MAC Repeater Functionality
The MAC devices in the nodes act as either the originator
of a packet or they can repeat the packet from one PHY to the other PHY.
The repeater function requires the use of a nibble wide
FIFO in each direction. The purpose of the FIFO is two-fold:
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- It isolates the clock domain of the receiver from the local
clock domain, which is also the transmitter clock.
- It allows slight differences in the
clock frequencies to be tolerated without losing data.
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The required depth of the FIFO depends upon the maximum
discrepancy between clock frequencies and the maximum packet size.
Since the frequency variation is +/- 100 PPM and the
maximum packet size is 1024 bytes, the FIFO must allow for (200/1,000,000)*(1024*8)
= 1.6 bits of variation in each packet. It is critical that the repeater
delay must be both consistent and known. The delay must be known in order
to correctly calculate SynqNet time slot assignments. Fluctuations in
repeater delay during cyclic operation may cause errors. Note that it
is not required that all nodes have identical delays.
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