5.4 Design overview
with a frequency
= 26.875 MHz.
The multipliers and divisors are programmable and can be controlled in the firmware. The values chosen ensure that the image sensor is clocked as close to its maximum clock rate as is possible.
Communications with the hub
number of camera modules can be connected to the central hub by a shared communica- tions bus. The bus transceivers are standard Universal synchronous Receiver/Transmitters (U RTs) with 3.3 V CMOS logic levels. The communications protocol and timing are de-
scribed in Chapter 6 and only the hardware aspects are noted here. The bus is shared in such a way that data transmitted by the hub is received by all of the camera modules. Data sent by any camera module is only received by the hub but the transmission line is shared by all camera modules. It is possible for any camera module to drive the bus at any time and this means a mechanism is required to avoid collisions on the bus when data is sent to the hub. Unfortunately the Blackfin U RT transmit line cannot enter a high impedance mode and therefore drives the bus continuously. This problem is overcome by the bus sharing circuit.
The hardware side of this mechanism protects the camera modules and stops excessively high currents flowing. It is a simple arrangement using a Shottky diode and resistor (Figure 5.8) and enables camera modules to enter a high impedance mode when not driving the bus. The resistor is used to pull the bus to 3.3 V and the Shottky diodes are in series with the Blackfins’ transmit lines. When a Black Spot module sets a logic zero the diode is forward biased and current flows pulling the bus low. In this arrangement if any module drives a logic zero then the bus registers a zero. When a module sets a logic one the diode is no longer forward biased and the bus sits at 3.3 V due to the pull-up resistor.
The communications header is shown in Figure 5.9. It is a standard 0.1 inch pitch 5 by 2 pin header. The header provides the Black Spot module with power (pin 9), communi- cations with the hub (pins 5, 7), bus in use signal (pin 3), and flow control (pin 1). The remaining pins are grounded. Pin 2 is used as a key and is removed from the header. The corresponding plug has the hole that corresponds to pin 2 filled and this assures that the plug is inserted in the correct orientation.
The printed circuit board was routed using four layers. This was done for two reasons. Firstly, the greater the number of layers that a board has, the easier it is to route. Secondly