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In Ethernet module , why do we need FIFO block before the mac layer ? Can we bypass it ? Thanx in advance

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The FIFO reduces the number of interrupts the attached processor has to service. In theory you could bypass it, but if you do so, the microprocessor will typically waste a lot of time on the overhead of interrupt servicing.

For a quick analogy, let's assume you were asked to move 40 gallons of water 20 miles. The FIFO is roughly equivalent to a truck with a 20 gallon water tank, so you make only two trips to get all the water to the destination. Without the FIFO, you can only carry one water bottle at a time, so you'd have to make around 500 trips back and forth instead.

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  • \$\begingroup\$ Your analogy is pretty good but does lack an important detail. Another part of the FIFO idea is that trip to carry the 1 gallon jugs may take a longer time than the rate that the gallons of water are required at the destination. It is also a more efficient to load up a 20 gallon tank in one go instead of having to fill a set of individual 1 gallon jugs. Back to the software world the FIFO can be loaded in full (or in part) more efficiently if a block is loaded at one go. The FIFO also also has delivers the data to the destination at the rate needed. \$\endgroup\$ – Michael Karas Nov 7 '14 at 18:48
  • \$\begingroup\$ Am making a processor on FPGA which processes incoming packets.The example design given by Xilinx has FIFO block. The FIFO block is delaying the incoming packets. Can I drop the FIFO module and speed ma design ? \$\endgroup\$ – Nijil Kadavathu Valapil Nov 7 '14 at 19:48
  • \$\begingroup\$ @NijilKadavathuValapil: removing the FIFO will reduce latency, but (at least usually) at the expense of substantially increasing processor overhead. \$\endgroup\$ – Jerry Coffin Nov 7 '14 at 19:49
  • \$\begingroup\$ My Ethernet speed is 1Gbps and ma system clock is 125 MHz.So system is getting one byte in one clock cycle.Do I still need a FIFO ? .I am confused. \$\endgroup\$ – Nijil Kadavathu Valapil Nov 7 '14 at 19:59
  • \$\begingroup\$ @NijilKadavathuValapil - think about what processor instructions are required to transfer a byte from the MAC to memory, plus the instructions for running that in a loop, and then compute how many processor cycles those instructions require. I'll give really good odds that is much more than 1 cycle! Also, if you miss even once (maybe you need to service an interrupt), you have an overrun and will lose bytes! \$\endgroup\$ – DoxyLover Nov 7 '14 at 20:16

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