Understanding Smart Sensors - Nomads.usp

98 Understanding Smart Sensorsinstalled. EPROM can be changed by erasing the contents with an ultravioletlight and then reprogramming new values. That nonvolatile memory has a limitednumber of erasure and reprogramming operations. One-time programmable(OTP) ROM is the same as EPROM except that it is packaged in a lowercost opaque package. Because ultraviolet light cannot penetrate the package,this memory cannot be erased after it is programmed. EEPROM is a nonvolatilememory that can be changed by using electrical signals. Typically anEEPROM location can be erased and reprogrammed thousands of times beforeit wears out. One of the newest types of memory is flash memory. Flash memoryis nonvolatile memory that is easily reprogrammed in the application fasterthan EEPROM. Once it is programmed, flash memory contents remain intactuntil software initiates an erase cycle. Program and erase voltages for EEPROMand flash memory are performed at approximately 12V. A next-generationmemory still in development is ferroelectric random access memory (FeRAMor FRAM). The ferroelectric effect is the tendency of dipoles within a crystal toalign in the presence of an electric field and to remain polarized after the field isremoved [4]. Reversing the field causes polarization in the opposite direction.No current is required to maintain either state providing a binary memorycapacitor with low power consumption. FeRAM for sensing applications hasseveral advantages:• Very fast write times (up to 20 times faster than EEPROM);• Write/erase (W/E) endurance up to 10 million times greater thanEEPROM;• Arrays up to eight times larger than prior versions;• Lower voltage, lower power operation to conserve battery life.Table 5.1 is a comparison of five different types of memory [4].5.3.3 Input/OutputI/O is a special type of memory that senses or changes based on external digitalelements and not the CPU [3]. I/O ports connect the external elements to theCPU and provide control capability for the system. I/O can be either parallel,transferring 8 bits at a time to the MCU, or serial, transferring data 1 bit at atime.General purpose I/O connections (pins) can be used as either an input oran output. A number of pins are typically grouped together and called a port.The program determines the function of each pin. Program instructions