TMS320C4X Datasheet, PDF (363/726 Pages) Texas Instruments – Digital Signal Processing Solutions

Operational Overview
The signals CREQx and CACKx handle the handshaking arbitration between
the two DSPs in two steps:
1) The PAU that does not own the data bus (CxD(7–0)) activates CREQx to
request bus ownership.
2) The PAU owning the bus then activates CACKx to acknowledge the re-
quest and relinquish bus ownership to the requesting PAU.
In this manner, these signals transfer a token (or priority) from one PAU to an-
other, and the PAU receiving the token gains ownership of the bus. See Sec-
tion 12.7, Token Transfer Operation, for a detailed description of token trans-
fer.
12.2.2 Data Transfer Operation
A data transfer operation takes four basic steps to complete:
1) The CPU or DMA coprocessor of the sending DSP writes a 32-bit data
word to the output FIFO (of a communication port) via a memory-mapped
address (listed in Figure 12–3).
2) The communication port then places the 32-bit data word on CxD(7–0) on
a byte-to-byte basis (LS byte first), activating CSTRBx to signal the receiv-
ing communication port that the bus contains a valid data byte.
3) Upon receiving each data byte, the receiving communication port acti-
vates CRDYx to indicate that it has received the data byte.
4) After receiving the 4 bytes of a 32-bit word, the CPU or DMA coprocessor
of the receiving DSP can then read the data from the input FIFO via a
memory-mapped address (listed in Figure 12–3).
Each of the input and output FIFOs can buffer a maximum of eight 32-bit
words.
Buffering provided by the input and output FIFOs is essential. This buffering
allows for a high degree of decoupling of computation and communication
overhead. When ’C4xs A and B are connected via their communication ports,
the effective length of the FIFOs becomes 16 levels. This occurs because the
output path from A to B is the concatenation of the eight levels of the output
FIFO of A with the eight levels of the input FIFO of B. This also applies for the
output path from B to A.
12-6