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TDM Interface
19.2.5 TDM Local Memory
Received data is temporarily stored in the TDM receive local memory until it is transferred to the
receive buffers mapped on the system I/F. A single data transfer from TDM local memory to a
memory-mapped region on the system I/F transfers at least 64 bits of data. Each channel can store
more than 64 bits before the data is written to the buffers mapped on the system I/F. The
TDMxRFP[RCDBL] field (see page 19-48) provides an upper boundary on the number of
receive bits that can be stored in the TDM local memory. The receive data latency is defined as
the time between receiving data and the time when it is available for processing by an SC3400
core. Reducing the TDMxRFP[RCDBL] value reduces the receive data latency. However,
reading the TDM local memory imposes more strict latency requirements on the system I/F. The
maximum receive data latency is calculated as follows: RCDBL /RCS × receive frame time.
When the amount of received data exceeds the size of the TDM receive local memory, the
TDMxRER[OLBE] bit is set (see page 19-69). If the TDMxRIER[OLBEE] bit is also set, an
error interrupt is generated. This error should not occur during normal operation. It indicates that
the TDM has not received enough bandwidth on the system I/F and therefore cannot write the
data into the destination memory (the data buffer).
Data transmitted from memories mapped on the system I/F is temporarily stored in the TDM
transmit local memory until it is transferred externally. A single data transfer from the system I/F
to TDM local memory transfers at least 64 bits of data. Each channel can store more than 64 bits
before it is transmitted externally. The TDMxTFP[TCDBL] field provides an upper boundary on
the number of transmit bits that can be stored in TDM local memory. The transmit data latency is
defined as the time between when the data is read from the buffers mapped to the system I/F and
when it is transmitted externally. Reducing the TDMxTFP[TCDBL] value reduces the transmit
data latency. However, writing the TDM local memory imposes more strict latency requirements
on the internal MBus. The maximum transmit data latency is calculated as follows: TCDBL /
TCS × transmit frame time.
When the TDM cannot transfer data from data buffers to TDM local memory, an underrun
occurs. When the TDM transmit local memory is empty, the TDMxTER[ULBE] bit (see
page 19-70) is set and the TDMxTIER[ULBEE] bit is also set, an error interrupt is generated.
This error should not occur during normal operation. It indicates that the TDM has not received
enough bandwidth on the system I/F and therefore cannot read the data from the source memory
into TDM transmit local memory. The minimum latency is achieved when the RCDBL/TCDBL
field is clear (only 64 bits are stored in the TDM local memory). For example, the minimum
latency for a T1 application with 8 bits per channel and a frame length of 125 μs is equal to 1 ms.
T1 minimum latency= 64/8 × 125 μs.
MSC8144E Reference Manual, Rev. 3
19-22
Freescale Semiconductor
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