Editing Microcontroller (section)

== Memory technology ==
Two different kinds of memory are commonly used with microcontrollers, a [[non-volatile memory]] for storing firmware and a [[read–write memory]] for temporary data.

=== Data ===
From the earliest microcontrollers to today, six-transistor SRAM is almost always used as the read/write working memory, with a few more transistors per bit used in the [[register file]].

In addition to the SRAM, some microcontrollers also have internal EEPROM and/or [[NVRAM]] for data storage; and ones that do not have any (such as the [[BASIC Stamp]]), or where the internal memory is insufficient, are often connected to an external EEPROM or flash memory chip.

A few microcontrollers beginning in 2003 have "self-programmable" flash memory.<ref name="flash"/>

=== Firmware ===
The earliest microcontrollers used mask ROM to store [[firmware]]. Later microcontrollers (such as the early versions of the [[Freescale 68HC11]] and early [[PIC microcontroller]]s) had [[EPROM]] memory, which used a translucent window to allow erasure via UV light, while production versions had no such window, being OTP (one-time-programmable). Firmware updates were equivalent to replacing the microcontroller itself, thus many products were not upgradeable.

Motorola [[Motorola 68HC05|MC68HC805]]<ref name="mc68ch805ref"/> was the first microcontroller to use [[EEPROM]] to store the firmware. EEPROM microcontrollers became more popular in 1993 when Microchip introduced [[PIC16x84|PIC16C84]]<ref name="pic16c84ref"/> and Atmel introduced an [[Intel MCS-51|8051-core]] microcontroller that was first one to use [[flash memory|NOR Flash memory]] to store the firmware.<ref name="flash"/> Today's microcontrollers almost all use flash memory, with a few models using FRAM and some ultra-low-cost parts still using OTP or Mask ROM.