Microcontroller Types and Applications

Key Takeaways

• The difference in system layout between embedded and external microcontrollers.

• The importance of programmability in microcontrollers – and why more isn’t always good.

• Microarchitecture and the interplay of the instruction set on performance.

The specializations offered by different microcontroller types and applications are integral to modern automation in electronics

The microprocessor is one of the triumphs of the 20th century. In the five decades since its debut, it has greatly reduced the size, cost, and complexity of computer design while affording a staggering amount of computational power within a small package. However, many devices simply do not require their robustness to perform as necessary and can achieve a more economical design with a microcontroller. 

Microcontroller types and applications span a wide range of products, offering basic automation to enable digital control. Especially in portables, the small footprint and low-power consumption of many microcontrollers endear them greatly for reliable, long-lasting performance.

Popular Microcontroller Architectures

Memory Sizes and Styles of Microcontrollers

A major distinction between microcontrollers is the size of the internal bus: 8, 16, or 32 bits. A higher bit count means a larger amount of data can be transferred during each clock cycle and allows for greater precision. Not every microcontroller application requires greater data precision and significant cost savings can be realized with a less sophisticated microcontroller. Furthermore, a smaller bus size means a smaller package, which may be preferred in dense assemblies with board size and weight restrictions.

Microcontrollers can also be defined by the location of the memory relative to the package. While a microcontroller differs from a microprocessor as a comprehensive computing function set (CPU, programmable I/O, etc.), there are some cases where memory is not included in the package and needs to be furnished elsewhere. These external memory microcontrollers differ from the more common embedded microcontrollers by having the memory located outside of the package. Separating memory in this way inhibits miniaturization and signal speeds, but external memory allows for much greater storage mechanisms than what would be available on-chip. Current trends in electronic design greatly favor an embedded model, but exceptions remain abound.

The style of non-volatile memory (which saves information even when powered down) can also differ between microcontrollers. Depending on the intended role of the microcontroller, the non-volatile memory’s (NVM) ability can differ tremendously:

• Programmable read-only memory (PROM) - PROM is a one-time programming (OTP) method typically used in final product assemblies. Unlike standard ROM, which has its programming set before manufacturing the chip, PROM allows for greater flexibility. Its single-shot programmability makes it a poor option for prototyping, but it fills a niche for certain low-volume assemblies where ROM may prove less economical or when a device’s parameters are set and testing needs to be confirmed before moving to ROM. The nature of the OTP NVM makes it highly reliable and repeatable, endearing it over more programmable NVMs for devices where low power and uptime are paramount. 

• Erasable programmable read-only memory (EPROM) - EPROM is a more expensive and less reliable option than PROM, but is highlighted by its ability to erase its stored programming through UV light. EPROM for full prototyping contains a transparent window over the silicon dies, but production EPROM needs to cover this window to prevent accidental erasure by ambient lighting. EPROM is hampered by a slow programming speed, relative inconvenience of erasure, and lack of specificity in the application.

• Electrically erasable programmable read-only memory (EEPROM/E²PROM) - E²PROM rapidly displaced EPROM due to its greater ease of erasability: the memory can be erased in-circuit through the application of unique signals. 

  •  Flash memory - A subset of E²PROM, flash makes some key tradeoffs: volumetric capacity for granularity of rewritability and fewer write cycles. Flash construction allows for much more memory density, as the device’s construction only allows for block (≥ 512 byte) erase/rewrite due to these cells of memory sharing read/write/erase functionality.

How Microarchitecture Impacts Microcontroller Types and Applications

As form follows function, the microarchitecture of various microprocessors allows specialization. Microarchitecture is designed around the instruction set architecture, which defines the structure of the microcontroller as seen by the assembler or compiler. Similar to a schematic at the system design level of electronic systems, the microarchitecture provides a roadmap for how data moves throughout the microcontroller and the relation between different on-chip functions. Today, most microcontrollers use a pipelined data format that parallelizes the sub-systems to maximize clock cycle efficiency. Methods to achieve better execution speed like predictive branching statistically optimize parallelization by prefetching for more commonly utilized data pathways.

Historically, microarchitecture design was built on two differing schools of thought: complex instruction set computing (CISC) and reduced instruction set computing (RISC). There is less distinction today; at the time, the two modes were coined with “simple” CISC models and “complex” RISC designs, but architectures differ in how the instruction set can address the low-level operators. CISC architectures can perform multi-step operations with a single instruction. RISC, meanwhile, has nearly uniform instruction length and isolates load and store instructions, which speeds up instruction processing time by increasing the speed of individual instructions. RISC leans heavily into parallelization for this reason.

Design goals will always dictate the preference between the microarchitectures, but RISC often has more favorable attributes. Although CISC has arguably higher data throughput, this is more of a preference for microprocessors than microcontrollers. Speed is the name of the game, but as microprocessors function as a quasi-OS, it is the interrupt latency that is most valuable to closely emulate real-time control in design. During an interrupt flag, the current state of the processes needs to be saved to the appropriate registers so they can be recovered once the interrupt exception is handled and standard processes resume; the longer it takes an individual process to finish, the larger proportion of time is spent storing/retrieving data instead of rectifying the interrupt.

Experience Greater Design Control With Cadence Solutions

Microcontroller types and applications differ tremendously based on the needs of the design, but keen development will be able to select the best features that support the device at hand. Programmability enables extensive in-circuit testing and prototyping without the need for continuous layout respins. Before this stage of the design, simulation is key to rapidly modeling different circuit parameters with high confidence in real-world accuracy. Cadence’s suite of PCB Design and Analysis Software lends electronic system developers a comprehensive toolset for optimizing design at the physical and logical levels. Combined with the power and efficiency of OrCAD PCB Designer, teams can improve reliability and performance without sacrificing time-to-market.

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