TMS320C4X Datasheet, PDF (370/726 Pages) Texas Instruments – Digital Signal Processing Solutions

Port Arbitration Units (PAUs)
If PAU B receives a request from its output buffer to use the bus, it activates
CREQ to request the token from PAU A. PAU A detects this request via the
state variable TOKRQ=1 and then activates the CACK line to transfer the bus
ownership token to PAU B. PAU B then generates an internal bus acknowledge
(BUSAK = 1) to indicate that it has gained bus ownership. As a result of this
token transfer operation, PAU A enters state 1 (idle without token), and PAU
B starts the word transfer and enters state 2 (active).
To prevent any communication port from monopolizing the communication-
port bus, the PAU always returns to state 0 (idle with token) and checks for a
token request (CREQ active) from the external device after each word transfer.
If the token request is active, the token is passed to the requesting device so
that it can transmit a word. As long as ’C4x A and ’C4x B have information to
send in their output FIFOs, they alternate use of the data bus to provide a bi-
directional data path.
If a token request is received at the end of a word transfer and the sender ’C4x
has another word in the output FIFO to send, two situations can occur:
- If the CREQ going low signal is received before CRDY low is received for
the last byte, the sender ’C4x releases the token at the end of the current
word transfer.
- If the CREQ going low signal is received after or at the same time as CRDY
goes low from the last byte, the sender ’C4x continues owning the token;
only after transferring the next word, will it release token ownership.
In summary, token transfer occurs only on word boundaries. The ’C4x will not
release the token until the transfer of the four bytes completes.
Communication Ports
12-13