TDM Basics
MSC8144E Reference Manual, Rev. 3
Freescale Semiconductor
19-23
19.2.6 Buffers Mapped on System Memory
Each receive or transmit data channel is stored in a different buffer mapped on the internal system
memory. This buffer can be located in any of the internal memories (such as the M2 or M3
memory) that are shared by all the SC3400 cores.
19.2.6.1 Data Buffer Size and A/
μ
-law Channels
Data buffer size is identical for all receive channels belonging to a TDM module and is indicated
in the TDMxRDBS[RDBS] field. Data buffer size is also identical for all the transmit data
buffers and is indicated in the TDMxTDBS[TDBS] field (see page 19-54). An exception is the
A/
μ
-law channels (buffer size
×
2). When the TDMxRCPRn[RCONV] field (see page 19-62)
indicates that a channel is an A-law channel, the received 8 bits are converted into a 13-bit PCM
sample padded with three zeros on the right. This channel therefore occupies 16 bits per 8
received bits, essentially occupying double the size. When the TDMxRCPRn[RCONV] field
indicates that a channel is a
μ
-law channel, the received 8 bits are converted into a 14-bit PCM
sample padded with two zeros on the right. This channel also occupies 16 bits per 8 received bits,
essentially occupying double the size.
When the TDMxTCPRn[TCONV] field (see page 19-63) indicates that a channel is an A-law
channel, the transmitted 13 bits are converted into an 8-bit PCM sample. This channel therefore
occupies 16 bits (13 bits padded with three zeros at the right) per 8 transmit bits, essentially
occupying double the size. When the TDMxTCPRn[TCONV] field indicates that a channel is a
μ
-law channel, the received 14 bits are converted into an 8-bit PCM sample. This channel also
occupies 16 bits (14 bits padded with two zeros at the right) per 8 transmit bits, essentially
occupying double the size. The A/
μ
-law conversion is performed according to the ITU-T
recommendation G.711.
Note:
The minimum buffer size for both transmit and receive is 16 bytes (that is, the
RDBS/TDBS value is 0x00000F). The maximum buffer size for both transmit and
receive is 16 MB (that is, the RDBS/TDBS value is 0xFFFFFF), but it can be further
limited by the main memory size according to the number of channels in the frame.
Figure 19-24 shows how the samples are stored in the receive main data buffer (if the receive
channel is A/
μ
law and TDMxRIR[RBOR] = 1) or the transmit main data buffer (if the transmit
channel is A/
μ
law and TDMxTIR[TBOR] = 1).
Figure 19-24. Receive/Transmit Main Data Buffer For A-Law/
μ
-Law Channel
smp #0
smp #1
smp #2
smp #3
smp #4
smp #5
smp #6
smp #7
smp #8
smp #9
smp #A
smp #B
smp #C
smp #D
smp #E
smp #F
16 bit
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