the Playback Interrupt bit, PI, (I24) is set
causing the INT bit (R2) to be set. The interrupt
is cleared by a write of any value to the Status
register (R2), or writing a "0" to the Playback
Interrupt bit, PI (I24).
DS213PP4
85