Microcontroller Embedded Memory Technology Information Technology Essay

Microcontroller Embedded Memory Technology Information Technology Essay A  microcontroller  is a small computer on a single  integrated circuit  containing a processor core, memory, and programmable  input/output  peripherals. Program memory in the form of  NOR flash  or  OTP ROM  is also often included on chip, as well as a typically small amount of  RAM. Microcontrollers are designed for embedded applications, in contrast to the  microprocessors  used inpersonal computers  or other general purpose applications. http://upload.wikimedia.org/wikipedia/commons/thumb/c/c7/153056995_5ef8b01016_o.jpg/230px-153056995_5ef8b01016_o.jpg Microcontrollers are used in automatically controlled products and devices, such as automobile engine control systems, implantable medical devices, remote controls, office machines, appliances, power tools, and toys. By reducing the size and cost compared to a design that uses a separate microprocessor, memory, and input/output devices, microcontrollers make it economical to digitally control even more devices and processes. Mixed signal microcontrollers are common, integrating analog components needed to control non-digital electronic systems. Some microcontrollers may use four-bit words and operate at  clock rate  frequencies as low as 4  kHz, for low power consumption (milliwatts or microwatts). They will generally have the ability to retain functionality while waiting for an event such as a button press or other interrupt; power consumption while sleeping (CPU clock and most peripherals off) may be just nanowatts, making many of them well suited for long lasting battery applications. Other microcontrollers may serve performance-critical roles, where they may need to act more like a  digital signal processor(DSP), with higher clock speeds and power consumption. Embedded design A microcontroller can be considered a self-contained system with a processor, memory and peripherals and can be used as an  embedded system.[1]  The majority of microcontrollers in use today are embedded in other machinery, such as automobiles, telephones, appliances, and peripherals for computer systems. These are called  embedded systems. While some embedded systems are very sophisticated, many have minimal requirements for memory and program length, with no operating system, and low software complexity. Typical input and output devices include switches,  relays,  solenoids,  LEDs, small or custom  LCD  displays, radio frequency devices, and sensors for data such as temperature, humidity, light level etc. Embedded systems usually have no keyboard, screen, disks, printers, or other recognizable I/O devices of a  personal computer, and may lack human interaction devices of any kind. 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  interruptsystem can signal the processor to suspend processing the current instruction sequence and to begin an  interrupt service routine  (ISR, or interrupt handler). The ISR 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 operated 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 Microcontroller programs must fit in the available on-chip program memory, since it would be costly to provide a system with external, expandable, memory. Compilers and assemblers are used to convert high-level language and assembler language codes into a compact  machine code  for storage in the microcontrollers memory. Depending on the device, the program memory may be permanent, read-only memory that can only be programmed at the factory, or program memory may be field-alterable flash or erasable read-only memory. 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. Many embedded systems need to read sensors that produce analog signals. This is the purpose of the  analog-to-digital converter  (ADC). Since processors 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 the analog to digital converter 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, the heater on, etc. Time Processing Unit  (TPU) is a sophisticated timer. In addition to counting down, the TPU can detect input events, generate output events, and perform other useful operations. A dedicated  Pulse Width Modulation  (PWM) block makes it possible for the CPU to control  power converters,  resistive  loads,  motors, etc., without using lots of CPU resources in tight timerloops. 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  I2C  and  Serial Peripheral Interface  (SPI). Higher integration In contrast to general-purpose CPUs, micro-controllers may not implement an external address or data bus as they integrate RAM and non-volatile memory on the same chip as the CPU. Using fewer pins, the chip can be placed in a much smaller, cheaper package. Integrating the memory and other peripherals on a single chip and testing them as a unit increases the cost of that chip, but often results in decreased net cost of the embedded system as a whole. Even if the cost of a CPU that has integrated peripherals is slightly more than the cost of a CPU and external peripherals, having fewer chips typically allows a smaller and cheaper circuit board, and reduces the labor required to assemble and test the circuit board. A micro-controller is a single  integrated circuit, commonly with the following features: central processing unit   ranging from small and simple 4-bit  processors to complex 32- or 64-bit processors discrete input and output bits, allowing control or detection of the logic state of an individual package pin serial  input/output  such as  serial ports  (UARTs) other  serial communications  interfaces  like  I ²C,  Serial Peripheral Interface  and  Controller Area Network  for system interconnect peripherals  such as  timers, event counters,  PWM generators, and  watchdog volatile memory (RAM) for data storage ROM,  EPROM,  EEPROM  or  Flash memory  for  program  and operating parameter storage clock generator   often an oscillator for a quartz timing crystal, resonator or  RC circuit many include analog-to-digital converters in-circuit programming and debugging support This integration drastically reduces the number of chips and the amount of wiring and  circuit board  space that would be needed to produce equivalent systems using separate chips. Furthermore, and on low pin count devices in particular, each pin may interface to several internal peripherals, with the pin function selected by software. This allows a part to be used in a wider variety of applications than if pins had dedicated functions. Micro-controllers have proved to be highly popular in  embedded systems  since their introduction in the 1970s. Some microcontrollers use a  Harvard architecture: separate memory buses for instructions and data, allowing accesses to take place concurrently. Where a Harvard architecture is used, instruction words for the processor may be a different bit size than the length of internal memory and registers; for example: 12-bit instructions used with 8-bit data registers. The decision of which peripheral to integrate is often difficult. The microcontroller vendors often trade operating frequencies and system design flexibility against time-to-market requirements from their customers and overall lower system cost. Manufacturers have to balance the need to minimize the chip size against additional functionality. Microcontroller architectures vary widely. Some designs include general-purpose microprocessor cores, with one or more ROM, RAM, or I/O functions integrated onto the package. Other designs are purpose built for control applications. A micro-controller instruction set usually has many instructions intended for bit-wise operations to make control programs more compact.[2]For example, a general purpose processor might require several instructions to test a bit in a register and branch if the bit is set, where a micro-controller could have a single instruction to provide that commonly-required function. Microcontrollers typically do not have a  math coprocessor, so  floating point  arithmetic is performed by software. Volumes About 55% of all  CPUs  sold in the world are  8-bit  microcontrollers and microprocessors. According to Semico, over four billion 8-bit microcontrollers were sold in 2006.[3] 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 as many as 30 or more microcontrollers. They can also be found in many electrical devices such as washing machines, microwave ovens, and telephones. http://upload.wikimedia.org/wikipedia/commons/thumb/1/18/PIC18F8720.jpg/220px-PIC18F8720.jpg A  PIC  18F8720  microcontroller  in an 80-pin  TQFP  package. 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 byultraviolet  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 increasingly 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 an economical option. These mask programmed parts have the program laid down in the same way as the logic of the chip, at the same time. Programming environments Microcontrollers were originally programmed only in  assembly language, but various  high-level programming languages  are now also in common use to target microcontrollers. These languages are either designed specially for the purpose, or versions of general purpose languages such as the  C programming language.  Compilers  for general purpose languages will typically have some restrictions as well as enhancements to better support the unique characteristics of microcontrollers. Some microcontrollers have environments to aid developing certain types of applications. Microcontroller vendors often make tools freely available to make it easier to adopt their hardware. Many microcontrollers are so quirky that they effectively require their own non-standard dialects of C, such as  SDCC for the 8051, which prevent using standard tools (such as code libraries or static analysis tools) even for code unrelated to hardware features. Interpreters are often used to hide such low level quirks. Interpreter  firmware is also available for some microcontrollers. For example,  BASIC  on the early microcontrollers  Intel  8052[4]; BASIC and  FORTH  on the  Zilog Z8[5]  as well as some modern devices. Typically these interpreters support  interactive programming. Simulators  are available for some microcontrollers, such as in Microchips  MPLAB  environment. These allow a developer to analyze what the behavior of the microcontroller and their program should be if they were using the actual part. A simulator will show the internal processor state and also that of the outputs, as well as allowing input signals to be generated. While on the one hand most simulators will be limited from being unable to simulate much other hardware in a system, they can exercise conditions that may otherwise be hard to reproduce at will in the physical implementation, and can be the quickest way to debug and analyze problems. Recent microcontrollers are often integrated with on-chip  debug  circuitry that when accessed by an  in-circuit emulator  via  JTAG, allow debugging of the firmware with a  debugger. Types of microcontrollers : Freescale 68HC11  (8-bit) Intel 8051 ARM  processors (from many vendors) using  ARM7  or Cortex-M3 cores are generally microcontrollers STMicroelectronics  STM8  (8-bit),  ST10  (16-bit) and  STM32  (32-bit) Atmel  AVR  (8-bit),  AVR32  (32-bit), and  AT91SAM  (32-bit) Freescale  ColdFire  (32-bit) and  S08  (8-bit) Hitachi H8,  Hitachi SuperH  (32-bit) Hyperstone  E1/E2 (32-bit, First full integration of  RISC  and  DSP  on one processor core [1996]  [1]) MIPS  (32-bit PIC32) NEC V850  (32-bit) PIC  (8-bit PIC16, PIC18, 16-bit dsPIC33 / PIC24) PowerPC  ISE PSoC (Programmable System-on-Chip) Rabbit 2000  (8-bit) Texas Instruments Microcontrollers  MSP430  (16-bit), C2000 (32-bit), and Stellaris (32-bit) Toshiba TLCS-870  (8-bit/16-bit) Zilog eZ8  (16-bit),  eZ80  (8-bit) and many others, some of which are used in very narrow range of applications or are more like applications processors than microcontrollers. The microcontroller market is extremely fragmented, with numerous vendors, technologies, and markets. Note that many vendors sell (or have sold) multiple architectures. Interrupt latency In contrast to general-purpose computers, microcontrollers used in embedded systems often seek to optimize  interrupt latency  over instruction throughput. Issues include both reducing the latency, and making it be more predictable (to support real-time control). When an electronic device causes an interrupt, the intermediate results (registers) have to be saved before the software responsible for handling the interrupt can run. They must also be restored after that software is finished. If there are more registers, this saving and restoring process takes more time, increasing the latency. Ways to reduce such context/restore latency include having relatively few registers in their central processing units (undesirable because it slows down most non-interrupt processing substantially), or at least having the hardware not save them all (this fails if the software then needs to compensate by saving the rest manually). Another technique involves spending silicon gates on shadow registers: one or more duplicate registers used only by the interrupt software, perhaps supporting a dedicated stack. Other factors affecting interrupt latency include: Cycles needed to complete current CPU activities. To minimize those costs, microcontrollers tend to have short pipelines (often three instructions or less), small write buffers, and ensure that longer instructions are continuable or restartable.  RISC  design principles ensure that most instructions take the same number of cycles, helping avoid the need for most such continuation/restart logic. The length of any  critical section  that needs to be interrupted. Entry to a critical section restricts concurrent data structure access. When a data structure must be accessed by an interrupt handler, the critical section must block that interrupt. Accordingly, interrupt latency is increased by however long that interrupt is blocked. When there are hard external constraints on system latency, developers often need tools to measure interrupt latencies and track down which critical sections cause slowdowns. One common technique just blocks all interrupts for the duration of the critical section. This is easy to implement, but sometimes critical sections get uncomfortably long. A more complex technique just blocks the interrupts that may trigger access to that data structure. This often based on interrupt priorities, which tend to not correspond well to the relevant system data structures. Accordingly, this technique is used mostly in very constrained environments. Processors may have hardware support for some critical sections. Examples include supporting atomic access to bits or bytes within a word, or other atomic access primitives like theLDREX/STREX  exclusive access primitives introduced in the  ARMv6  architecture. Interrupt nesting. Some microcontrollers allow higher priority interrupts to interrupt lower priority ones. This allows software to manage latency by giving time-critical interrupts higher priority (and thus lower and more predictable latency) than less-critical ones. Trigger rate. When interrupts occur back-to-back, microcontrollers may avoid an extra context save/restore cycle by a form of  tail call  optimization. Lower end microcontrollers tend to support fewer interrupt latency controls than higher end ones. History The first single-chip microprocessor was the 4-bit  Intel 4004  released in 1971. With the  Intel 8008  and more capable microprocessors available over the next several years. These however all required external chip(s) to implement a working system, raising total system cost, and making it impossible to economically computerize appliances. The first computer system on a chip optimized for control applications was the  Intel 8048  released in 1975,[citation   with both  RAM  and  ROM  on the same chip. This chip would find its way into over one billion PC keyboards, and other numerous applications. At this time Intels President, Luke J. Valenter, stated that the (Microcontroller) was one of the most successful in the companies history, and expanded the divisions budget over 25%. Most microcontrollers at this time had two variants. One had an erasable  EPROM  program memory, which was significantly more expensive than the  PROM  variant which was only programmable once. In 1993, the introduction of  EEPROM  memory allowed microcontrollers (beginning with the Microchip  PIC16x84)  [2][citation needed]) to be electrically erased quickly without an expensive package as required for  EPROM, allowing both rapid prototyping, and  In System Programming. The same year, Atmel introduced the first microcontroller using  Flash memory.[6] Other companies rapidly followed suit, with both memory types. Cost has plummeted over time, with the cheapest 8-bit microcontrollers being available for under $0.25 in quantity (thousands) in 2009,[citation needed]  and some 32-bit microcontrollers around $1 for similar quantities. Nowadays microcontrollers are low cost and readily available for hobbyists, with large online communities around certain processors. In the future,  MRAM  could potentially be used in microcontrollers as it has infinite endurance and its incremental semiconductor wafer process cost is relatively low. Microcontroller embedded memory technology Since the emergence of microcontrollers, many different memory technologies have been used. Almost all microcontrollers have at least two different kinds of memory, 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.  MRAMcould potentially replace it as it is 4-10 times denser which would make it more cost effective. In addition to the SRAM, some microcontrollers also have internal EEPROM for data storage; and even ones that do not have any (or not enough) are often connected to external serial EEPROM chip (such as the  BASIC Stamp) or external serial flash memory chip. A few recent microcontrollers beginning in 2003 have self-programmable flash memory[6]. Firmware The earliest microcontrollers used hard-wired or mask ROM to store firmware. Later microcontrollers (such as the early versions of the  Freescale 68HC11  and early  PIC microcontrollers) had quartz windows that allowed ultraviolet light in to erase the  EPROM. The Microchip  PIC16C84, introduced in 1993,[7]  was the first microcontroller to use  EEPROM  to store firmware. Also in 1993, Atmel introduced the first microcontroller using  NOR Flash memory  to store firmware.[6] PSoC  microcontrollers, introduced in 2002, store firmware in  SONOS  flash memory. MRAM  could potentially be used to store firmware.