Transmission Error Detection - Analog Devices ADSP-2106x SHARC User Manual

9.7

TRANSMISSION ERROR DETECTION

Transmission errors on the link ports may be detected by reading the
LRERRx bits in the LCOM register. These bits reflects the status of each
nibble counter. The LRERRx bit will be zero if the pack counter of the
corresponding link buffer is zero (i.e. a multiple of 8 or 12 nibbles have
been received). If LRERR is high when a transmission has completed,
then an error occurred during transmission. (Note that the DMA word
count provides an exact count of the number of words to be
transferred.)
To allow checking of this status, the transmitter and receiver should
follow a protocol such as the one described below:
Transmitter Protocol—To make use of the LRERRx status, one
additional dummy word should always be transmitted at the end of a
block transmission. The transmitter must then deselect the link port to
allow the receiver to send an appropriate message back to the
transmitter.
Receiver Protocol—When the receiver has received the data block,
indicated by a DMA done interrupt, it checks that it has received an
additional word in the link buffer and then reads the LRERR bit. The
receiver may then clear the link buffer (LxEN=0) and transmit the
appropriate message back to the transmitter on the same, or a
different, link port.
9.8 TOKEN PASSING
Two processors wishing to communicate on a link port need to know
which of them is currently the transmitter and which is the receiver,
otherwise they might both try to transmit at the same time. Token
passing is a way of accomplishing this. Figure 9.7 shows a flow chart of
the token passing process.
Token passing is a way of establishing which of two ADSP-2106xs is
the current transmitter. The token is a software flag, similar to a
semaphore, that is passed between the processors. At reset, by
convention, the token (flag) is set to reside in the link port of one
device, making him the master and the transmitter. When a receiver
link port (slave) wants to become the master, he may assert his LxACK
line (request data) to get the master's attention. The master will know,
through software protocol, whether he is supposed to respond with
actual data or whether he is being asked for the token.
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Link Ports 9
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