The configuration of the crossbar (which input lane is connected with which output lane) is stored in a local configuration memory. Per output lane the input lane is stored plus an activation bit. The configuration memory size is 5x20 = 100 bits. To minimize energy consumption the circuit switching has fully separated data and control paths. Because a data packet cannot include routing information, we cannot serve best effort traffic. We configure the configuration memory via a small additional interface. Configuration of 1 lane requires 10 bits that are generated by the CCN based on the optimal applications mapping [3]. The configuration interface is connected to the separate BE network. We aim to transport the reconfiguration data in less than 1 ms over the BE network, because the configuration of the crossbar will not change frequently due to the long-life data streams between tiles. One single router can than be fully reconfigured within 20 ms.
5.2. Data and Flow Control
The circuit-switched network can handle synchronization of information in the data-packets. To enable this we included a small four bits header with every data-word. The header is combined with a 16-bit data-word of the tile. The result is a packet of 5x4 bits, which can be transported over a lane. The organization of this 20 bit packet is given in Fig. 6.
With only a four bit forward lane from source to destination and no feedback, we have to assume the destination can consume the data. In this case we do not support end-to-end flow control.