5.4 Design overview
TWI data TWI clock
/ 8 b i t i m a g e d a t a P P I C l o c k Line Valid Frame Valid
MT9V023 CMOS Sensor
Figure 5.7 The Blackfin DSP is connected to the image sensor by an image data bus and control signals and a two wire data bus for control of the sensor by the DSP.
Figure 5.7 shows the Parallel Peripheral Interface (PPI) bus and TWI bus connecting the
image sensor and Blackfin in more detail. The parallel buses five timing signals are the sensor clock, PPI clock, line valid, frame valid, and exposure signals. The sensor clock is the main clock for the sensor. This signal dictates the maximum frame rate of the sensor. It is 26.6 MHz for the maximum 60 fps that the sensor supports. Lower frequencies lead to lower frame rates. This signal is controlled by a line output from the Blackfin and the frame rate can be set by the DSP.
The timing signals are used to synchronise the image frames with the Blackfin. The PPI clock is the return signal from the image sensor and is derived from the sensor clock signal. This is used to clock the parallel peripheral interface on the Blackfin. The line valid and frame valid signals instruct the PPI port as to when the bus contains valid image data. This
is used internally by the Blackfin’s DM
controller to copy image data to memory.
The TWI bus is a multi-drop bus compatible with the Inter-Integrated Circuit (IIC) bus from NXP Semiconductors  (formerly Philips Semiconductors). The bus is half-duplex and the data lines and clock are shared between upstream and downstream devices. The bus is based around a master and slave configuration. In this application, the Blackfin is the bus master and the image sensor is the slave. The slave drives the bus only when the master has requested to read data from the slave or when the master is writing to the slave and the slave is acknowledging the written data. The BF533 does not have a hardware TWI so the TWI lines are connected to general purpose I/O lines and the interface is implemented in software.
Flash chip interface
The Blackfin processor is connected to a Serial Peripheral Interface (SPI) flash memory
chip. The serial flash chip is used to store program code so that the camera module can run without a PC attached. The chip is programmed using software running on a PC and a hardware JT G debugger interface. In theory it is also possible to program the flash via