Example Token Passing - Analog Devices SHARC ADSP-214 Series Hardware Reference Manual

Example Token Passing

When two ADSP-2146x processors communicate using a link port only
one can be the transmitter or receiver. Token passing is a protocol that
assists the DSPs alternate control.
token passing process.
In token passing, the token is a software flag that passes between the pro-
cessors. At reset, the token (flag) is set to reside in the link port of one
device, making it the master and the transmitter. When a receiver link
port (slave) wants to become the master, it may assert its
(request data) to get the master's attention. The master knows, through
software protocol, whether it is supposed to respond with actual data or
whether it is being asked for the token.
The token release word can be any user-defined value. Since both the
transmitter and receiver are expecting a code word, this does not need to
be exclusive of normal data transmission.
If the master wishes to give up the token, it may send back a user-defined
token release word and thereafter clear its token flag. Simultaneously, the
slave examines the data sent back and if it is the token release word, the
slave will set its token, and can thereafter transmit. If the received data is
not the token release word, then the slave must assume the master was
beginning a new transmission.
Through software protocol, the master can also request data by sending
the token release word (TRW) without the
first.
ADSP-214xx SHARC Processor Hardware Reference
www.BDTIC.com/ADI
Link Ports—ADSP-2146x
Figure 4-8 shows a flow chart of the
(data request) going low
LACKx
line
LACKx
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