Network-on-Chip¶

In OpTiMSoC we ship our own, minimalistic network-on-chip. It is a plain, straight-forward implementation with the following features:

wormhole-based: Packets are preserved in the network
dimension-routing: Deadlock free XY-routing
strict ordering: Packets in one stream leave the network in the same order as they entered it
configurable: Easily create your own topology from the building blocks

In this part of the documentation you can learn more about the network-on-chip.

Transmission & Packet Format¶

The packet width is parameterized in the network-on-chip, but in OpTiMSoC we currently rely on 32-bit links. This may change in the future, but is abstracted in the network-on-chip. The transmission protocol is a very simple handshake signal with the following signals:

valid signals from the master that it currently assigns valid data.
ready signals from the slave that it can accept a data item in this cycle.
flit is the actual data and assigned by the master.
last signals from the master that this is the last flit.

Packets are formed by multiple flits and the last bit marks the end of a packet. The network-on-chip is agnostic to the packet content, except for the first flit in a packet: the header. The first 5 bits are the destination of this packet and those bits are used to compute the output port at each hop.

Beside the destination two extra fields are standardized in OpTiMSoC as depicted in the following table:

Bit(s)	Field	Description
31:27	`DEST`	Packet Destination The ID of this packet’s destination network-on-chip port.
26:24	`CLASS`	Packet Class Packet classes are used to differentiate between different services in the network-on-chip. Services are commonly associated with a specific module in the network adapter of the tiles.
23:19	`SRC`	Packet Source The ID of this packet’s source network-on-chip port.
18:0	`...`	Class specific The remaining part of the header is used by the classes.

Network-on-Chip Router¶

Fig. 1 The network-on-chip router¶

Figure 1 sketches the anatomy of the basic network-on-chip router: It has a configurable number of input ports and output ports (that don’t have to be the same). An input port stores incoming flits into a buffer, then a route lookup module determines the output port based on the destination of a packet and forwards it to the switch with that routing information. The output ports get the requests from the input ports and an arbiter decides which input port to serve, forwarding the packets to an output buffer then.

Virtual Channels¶

Virtual channels are a bit hard to grasp if you are new to networks-on-chip. First of all, you need to understand the concept of channels: Channels are independent links between routers in networks-on-chip. As we use so called strict ordering, the channel number of the output port link is the same as the input port link. Hence, this notion does not really exist in our network-on-chip. Having two channels actually means having two physical networks.

Channels are useful to avoid so called message-dependent deadlocks and to divert different traffic classes in general. As the point-to-point links between routers have historically been more costy in an ASIC, the idea of virtual channels is to share the links between two routers virtually. So, instead of having two networks they overlap on the links. This means that the inner parts of the router are replicated, but with some extra logic to share the links on input and output.

Configuration¶

The router has the following configuration parameters:

Name	Type	Description
`FLIT_WIDTH`	integer	Width of the flit data Number of bits in each data item during a network-on-chip transfer.
`VCHANNELS`	integer	Number of virtual channels
`INPUTS`	integer	Number of input ports
`OUTPUTS`	integer	Number of output ports
`DESTS`	integer	Number of destinations in network
`ROUTES`	`[OUTPUTS*DESTS-1:0]` bits	Output destinations This is a flat field of bitvectors, with one entry for each possible destination. Each of the bitvectors is a one-hot encoding of the output to route the packet for. Be careful with ordering when assigning to it (LSB is index 0).

Instantiating a Network-on-Chip¶

Actually, you will rarely build the network-on-chip out of the routers. Instead you will use a topology toplevel to generate the router layout for you. The topology we use by default is a mesh.

Mesh¶

The module noc_mesh is instantiated to build a regular two-dimensional mesh. It has the following configuration parameters:

Name	Type	Description
`FLIT_WIDTH`	integer	Width of the flit data Number of bits in each data item during a network-on-chip transfer.
`CHANNELS`	integer	Number of channels This is the number of independent channels in the network. The parameter `ENABLE_VCHANNELS` switches between physical and virtual channels.
`ENABLE_VCHANNELS`	1 bit	Activate virtual channels Setting this to `1` will activate link-sharing between the routers in the mesh, while setting it to `0` will instantiate as many networks as configured by `CHANNELS`.
`X`, `Y`	integer	Dimensions of the mesh