Electronics Guide

GPIO Architecture

A typical GPIO pin includes several configurable elements that determine its electrical behavior:

• Direction register: Controls whether the pin operates as an input (high impedance) or output (driven low or high)
• Output data register: Holds the value to be driven on output pins
• Input data register: Captures the current state of input pins for software reading
• Pull-up/pull-down resistors: Internal resistors that establish a default logic level when the pin is unconnected
• Open-drain/open-collector mode: Output configuration that can only pull the line low, enabling wired-OR connections
• Drive strength control: Adjustable output current capability for impedance matching and EMI control
• Slew rate control: Limits edge transition speed to reduce electromagnetic emissions

These configuration options allow GPIO pins to interface with a wide variety of external circuits while maintaining signal integrity and electromagnetic compatibility.

Input Conditioning

GPIO inputs often include hardware features that improve reliability in noisy environments:

• Schmitt triggers: Hysteresis in the input threshold prevents noise from causing multiple transitions
• Digital filters: Programmable filters that require stable input levels for a specified number of clock cycles
• Interrupt generation: Hardware detection of rising edges, falling edges, or level changes that trigger processor interrupts
• Wake-up capability: Ability to wake the processor from low-power sleep modes when input conditions change

Proper input conditioning is essential for reliable operation in industrial and automotive environments where electrical noise is prevalent.

Port Organization

GPIO pins are typically organized into ports of 8, 16, or 32 pins that can be accessed simultaneously:

• Parallel access: Read or write entire port values in a single operation for fast data transfer
• Bit manipulation: Set, clear, or toggle individual pins without affecting others using atomic operations
• Alternate functions: Most GPIO pins can be multiplexed with other peripheral functions, sharing physical pins

Efficient port access is critical for bit-banging protocols and high-speed parallel interfaces.

ADC Architecture Types

Microcontrollers employ various ADC architectures, each with distinct characteristics:

• Successive approximation register (SAR): The most common type in microcontrollers, offering a good balance of speed, resolution, and power consumption. SAR ADCs typically achieve 10 to 16 bits of resolution at sample rates from tens of kilosamples to several megasamples per second.
• Sigma-delta (oversampling): Trades speed for resolution by sampling at very high rates and filtering the result. Sigma-delta ADCs can achieve 24-bit resolution but with lower effective sample rates, making them ideal for precision measurement applications.
• Pipeline: Achieves high sample rates by dividing conversion into stages. Less common in microcontrollers but found in high-performance devices requiring tens of megasamples per second.
• Flash: The fastest architecture, comparing the input against all possible levels simultaneously. Rarely integrated in microcontrollers due to high power and area requirements.

ADC Specifications

Understanding ADC specifications is crucial for selecting and using converters effectively:

• Resolution: The number of bits in the digital output, determining the smallest detectable voltage change. A 12-bit ADC divides the input range into 4096 levels.
• Sample rate: Maximum conversions per second, determining the highest frequency signal that can be accurately captured per the Nyquist criterion
• Input range: The voltage span from minimum to maximum measurable values, often programmable through gain selection
• Reference voltage: The voltage that corresponds to full-scale output, critical for absolute accuracy
• Integral nonlinearity (INL): Maximum deviation from an ideal straight-line transfer function
• Differential nonlinearity (DNL): Variation in step size between adjacent codes, with values exceeding one LSB indicating missing codes
• Effective number of bits (ENOB): Actual resolution accounting for noise and distortion, always less than nominal resolution

Input Multiplexing

Microcontroller ADCs typically share a single converter among multiple input channels through analog multiplexing:

• Channel selection: Software selects which input pin connects to the ADC
• Settling time: Time required after switching channels for the input to stabilize
• Scan modes: Automatic sequential conversion of multiple channels
• Simultaneous sampling: Some devices include sample-and-hold circuits on multiple channels to capture signals at the same instant

Conversion Triggering

ADC conversions can be initiated through various mechanisms:

• Software trigger: Conversion starts when firmware writes to a control register
• Timer trigger: Conversions occur at precise intervals synchronized to timer events
• External trigger: An external signal initiates conversion for synchronization with external events
• Continuous mode: Conversions repeat automatically without software intervention

Timer-triggered conversions are essential for consistent sample timing in signal processing applications.

DAC Architecture Types

Several architectures are used in integrated DACs:

• Resistor string (R-string): A series of equal resistors divides a reference voltage. Switches select tap points corresponding to each output code. Simple and monotonic but requires many resistors for high resolution.
• R-2R ladder: Uses only two resistor values arranged in a ladder network. More area-efficient than resistor strings for higher resolutions.
• Sigma-delta: Generates a high-frequency pulse stream whose average value equals the desired output. External filtering produces the analog signal. Achieves high resolution with simple circuitry.
• Current steering: Multiple weighted current sources are summed, with switches directing current to output or ground. Offers fast settling times for high-speed applications.

DAC Specifications

Key DAC parameters include:

• Resolution: Number of bits determining output voltage granularity. A 12-bit DAC can produce 4096 distinct output levels.
• Settling time: Time required for output to reach final value within specified accuracy after code change
• Output range: Span from minimum to maximum output voltage, often rail-to-rail or buffered
• Output impedance: Source impedance seen by the load, affecting drive capability
• Glitch energy: Transient disturbance during code transitions, critical for audio applications
• Monotonicity: Guarantee that output always increases (or stays same) as code increases

DAC Applications in Microcontrollers

Integrated DACs serve numerous embedded applications:

• Audio generation: Playing back stored audio samples or generating tones
• Setpoint references: Providing reference voltages for comparators and control loops
• Waveform generation: Creating sine waves, triangles, and arbitrary waveforms when combined with timer interrupts
• Offset calibration: Trimming sensor interface circuits
• Motor control: Setting speed references for analog motor drives

PWM Fundamentals

PWM signals are characterized by two primary parameters:

• Period (frequency): The time for one complete on/off cycle. Higher frequencies reduce audible noise in motor control and improve filtering efficiency but increase switching losses.
• Duty cycle: The percentage of time the signal is high. A 75% duty cycle means the signal is high for three-quarters of each period.

The average voltage delivered to a load equals the supply voltage multiplied by the duty cycle. A 50% duty cycle on a 5V supply delivers an average of 2.5V.

PWM Timer Architecture

PWM outputs are typically generated using timer/counter peripherals configured in specific modes:

• Edge-aligned PWM: The counter counts up from zero to a maximum value, then resets. Output changes at zero and at the compare match point. Simple but creates aligned edges that can cause current spikes.
• Center-aligned PWM: The counter counts up to maximum then back down to zero. Output changes symmetrically around the center, spreading switching noise and reducing torque ripple in motor control.
• Asymmetric PWM: Different compare values for rising and falling edges enable phase-shifted outputs for advanced control schemes.

Dead-Time Generation

In motor drives and power converters, complementary PWM outputs must include dead time to prevent shoot-through:

• Shoot-through: Both high-side and low-side switches conducting simultaneously, causing destructive current flow
• Dead-time insertion: Hardware automatically delays turn-on of each switch until the complementary switch has fully turned off
• Programmable delay: Dead time adjusted to match transistor switching characteristics

Hardware dead-time generation ensures safe operation even if software timing varies.

PWM Applications

PWM peripherals enable efficient control across many domains:

• DC motor control: Varying speed by adjusting duty cycle, with H-bridge configurations enabling direction control
• LED dimming: Controlling brightness perception through rapid on/off switching faster than the eye can perceive
• Servo control: Standard hobby servos use PWM pulse width (typically 1-2ms within a 20ms period) to set position
• Power supply regulation: Switch-mode power supplies use PWM to regulate output voltage
• Audio generation: Class D audio amplifiers use PWM with filtering to reproduce audio signals
• Heater control: Proportional temperature control by varying heater duty cycle

PWM Resolution

The resolution of PWM duty cycle control depends on the timer resolution and PWM frequency:

• Timer bits: More counter bits enable finer duty cycle steps
• Frequency trade-off: Higher PWM frequencies require faster timer clocks to maintain resolution
• Dithering: Alternating between adjacent duty cycles can simulate higher resolution for slowly changing outputs

For example, a 16-bit timer at 48 MHz can generate a 10 kHz PWM with 4800 discrete duty cycle levels.

Input Capture Mode

In capture mode, the timer captures its current count value when an input signal edge occurs:

• Frequency measurement: Capturing consecutive edges and computing the difference gives the signal period
• Pulse width measurement: Capturing rising and falling edges measures pulse duration
• Time stamping: Recording when events occur relative to a continuous time reference
• Input capture prescaler: Dividing input frequency for measuring very high-frequency signals

Input capture provides accurate timing measurement without polling overhead, as hardware captures the exact count at the moment of the edge.

Output Compare Mode

In compare mode, the unit generates output signals when the timer count matches programmed values:

• Toggle output: Output pin changes state at each compare match
• Set or clear: Output driven high or low at match
• PWM generation: Combining compare matches with timer overflow generates PWM waveforms
• Interrupt generation: Compare match triggers an interrupt for software timing

Output compare enables precise timing of external events with microsecond or better accuracy.

Multiple Capture/Compare Channels

Timers typically include multiple capture/compare channels sharing a single time base:

• Multi-channel PWM: Generate multiple PWM outputs with different duty cycles but synchronized periods
• Phase measurement: Capture multiple input signals to determine their relative timing
• Complex waveforms: Multiple compare events per period create complex output patterns

DMA Integration

Advanced capture/compare units can trigger DMA transfers for high-throughput applications:

• Capture to memory: Automatically store captured timestamps without CPU intervention
• Compare from memory: Load new compare values from a buffer for arbitrary waveform generation
• Circular buffers: Continuous data flow for extended measurements or waveform sequences

DMA integration enables capturing or generating thousands of events per second with minimal processor overhead.

Quadrature Signal Encoding

Quadrature encoders produce two output channels, typically labeled A and B:

• Phase relationship: Channel B leads or lags channel A by 90 degrees depending on rotation direction
• Pulses per revolution: Encoder resolution determines position accuracy, from tens to thousands of counts per revolution
• Index pulse: Many encoders include a Z or index channel that pulses once per revolution for absolute position reference

Hardware Decoding

Microcontroller quadrature interfaces decode encoder signals in hardware:

• Direction detection: Hardware examines which channel leads to determine rotation direction
• Count modes: Count on A edges only (1x), on A and B edges separately (2x), or on all edges (4x quadrature)
• Position counter: Hardware counter tracks cumulative position, incrementing or decrementing based on direction
• Velocity calculation: Timer capture of position at fixed intervals enables velocity computation

Hardware decoding handles high pulse rates that would overwhelm software interrupt processing.

Noise Filtering

Real encoder signals contain noise that can cause false counts:

• Digital filters: Require stable input levels for multiple clock cycles before recognizing edges
• Glitch rejection: Ignore pulses shorter than a minimum duration
• Quadrature error detection: Detecting illegal state transitions (both channels changing simultaneously) indicates noise or wiring problems

Encoder Applications

Quadrature encoders provide position feedback in numerous applications:

• Motor control: Closed-loop servo systems use encoder feedback for precise positioning
• User interfaces: Rotary knobs with quadrature output provide smooth parameter adjustment
• Linear motion: Linear encoders measure position in CNC machines, printers, and robotics
• Flow measurement: Turbine flow meters output quadrature signals proportional to flow rate

Capacitive Sensing Principles

Capacitive touch sensing exploits the electrical properties of the human body:

• Self-capacitance: A touch electrode has capacitance to ground. A finger near the electrode adds capacitance in parallel, increasing total capacitance.
• Mutual capacitance: Two electrodes form a capacitor. A finger between them absorbs some electric field, reducing measured capacitance.
• Typical changes: Touch typically changes capacitance by a few picofarads, requiring sensitive measurement techniques

Measurement Techniques

Microcontroller touch peripherals employ various methods to measure capacitance:

• Charge transfer: Repeatedly charge the touch electrode and transfer charge to a reference capacitor, counting cycles until threshold voltage reached
• Relaxation oscillator: Touch electrode forms part of an RC oscillator whose frequency changes with capacitance
• Charge integration: Measure current flowing into electrode during voltage ramp
• Sigma-delta: Oversampling techniques provide high resolution and noise rejection

Touch Interface Features

Integrated touch controllers provide features that simplify application development:

• Automatic calibration: Baseline tracking to compensate for environmental drift
• Threshold detection: Hardware comparison against programmable touch thresholds
• Proximity detection: Detect approaching finger before actual touch
• Multi-touch support: Mutual capacitance scanning enables detecting multiple simultaneous touches
• Noise suppression: Frequency hopping and filtering to reject interference
• Low power scanning: Periodic scanning from sleep mode to detect wake-up touch

Touch Electrode Design

Effective touch sensing requires careful electrode design:

• Electrode size: Larger electrodes provide more capacitance change but reduce spatial resolution
• Overlay material: Thinner, higher-dielectric overlays improve sensitivity
• Guard rings: Driven shields around electrodes reduce parasitic coupling
• Ground planes: Reference planes provide stable capacitance baseline
• Trace routing: Minimize parasitic capacitance in traces connecting electrodes to the microcontroller

Touch Applications

Capacitive touch interfaces enable modern user interfaces:

• Touch buttons: Replace mechanical switches with sealed, durable touch points
• Sliders: Linear electrode arrangements detect finger position for continuous control
• Rotary controls: Circular electrode patterns simulate rotary knob behavior
• Touch screens: Grid patterns enable x-y position detection for interactive displays
• Gesture recognition: Pattern analysis detects swipes, pinches, and other gestures

LCD Technology Basics

Understanding LCD operation is essential for proper controller configuration:

• Liquid crystal behavior: Liquid crystals twist or align in response to electric fields, controlling light transmission
• AC drive requirement: DC voltages damage LCD materials; displays require alternating voltage with zero average DC component
• Segments and commons: Display elements are organized as intersections between segment and common electrodes
• Multiplexing: Multiple commons allow addressing more segments with fewer pins through time-division scanning

LCD Drive Waveforms

LCD controllers generate precise waveforms to control display elements:

• Static drive: Each segment has its own pin; simplest but requires many pins
• Multiplexed drive: Commons are scanned sequentially; segments are driven based on which common is active
• Duty cycle: Ratio 1/N where N is the number of commons; higher multiplex ratios reduce contrast
• Bias voltage: Intermediate voltage levels improve contrast in multiplexed displays
• Frame rate: Display update frequency; too slow causes visible flicker, too fast increases power

LCD Controller Features

Integrated LCD controllers provide hardware support for display driving:

• Voltage generation: Internal charge pumps generate bias voltages from a single supply
• Waveform timing: Hardware generates proper frame and segment timing
• Display memory: Dedicated RAM holds segment on/off states
• Blink control: Hardware blinking of selected segments without CPU intervention
• Contrast adjustment: Software-controlled bias voltage for contrast optimization
• Low power modes: Display continues operating while main CPU sleeps

Display Types

LCD controllers support various display configurations:

• Segment displays: Seven-segment numerals, icons, and custom segments for dedicated information display
• Character displays: Dot-matrix character modules with built-in character generators
• Graphic displays: Pixel-addressable displays for arbitrary graphics, though typically small due to drive complexity

LCD Interface Considerations

Practical LCD implementation requires attention to several factors:

• Temperature effects: LCD response speed and contrast vary with temperature
• Viewing angle: Display visibility depends on viewing direction; design orientation accordingly
• Backlight control: PWM output can control LED backlight brightness
• EMI: Rapidly changing segment lines can radiate; proper filtering and layout minimize interference

Clock and Power Management

Peripherals require careful clock and power configuration:

• Peripheral clocks: Each peripheral has a clock gate that must be enabled before use
• Clock selection: Some peripherals can select from multiple clock sources for optimal frequency
• Power domains: Peripherals may reside in separate power domains for selective sleep operation
• Peripheral reset: Resetting peripherals to known state before configuration ensures predictable behavior

Pin Multiplexing

Physical pins are shared among multiple peripheral functions:

• Alternate function selection: Registers select which peripheral controls each pin
• Pin mapping: Some peripherals can be remapped to different pins for PCB layout flexibility
• Conflict avoidance: Designers must track which pins are used for which functions

Interrupt and DMA Configuration

Peripheral events can trigger interrupts or DMA transfers:

• Event sources: Each peripheral generates specific events (conversion complete, compare match, receive ready, etc.)
• Interrupt priority: Assigning appropriate priorities ensures timely handling of critical events
• DMA channels: Mapping peripherals to DMA channels enables automated data transfer
• Event routing: Some devices allow routing peripheral events to trigger other peripherals directly

Peripheral Libraries and Abstraction

Modern development often uses hardware abstraction layers:

• Register access: Direct register manipulation provides full control but is device-specific
• Vendor libraries: Manufacturer-provided libraries abstract common operations
• HAL layers: Hardware abstraction layers provide consistent APIs across device families
• RTOS drivers: Real-time operating systems often include standardized peripheral drivers

Abstraction simplifies development but understanding underlying hardware remains important for optimization and debugging.

Signal Integrity

Maintaining clean signals between microcontroller pins and external circuits:

• Impedance matching: Termination for high-speed signals to prevent reflections
• Decoupling: Capacitors near pins filter supply noise from analog peripherals
• Ground paths: Clean return paths for both digital and analog signals
• Isolation: Separating sensitive analog inputs from noisy digital sections

Protection Circuits

External circuits protect peripheral pins from damage:

• ESD protection: TVS diodes clamp electrostatic discharge transients
• Overvoltage protection: Series resistors and clamp diodes limit input voltages
• Reverse polarity: Protection against incorrect power connection
• Isolation: Optocouplers or digital isolators for galvanically separated interfaces

Calibration and Testing

Achieving specified accuracy requires proper calibration:

• Factory calibration: Many microcontrollers store calibration data in non-volatile memory
• In-system calibration: Calibrating against known references during production
• Self-test modes: Built-in test features verify peripheral operation
• Loopback testing: Connecting outputs to inputs for functional verification