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Text of the page (random words):
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 micro controller 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 micro controllers 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light ready for reprogramming after a programming burn and test cycle since 1998 eprom versions are rare and have been replaced by eeprom and flash which are easier to use can be erased electronically and cheaper to manufacture other versions may be available where the rom is accessed as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 micro controller 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 micro controller 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 edit 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 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 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 25 higher integration edit die of a pic12c508 8 bit fully static eeprom eprom rom based cmos microcontroller manufactured by microchip technology using a 1200 nanometer process die of a stm32f100c4t6b arm cortex m 3 microcontroller with 16 kilobytes flash memory 24 mhz central processing unit cpu motor control and consumer electronics control cec functions manufactured by stmicroelectronics 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 in addition to tending to decrease the defect rate for the finished assembly 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 bit or 64 bit processors volatile memory ram for data storage rom eprom eeprom or flash memory for program and operating parameter storage 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 i²c serial peripheral interface and controller area network for system interconnect peripherals such as timers event counters pwm generators and watchdog clock generator often an oscillator for a quartz timing crystal resonator or rc circuit many include analog to digital converters some include digital to analog converters in circuit programming and in circuit 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 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 manipulation bit wise operations to make control programs more compact 26 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 traditionally do not have a math coprocessor so floating point arithmetic is performed by software however some recent designs do include an fpu and dsp optimized features an example would be microchip s pic32 mips based line programming environments edit microcontrollers were originally programmed only in assembly language but various high level programming languages such as c python and javascript are now also in common use to target microcontrollers and embedded systems 27 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 microcontrollers with specialty hardware may 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 may also contain nonstandard features such as micropython although a fork circuitpython has looked to move hardware dependencies to libraries and have the language adhere to a more cpython standard interpreter firmware is also available for some microcontrollers for example basic on the early microcontrollers intel 8052 28 basic and forth on the zilog z8 29 as well as some modern devices typically these interpreters support interactive programming simulators are available for some microcontrollers 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 ice via jtag allow debugging of the firmware with a debugger a real time ice may allow viewing and or manipulating of internal states while running a tracing ice can record executed program and mcu states before after a trigger point types edit see also list of common microcontrollers as of 2008 update there are several dozen microcontroller architectures and vendors including arm core processors many vendors arm cortex m cores are specifically targeted toward microcontroller applications microchip technology atmel avr 8 bit avr32 32 bit and at91sam 32 bit cypress semiconductor s m8c core used in their psoc programmable system on chip freescale coldfire 32 bit and s08 8 bit freescale 68hc11 8 bit and others based on the motorola 6800 family intel 8051 also manufactured by nxp semiconductors infineon and many others infineon 8 bit xc800 16 bit xe166 32 bit xmc4000 arm based cortex m4f 32 bit tricore and 32 bit aurix tricore bit microcontrollers 30 maxim integrated max32600 max32620 max32625 max32630 max32650 max32640 mips microchip technology pic 8 bit pic16 pic18 16 bit dspic33 pic24 32 bit pic32 nxp semiconductors lpc1000 lpc2000 lpc3000 lpc4000 32 bit lpc900 lpc700 8 bit parallax propeller powerpc ise rabbit 2000 8 bit renesas electronics rl78 16 bit mcu rx 32 bit mcu superh v850 32 bit mcu h8 r8c 16 bit mcu silicon laboratories pipelined 8 bit 8051 microcontrollers and mixed signal arm based 32 bit microcontrollers stmicroelectronics stm8 8 bit st10 16 bit stm32 32 bit spc5 automotive 32 bit texas instruments ti msp430 16 bit msp432 32 bit c2000 32 bit toshiba tlcs 870 8 bit 16 bit many others exist 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 edit 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 during the context switch 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 interrupt handler is finished if there are more processor registers this saving and restoring process may take more time increasing the latency if an isr does not require the use of some registers it may simply leave them alone rather than saving and restoring them so in that case those registers are not involved with 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 is 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 cri...
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