Port Arbitration Units (PAUs)
12-11
Communication Ports
12.4 Port Arbitration Units (PAUs)
The PAU arbitrates between two devices to determine which device has pos-
session of the communication port data bus at any given time. This arbitration
uses CREQ and CACK signals to pass the bus ownership token back and forth
between two devices connected via their communication ports. Token transfer
operation is covered in detail in Section 12.7, Token Transfer Operation
After system reset, half of the communication channels associated with a par-
ticular ’C4x have token ownership (communication ports 0, 1, 2), and the other
half (communication ports 3, 4, 5) do not.
The PAU is a synchronous state machine with four states, as shown in
Table 12–2. These states are not software-accessible by the CPU or the DMA
coprocessor.
Table 12–2.PAU State Definitions
PAU State
Summary
PAU Status
State 0:
Idle with token
1.PAU has token (PORT DIR = 0).
2.Channel not in use.
The PAU currently has possession of the bus own-
ership token, and its associated communication
channel is not in use. Under this condition, the
PORT DIR bit of the associated CPCR is 0
(output). This is the state of communication ports
0, 1, and 2 after system reset.
State 1:
Idle without
token
1.PAU does not have token
(PORT DIR = 1).
2.Token not requested by PAU
(OUTPUT LEVEL = 0).
The PAU currently does not have possession of
the bus ownership token and has not requested
the token. Under this condition, the PORT DIR bit
equals 1 (input), and the OUTPUT LEVEL field
equals 0 (empty output FIFO). This is the state of
communication ports 3, 4, and 5 after system re-
set.
State 2:
Active
1.PAU has token (PORT DIR= 0).
2.Channel is in use (OUTPUT
LEVEL
≠
0).
The PAU currently has possession of the bus own-
ership token, and its associated communication
channel is in use. Under this condition, the PORT
DIR bit equals 0 (output), and the OUTPUT LEVEL
field does notequal 0).
State 3:
Waiting for
token
1.PAU does not have token
(PORT DIR = 1).
2.Token requested by PAU
(OUTPUT LEVEL
≠
0).
The PAU currently does not have the bus owner-
ship token but has requested it. Under this condi-
tion, the PORT DIR bit equals 1 (input), and the
OUTPUT LEVEL field does notequal 0.