Disadvantages of 8-Bit Microcontrollers

Key Takeaways

• Higher-bit microcontrollers offer additional space

• 8-bit microcontrollers may not be as fast or responsive as higher-bit microcontrollers

• It is possible to bring 8-bit architecture into higher-bit architecture without losing functionality

The 8-bit microcontroller is a wonderful, lightweight design that, to this day, provides excellent functionality in a relatively small package. Cheap and well-documented, it sees wide adoption by hobbyists and professionals alike for designs where the size of the data is not a primary concern. While the 8-bit microcontroller is appropriate in certain applications, it lacks some of the robustness of higher-bit microcontrollers like a 16-bit or 32-bit microcontroller. 

Disadvantages of 8-bit microcontrollers relative to 16-bit, 32-bit, or greater architectures generally stem from the former’s inability to offer the byte-addressable space found among the latter entries. As IoT increases prevalence in designs among all notable industries, there is little advantage to remaining with 8-bit architectures when the demand for data and communication grows ever greater.

The Disadvantages of 8-Bit Microcontroller Data Bus and Byte-Addressable Space

Memory

Memory is an immediate disadvantage of 8-bit microcontrollers. It is important to understand that the space allotted to the data bus does not necessarily represent the amount of data that will be packaged and sent; 8-bits is simply the amount of data that can be accessed each machine cycle instruction. Typically, address busses for 8-bit microcontrollers will be double octet wide, allowing for 65,536 (or 2562) memory addresses.

Although this allows for significantly more space, 8-bit microcontroller operations are best suited for lightweight programming. 8-bit code design does have an advantage in overall portability, but it is more susceptible to data sizing issues. Data handling will require more care when a data bus is performing operations on byte sizes larger than itself using specific definitions at the high-language level. Statements involving these critical sections of code ambiguity to the microcontroller must be treated atomically; failure to introduce interrupts into code is a likely cause of runtime errors. 

Also along these lines, microcontrollers performing operations on data sizes larger than its bus will require far more lines of machine code to achieve the same result as a larger microcontroller performing operations on appropriately sized data. Not only does this eat further into the limited memory space of an 8-bit microcontroller, but potential speed gains that may have been possible due to performing fewer operations in total are negated, as additional cycles are required at the hardware level to account for the data/bus mismatch. To put a neat bow on it, 8-bit code is usually smaller overall, while larger-bit microcontrollers have denser code, as they can perform steps in a single cycle that would occupy several commands on an 8-bit microcontroller.

Speed 

Speed is an important factor in any microcontroller choice. Speaking from surface-level specifications, most higher-bit microcontrollers have oscillators that can operate into the gigahertz range. Comparatively, 8-bit microcontrollers will typically only reach megahertz oscillation speeds, although exact values will depend on the specific IC. While the potential for higher clock speeds is never a detriment, a particular design’s clock needs may necessitate a speed slower than the max available. Even operating on the same clock speed, higher-bit microcontrollers possess a speed advantage, as they can perform more computations per individual cycle. Higher-bit microcontrollers, therefore, tend to have an overall greater capacity for faster clocking speeds while still maintaining a processing advantage over the range of clock speeds they share with 8-bit microcontrollers.

Real-Time Considerations Improve Circuit Responsiveness

Real-time consideration is a measure of how responsive a microcontroller is to external stimuli. The quicker a microcontroller can receive and resolve the stimuli, the quicker it can return to its standby state. Some of the attributes responsible for real-time reactions are listed below:

• Instruction latency: It is rare to be able to interrupt instructions while running; the interrupt will insert at the next possible opening instead. In general, the more complex the instructions, the greater the latency. These complex instructions could be the result of an 8-bit microcontroller handling data larger than its data bus.
• Interrupt efficiency: How is the interrupt handled? In 32-bit microcontrollers, it is typically via vectored interrupts, where interrupts are given vector numbers that point to a specific interrupt routine. An 8-bit microcontroller may instead use fixed routine addresses, which is faster than the vectored interrupt approach. The caveat for fixed routine addressing is that multiple interrupts decrease the real-time potential.

For speed and responsiveness, 8-bit microcontrollers have difficulty keeping up with higher-bit microcontrollers. 

Portability Extends to Higher-Bit Architectures

When translating between architectures or implementing futureproof design methodology, the simpler the code is, the easier it will be to add to it. Especially if it comes time to redesign machine-level instructions to a more readable and higher-level language like C/C++, well-commented and lightweight code aids in this endeavor. Higher-bit microcontrollers, having 8-bit microcontrollers as a subset, are positioned to be able to handle any porting changes. 32-bit microcontrollers can emulate 8-bit microcontroller functions that are not included in their standard libraries. This allows 32-bit microcontrollers to be a safe upgrade, as they can incorporate the functionality of the 8-bit microcontroller while offering additional features. Data alignment issues are a concern during runtime, a converse situation to the disadvantage of an 8-bit microcontroller attempting to perform operations on data too large for its data bus. To avoid this, efforts should be made to access the structure access syntax as opposed to operating on the byte level.

The key to translating from 8-bit to higher architectures is code portability

The Cadence family of products can meet your design needs from beginning to end. Ensure your layout and design are well-suited for the timing needs and signal integrity of your microcontrollers using the powerful tools contained within Allegro PCB Editor. 

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