Output for Microcontrollers: Registers and Protocols

Key Takeaways

• Microcontroller output can be analog, which uses a novel resistor arrangement instead of costlier and less reliable components.

• Microcontroller register configurations use different I/O settings for different functionality.

• MIcrocontrollers can interface with multiple secondary devices to broaden system features.

Outputs for microcontrollers include seven-segment displays.

Microcontrollers are ubiquitous in electronics because they offer a stripped-down approach to computer interfacing for various system applications. Cheap, highly customizable, and widely supported, they are the backbone of system control and communications for devices, and their role should only grow with a continued emphasis on greater connectivity through the Internet of Things (IoT). Properly interfacing with these devices requires knowledge of electrical ratings and communication standards that offer reliable transceiving even over great distances with a reduced crosstalk susceptibility. While constraints may differ considerably between particular devices, designers should be mindful of how the output for microcontrollers operates in a general sense.

Input/Output Register Configuration Applications

No DAC? How to Generate Analog Output for Microcontrollers

Given the wide variety of circuit applications microcontrollers interface with, an equally vast amount of output configurations is necessary. The most overarching dichotomy of outputs is digital and analog: the former represents discrete logic levels determined by the hardware, and the latter a continuous range of values. Microcontroller output represents system performance or feedback to facilitate user input. These could be as simple as lighting an LED or tripping a buzzer to more advanced output devices like monitors, displays, and speakers. 

Microcontrollers need sensors and an analog-to-digital (ADC) converter on the input side to interact meaningfully with our analog world for sampling and quantization (some sensors provide digital output, but these are less common). However, a mirrored function may not be available at the output owing to cost, miniaturization, or redundancy. Instead, leaner hardware approaches can simulate analog signals using an R-2R ladder to convert a binary voltage into an analog output signal. 

The R-2R ladder uses a parallel arrangement of resistors of some resistance 2R on the data lines (the least significant bit or LSB also has a 2R series pull-down resistor). Between each data line are resistors of value R, with the analog output coming off the most significant bit (MSB). In this topology, the weight of each bit is half that of the preceding bit (moving from MSB to LSB) as a series of consecutive half-voltage dividers. Because of its straightforward manufacturing implementation, high reliability, low cost, and accuracy that exceeds most usage requirements, the R-2R ladder is found more often in microcontroller output than a corresponding DAC. 

Microcontroller output uses I/O pins, which are bi-directionally programmable to accept incoming signals or transmit outgoing ones. One of the most important first steps for peripheral integration and microcontrollers is establishing both the logic high and low voltage levels, ensuring that signal levels are safe and meet the minimum detectability threshold. Similarly, a current check between the two devices is necessary to prevent sinking or sourcing too much (or too little current) that would either endanger operation or fail to meet minimum thresholds. Of course, devices can’t source a voltage or current higher than their maximum ratings, which is device-dependent. 

Microcontroller Register Configurations

Synonymous with microcontroller design is the use of registers, which are pervasive throughout microcontroller system architecture, including I/O registers. Output formats come either parallel or serial, with the former denoting distinct data lines out of some cascaded number of flip-flop registers and the latter used to produce a single output only at the end of the cascaded registers. Combined with the input, they make four possible arrangements:

• Serial in/serial out (SISO) - Typically used for buffering and temporary data storage between source and destination, SISO starts at the input of the first flip-flop and continues sequentially through the remainder of the series. The design takes the longest time for transmission and data accessibility from the point of input.
• Parallel in/parallel out (PIPO) - Used as a general-purpose I/O register, PIPO data is immediately accessible at each flip-flop simultaneously (for input and output signals). Each flip-flop’s output updates with each clock cycle and reflects the most recent change on the input (unlike SISO, which is staggered by the number of cycles equivalent to the preceding flip-flop count). Due to the simultaneous input at each flip-flop, PIPOs can also operate as bit-shift registers.
• Serial in/parallel out (SIPO) - A SIPO register combines serial communication protocols with simultaneous output, functioning very similar to  SISO but with the added benefit of data lines representing the cascaded data between the flip-flops (instead of just the output from the final flip-flop). In addition to established circuit applications, it makes troubleshooting easier as designers can keep track of the data at each flip-flop “step.”
• Parallel in/serial out (PISO) - Circuit designers can use many gates and other logic controls to select between input states for a single serial output. 

Communication Protocols to Integrate Devices

The topic of serial and parallel lines is also crucial to communication protocols, which can greatly simplify the wiring between devices. Unlike parallel communications that map microcontroller outputs to receiver inputs 1:1 physically (i.e., each I/O bit has a dedicated wire, which limits interfacing capabilities by reserving most or all available I/O pins). A serial line uses one wire for simplex (one-way) or half-duplex (asynchronous bidirectional) or two wires for full-duplex (synchronous bidirectional). However, managing signal transceiving can require additional wires depending on the protocol:

• Universal synchronous/asynchronous receiver/transmitter (USART) - Supports clock-driven (synchronous) transceiving or start/stop bit signaling (asynchronous) within a full-duplex communication framework. USART can send small chunks of bit data while performing basic error checking, such as data overrun and framing.
• Serial peripheral interface (SPI) - A full-duplex communication mode uses two wires to communicate (using a primary and secondary designation to indicate hierarchy) and a clock and select line. There can be multiple select lines for a primary device to interface with multiple secondary devices independently or a daisy-chain configuration communicating in a cascading fashion.
• Two-wire interface (TWI) - Communicates with a half-duplex method that uses only two wires (one data, one clock) to connect a primary device to one or multiple secondary devices. 

Cadence Solutions Offer Unparalleled Control

Output for microcontrollers is various and requires a thorough understanding of the architecture (both hardware and software). Equally important to success is accurate circuit simulation to model performance characteristics and determine baseline parasitics (and other detriments) that undermine signal quality. Cadence’s PCB Design and Analysis Software suite makes it easy for electronic developers to rapidly test and build circuits when designs are in high flux. Combined with the powerful OrCAD PCB Designer, circuit layout has never had a better mix of functionality and simplicity.

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