communication medium is assumed to have a negligibly small error rate and error cor- recting codes are not used. The protocol should be extensible in its ability to transport new types of data with few changes required. Provided sufficient bandwidth is available, the protocol should be human readable to allow the system to be debugged easily using a terminal program. The physical layer and transport layer are described in the next section.
The physical layer is a three wire serial bus using U RTs as shown in Figure 6.1. From left to right the figure shows the Soft Hub connected via USB to a bridge chip. This chip converts the USB data stream to a standard U RT stream. This is shared by the modules connected to the bus. The three bus wires are a transmit line (Tx), a receive line (Rx), and a flow control line (CTS). These provide full duplex communications between the cam- era modules and the Soft Hub. The U RTs are built into each of the Blackfin DSP chips and into the FT2232C  USB bridge chip from Future Technology Devices International (FTDI) Ltd. The bridge chip provides a bandwidth limited to approximately 1 M baud  (921600 baud by the PC drivers). Using the common configuration of 8 data bits, no parity bits, and 1 stop bit (8N1), this gives a bandwidth of
= 90 K bytes/s.
The Blackfin U RTs are limited to the system clock divided by 16 . The Black Spot system clock is SCLK = 107.5 MHz (see Equation 5.9). Therefore, the maximum baud rate is SCLK /16 ≈ 6.7 MBaud. Using a 8N1 configuration, this gives
SCLK 16 × 10
= 688 K bytes/s,
so a system utilising a Blackfin chip for the Hub could achieve a much higher throughput.
The communications channel is full duplex meaning that the data from the Soft Hub does not share the same path as the data directed to the Soft Hub. ny transmission from the bridge chip is received by all camera modules. Each module may drive the shared data path to the Soft Hub and this makes it necessary to have some way to control which module may use the bus at any given time. This is required to avoid collisions when two modules attempt to use the bus at the same time, otherwise the data that they are transmitting will be lost. In this scheme the camera modules are directed by the Soft Hub when they may
use the bus. This is described in more detail in the following bus arbitration section.