LIS Network-on-Chip Implementation

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Router

lisnoc_router is the basic router implementation. It has the following basic parameters:

  • FLIT_DATA_WIDTH is the actual data transfer width per flit. Default: 32
  • FLIT_TYPE_WIDTH is the extra bits for packet control. The default 2 should not be overwritten at the moment
  • VCHANNELS is the number of virtual channels. Default: 1 that is no virtual channels.
  • IN_FIFO_LENGTH and OUT_FIFO_LENGTH define the number of flits that the buffers at input and output port can hold. Default: 4

The number of input and output ports can be configured with INPUT_PORTS and OUTPUT_PORTS. Both of them default to 5 (mesh router).

Beside this the overall NoC parameters need to be set:

  • NUM_DESTS is the number of destinations. Default: 32
  • PH_DEST_WIDTH is the width of the destination field in the header flit. It must match NUM_DESTS. Default: 5

The output port lookup for packets is defined with the LOOKUP parameter which has the width NUM_DESTS*OUTPUT_PORTS. For each destination it has a one-hot field that selects the output port. Such a lookup “table” is of course not a table, but usually synthesized to a combinational function by your synthesis tool. As the LOOKUP is most easy configured with concattenation, so that the order of ports is reversed internally.

The router also allows for priorization of packets, meaning that when two packets want to use the same output buffer it always chooses the one with the higher priority (contrary to the default policy round robin). Priorities are enabled with setting USE_PRIO to 1 and then the next PH_PRIO_WIDTH bits after the PH_DEST_WIDTH bits are used to define priorities. You should be careful when using priorities.

Router Overview

The following sketch shows an overview of the router with INPUT_PORTS number of input stages and OUTPUT_PORTS number of output stages. Each input stage is a router and the lookup function. The switch can switch at maximum OUTPUT_PORTS*VCHANNELS flits er cycle, which is the maximum. The output stages contain the output port arbitration. The flits are then buffered and the output arbiter selects the virtual channel to transfer next.

Router

On the outside there are flat signals with ordering port number, then virtual channel. For example a router with two input ports and two output ports and two vitual channels is connected like:

[...]
.in_valid({in_link_1_valid, in_link_0_valid}),
.in_ready({in_link_1_ready, in_link_0_ready})
.in_flit({in_link_1_flit, in_link_0_flit}),

.out_valid({out_link_1_valid, out_link_0_valid}),
.out_ready({out_link_1_ready, out_link_0_ready})
.out_flit({out_link_1_flit, out_link_0_flit}),
[...]

where the signals are defined like

wire [VCHANNELS-1:0] in_link_0_valid;
wire [VCHANNELS-1:0] in_link_0_ready;
wire [FLIT_WIDTH-1:0] in_link_0_flit;