PXD20RM Datasheet, PDF (1375/1628 Pages) Freescale Semiconductor, Inc – PXD20 Microcontroller

For Wave mode the volume is directly controlled by the volume control register for each channel. For DDS
mode, the volume is controlled by the ASR envelope controller.
Figure 39-46. Volume Controller for One Channel
39.7.6 Re-sampling Block
The re-sampling block is used to unify the sample rates for all channels before entering the mixers. For
DDS mode, different sample clocks can be used for each channel and the re-sampling clock can be used
to choose the current sample value on each channel as its output. For Wave mode, the algorithm for
SRC(Sample Rate Conversion) would be very complicated with linear/cubic interpolation and therefore
all wave mode channels must share a sample clock.
39.7.7 Mixer
The mixers are used to mix the data from the four channels together and generate the PCM outputs for the
external audio DAC and/or PWM (as the duty). Two mixers, right mixer and left mixer, are provided to
generate the PCM data for 2 output channels: Channel-R and Channel-L.
Each mixer can be configured to mix any 1 to 4 of all 4 input channels together. Consider, for example,
channel-0 and channel-1 in Wave mode and channel-2 and channel-3 in DDS mode. The left mixer can be
configured to mix channel-0 and channel-3 together while the right mixer can be configured to mix
channel-1and channel-2 together.
The mixer implements an arithmetic add for all the 4 input data (represented by 2' s complement) and a
division with the rule below:
If 1 channel is selected, division by 1;
If 2 channels are selected, division by 2;
If 3channels are selected, division by 4;
Freescale Semiconductor
PXD20 Microcontroller Reference Manual, Rev. 1
Preliminary—Subject to Change Without Notice
39-51