CCD and CMOS/CCD

 

CCD processes have originally been developed for analog signal processing and imaging devices. Although this may have facilitated the design of vision chips, due to their drawbacks there has been limited success in achieving functional and reliable vision chips. Major drawbacks of CCD and CCD/CMOS with respect to CMOS are:

• Clocking: To perform even simple operations large number of clock phases are required, these clock phases should be distributed to all cells
• Process optimization: Special CCD processes do not have optimized CMOS devices and CCD/CMOS processes do not have optimized CCD structures
• Special read and write circuit: For transferring signals between CMOS and CCD parts in a CCD/CMOS circuit special read/write circuits are necessary
• Large area: Occupying large area per cell due to the above items
• Digital noise: Massive clock-induced-noise to analog circuits in mixed CCD/CMOS approach
• Power: Power consumption due to large voltage transients required for clocking the gates of CCD structures (large capacitive loads)

Despite these numerous drawbacks, CCDs offer easier solutions for some operations. For example, in a smoothing CCD vision chip the smoothing width can be increased by only leaving the circuit to operate over more clock cycles. In other words, CCDs are capable of iterating a function without demands on additional space.