Editing Microcontroller (section)

=== Programs ===
Typically microcontroller programs must fit in the available on-chip memory, since it would be costly to provide a system with external, expandable memory. Compilers and assemblers are used to convert both [[High-level programming language|high-level]] and [[assembly language]] code into a compact [[machine code]] for storage in the microcontroller's memory. Depending on the device, the program memory may be permanent, [[read-only memory]] that can only be programmed at the factory, or it may be field-alterable [[Flash memory|flash]] or erasable read-only memory.

Manufacturers have often produced special versions of their microcontrollers in order to help the hardware and [[software development]] of the target system. Originally these included [[EPROM]] versions that have a "window" on the top of the device through which program memory can be erased by [[ultraviolet]] light, ready for reprogramming after a programming ("burn") and test cycle. Since 1998, EPROM versions are rare and have been replaced by [[EEPROM]] and flash, which are easier to use (can be erased electronically) and cheaper to manufacture.

Other versions may be available where the ROM is accessed as an external device rather than as internal memory, however these are becoming rare due to the widespread availability of cheap microcontroller programmers.

The use of field-programmable devices on a microcontroller may allow field update of the [[firmware]] or permit late factory revisions to products that have been assembled but not yet shipped. Programmable memory also reduces the lead time required for deployment of a new product.

Where hundreds of thousands of identical devices are required, using parts programmed at the time of manufacture can be economical. These "[[mask ROM|mask-programmed]]" parts have the program laid down in the same way as the logic of the chip, at the same time.

A customized microcontroller incorporates a block of digital logic that can be personalized for additional processing capability, [[peripherals]] and [[Interface (computer science)|interfaces]] that are adapted to the requirements of the application. One example is the [[AT91CAP]] from [[Atmel]].