Microcontroller

Background

The first multi-chip microprocessors, the Four-Phase Systems AL1 in 1969 and the Garrett AiResearch MP944 in 1970, were developed with multiple MOS LSI chips. The first single-chip microprocessor was the Intel 4004, released on a single MOS LSI chip in 1971. It was developed by Federico Faggin, using his silicon-gate MOS technology, along with Intel engineers Marcian Hoff and Stan Mazor, and Busicom engineer Masatoshi Shima.^[2] It was followed by the 4-bit Intel 4040, the 8-bit Intel 8008, and the 8-bit Intel 8080. All of these processors required several external chips to implement a working system, including memory and peripheral interface chips. As a result, the total system cost was several hundred (1970s US) dollars, making it impossible to economically computerize small appliances.

MOS Technology introduced its sub-$100 microprocessors in 1975, the 6501 and 6502. Their chief aim was to reduce this cost barrier but these microprocessors still required external support, memory, and peripheral chips which kept the total system cost in the hundreds of dollars.

Development

One book credits TI engineers Gary Boone and Michael Cochran with the successful creation of the first microcontroller in 1971. The result of their work was the TMS 1000, which became commercially available in 1974. It combined read-only memory, read/write memory, processor and clock on one chip and was targeted at embedded systems.^[3]

During the early-to-mid-1970s, Japanese electronics manufacturers began producing microcontrollers for automobiles, including 4-bit MCUs for in-car entertainment, automatic wipers, electronic locks, and dashboard, and 8-bit MCUs for engine control.^[4]

Partly in response to the existence of the single-chip TMS 1000,^[5] Intel developed a computer system on a chip optimized for control applications, the Intel 8048, with commercial parts first shipping in 1977.^[5] It combined RAM and ROM on the same chip with a microprocessor. Among numerous applications, this chip would eventually find its way into over one billion PC keyboards. At that time Intel's President, Luke J. Valenter, stated that the microcontroller was one of the most successful products in the company's history, and he expanded the microcontroller division's budget by over 25%.

Various PIC microcontrollers with
integrated EPROM

Piggyback microcontroller from MOSTEK

Most microcontrollers at this time had concurrent variants. One had EPROM program memory, with a transparent quartz window in the lid of the package to allow it to be erased by exposure to ultraviolet light. These erasable chips were often used for prototyping. The other variant was either a mask-programmed ROM or a PROM variant which was only programmable once. For the latter, sometimes the designation OTP was used, standing for "one-time programmable". In an OTP microcontroller, the PROM was usually of identical type as the EPROM, but the chip package had no quartz window; because there was no way to expose the EPROM to ultraviolet light, it could not be erased. Because the erasable versions required ceramic packages with quartz windows, they were significantly more expensive than the OTP versions, which could be made in lower-cost opaque plastic packages. For the erasable variants, quartz was required, instead of less expensive glass, for its transparency to ultraviolet light—to which glass is largely opaque—but the main cost differentiator was the ceramic package itself. Piggyback microcontrollers were also used.^[6][7][8]

In 1993, the introduction of EEPROM memory allowed microcontrollers (beginning with the Microchip PIC16C84)^[9] to be electrically erased quickly without an expensive package as required for EPROM, allowing both rapid prototyping, and in-system programming. (EEPROM technology had been available prior to this time,^[10] but the earlier EEPROM was more expensive and less durable, making it unsuitable for low-cost mass-produced microcontrollers.) The same year, Atmel introduced the first microcontroller using Flash memory, a special type of EEPROM.^[11] Other companies rapidly followed suit, with both memory types.

Nowadays microcontrollers are cheap and readily available for hobbyists, with large online communities around certain processors.

Volume and cost

In 2002, about 55% of all CPUs sold in the world were 8-bit microcontrollers and microprocessors.^[12]

Over two billion 8-bit microcontrollers were sold in 1997,^[13] and according to Semico, over four billion 8-bit microcontrollers were sold in 2006.^[14] More recently, Semico has claimed the MCU market grew 36.5% in 2010 and 12% in 2011.^[15]

A typical home in a developed country is likely to have only four general-purpose microprocessors but around three dozen microcontrollers. A typical mid-range automobile has about 30 microcontrollers. They can also be found in many electrical devices such as washing machines, microwave ovens, and telephones.

Historically, the 8-bit segment has dominated the MCU market [..] 16-bit microcontrollers became the largest volume MCU category in 2011, overtaking 8-bit devices for the first time that year [..] IC Insights believes the makeup of the MCU market will undergo substantial changes in the next five years with 32-bit devices steadily grabbing a greater share of sales and unit volumes. By 2017, 32-bit MCUs are expected to account for 55% of microcontroller sales [..] In terms of unit volumes, 32-bit MCUs are expected account for 38% of microcontroller shipments in 2017, while 16-bit devices will represent 34% of the total, and 4-/8-bit designs are forecast to be 28% of units sold that year. The 32-bit MCU market is expected to grow rapidly due to increasing demand for higher levels of precision in embedded-processing systems and the growth in connectivity using the Internet. [..] In the next few years, complex 32-bit MCUs are expected to account for over 25% of the processing power in vehicles.

Cost to manufacture can be under US$0.10 per unit.

Cost has plummeted over time, with the cheapest 8-bit microcontrollers being available for under US$0.03 in 2018,^[17] and some 32-bit microcontrollers around US$1 for similar quantities.

In 2012, following a global crisis—a worst ever annual sales decline and recovery and average sales price year-over-year plunging 17%—the biggest reduction since the 1980s—the average price for a microcontroller was US$0.88 (US$0.69 for 4-/8-bit, US$0.59 for 16-bit, US$1.76 for 32-bit).^[16]

In 2012, worldwide sales of 8-bit microcontrollers were around US$4 billion, while 4-bit microcontrollers also saw significant sales.^[18]

In 2015, 8-bit microcontrollers could be bought for US$0.311 (1,000 units),^[19] 16-bit for US$0.385 (1,000 units),^[20] and 32-bit for US$0.378 (1,000 units, but at US$0.35 for 5,000).^[21]

In 2018, 8-bit microcontrollers could be bought for US$0.03,^[17] 16-bit for US$0.393 (1,000 units, but at US$0.563 for 100 or US$0.349 for full reel of 2,000),^[22] and 32-bit for US$0.503 (1,000 units, but at US$0.466 for 5,000).^[23]

In 2018, the low-priced microcontrollers above from 2015 were all more expensive (with inflation calculated between 2018 and 2015 prices for those specific units) at: the 8-bit microcontroller could be bought for US$0.319 (1,000 units) or 2.6% higher,^[19] the 16-bit one for US$0.464 (1,000 units) or 21% higher,^[20] and the 32-bit one for US$0.503 (1,000 units, but at US$0.466 for 5,000) or 33% higher.^[21]

Smallest computer

On 21 June 2018, the "world's smallest computer" was announced by the University of Michigan. The device is a "0.04 mm³ 16 nW wireless and batteryless sensor system with integrated Cortex-M0+ processor and optical communication for cellular temperature measurement." It "measures just 0.3 mm to a side—dwarfed by a grain of rice. [...] In addition to the RAM and photovoltaics, the new computing devices have processors and wireless transmitters and receivers. Because they are too small to have conventional radio antennae, they receive and transmit data with visible light. A base station provides light for power and programming, and it receives the data."^[24] The device is 1⁄10th the size of IBM's previously claimed world-record-sized computer from months back in March 2018,^[25] which is "smaller than a grain of salt",^[26] has a million transistors, costs less than $0.10 to manufacture, and, combined with blockchain technology, is intended for logistics and "crypto-anchors"—digital fingerprint applications.^[27]

Interrupts

Microcontrollers must provide real-time (predictable, though not necessarily fast) response to events in the embedded system they are controlling. When certain events occur, an interrupt system can signal the processor to suspend processing the current instruction sequence and to begin an interrupt service routine (ISR, or "interrupt handler") which will perform any processing required based on the source of the interrupt, before returning to the original instruction sequence. Possible interrupt sources are device-dependent and often include events such as an internal timer overflow, completing an analog-to-digital conversion, a logic-level change on an input such as from a button being pressed, and data received on a communication link. Where power consumption is important as in battery devices, interrupts may also wake a microcontroller from a low-power sleep state where the processor is halted until required to do something by a peripheral event.

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 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 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-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 interfaces that are adapted to the requirements of the application. One example is the AT91CAP from Atmel.

Other microcontroller features

Microcontrollers usually contain from several to dozens of general purpose input/output pins (GPIO). GPIO pins are software configurable to either an input or an output state. When GPIO pins are configured to an input state, they are often used to read sensors or external signals. Configured to the output state, GPIO pins can drive external devices such as LEDs or motors, often indirectly, through external power electronics.

Many embedded systems need to read sensors that produce analog signals. However, because they are built to interpret and process digital data, i.e. 1s and 0s, they are not able to do anything with the analog signals that may be sent to it by a device. So, an analog-to-digital converter (ADC) is used to convert the incoming data into a form that the processor can recognize. A less common feature on some microcontrollers is a digital-to-analog converter (DAC) that allows the processor to output analog signals or voltage levels.

In addition to the converters, many embedded microprocessors include a variety of timers as well. One of the most common types of timers is the programmable interval timer (PIT). A PIT may either count down from some value to zero, or up to the capacity of the count register, overflowing to zero. Once it reaches zero, it sends an interrupt to the processor indicating that it has finished counting. This is useful for devices such as thermostats, which periodically test the temperature around them to see if they need to turn the air conditioner on/off, the heater on/off, etc.

A dedicated pulse-width modulation (PWM) block makes it possible for the CPU to control power converters, resistive loads, motors, etc., without using many CPU resources in tight timer loops.

A universal asynchronous receiver/transmitter (UART) block makes it possible to receive and transmit data over a serial line with very little load on the CPU. Dedicated on-chip hardware also often includes capabilities to communicate with other devices (chips) in digital formats such as Inter-Integrated Circuit (I²C), Serial Peripheral Interface (SPI), Universal Serial Bus (USB), and Ethernet.^[29]

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.^[11]

Firmware

The earliest microcontrollers used mask ROM to store firmware. Later microcontrollers (such as the early versions of the Freescale 68HC11 and early PIC microcontrollers) 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 MC68HC805^[10] was the first microcontroller to use EEPROM to store the firmware. EEPROM microcontrollers became more popular in 1993 when Microchip introduced PIC16C84^[9] and Atmel introduced an 8051-core microcontroller that was first one to use NOR Flash memory to store the firmware.^[11] 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.