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== History ==
Starting from the early 1990s, an important technological evolution in the architecture of the microcontrollers was witnessed. At first, they were realized in two possible solutions: with [[Programmable read-only memory|OTP (one-time programmable)]] or with [[EPROM|EPROM memories]]. In EPROM, memory-erasing process requires the chip to be exposed to ultraviolet light through a specific window above the package. In 1993 [[Microchip Technology]] introduced the first microcontroller with [[EEPROM|EEPROM memory]]: the PIC16C84. EEPROM memories can be electrically erased. This feature allowed to lower the realization costs by removing the erasing window above the package and initiate in-system programming technology. With ISP flashing process can be performed directly on the board at the end of the production process. This evolution gave the possibility to unify the programming and functional test phase and in production environments and to start the preliminary production of the boards even if the firmware development has not yet been completed. This way it was possible to correct bugs or to make changes at a later time. In the same year, [[Atmel]] developed the first microcontroller with flash memory, easier and faster to program and with much longer life cycle compared to EEPROM memories.
Microcontrollers that support ISP are usually provided with pins used by the serial communication peripheral to interface with the programmer, a flash/EEPROM memory and the circuitry used to supply the voltage necessary to program the microcontroller. The communication peripheral is in turn connected to a programming peripheral which provides commands to operate on the flash or EEPROM memory.
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