de (%) | Timing Jitter (μs) | Avg Power Draw (mA) | |----------|-------------------------|-----------------------------|------------------------------|--------------------|---------------------| | Monolithic SoC-Only (Pi 4) | 340 ± 85 | 78.4 | 89.2 | ±420 | 850 | | Fragmented Commercial Setup | 520 ± 110 | 82.1 | N/A (Distributed) | ±680 | 1120 | | UACS Dual-Layer (Pi 4 + ATmega328P) | 115 ± 18 | 96.7 | 34.5 | ±22 | 410 |

Key Findings:

• Deterministic Timing Isolation: Offloading sensor polling and Morse segmentation to the ATmega328P reduced timing jitter by 94.7%, eliminating OS-induced packet loss in haptic rendering.
• Compute Offloading: Separating STT/TTS from real-time I/O dropped Pi CPU utilization from ~89% to ~34.5%, enabling stable background processing without thermal throttling.
• Sweet Spot: The dual-layer architecture achieves optimal balance when UART/Bluetooth bridge operates at 115200 baud with hardware flow control disabled but software ring-buffering enabled. Morse timing windows calibrated to 60ms (dot) / 180ms (dash) / 420ms (character gap) yield >96% accuracy across variable user input speeds.

Core Solution

The UACS implements a deterministic, event-driven pipeline that treats Morse code as a universal translation layer between continuous (speech/audio) and discrete (tactile/switch) modalities.

Architecture Decisions

• Dual-Layer Split: The Raspberry Pi 4 handles compute-intensive tasks (STT, TTS, text normalization, Morse encoding/decoding algorithms). The ATmega328P manages time-critical operations (interrupt-driven switch polling, timing classification, PWM haptic control, UART/HC-05 bridging).
• Separation of Concerns: Prevents OS scheduling variability from affecting tactile feedback. The MCU acts as a compact HMI device, while the SoC remains external or docked, improving portability and thermal management.
• Hardware Protections: Integrated overcharge, deep discharge, short-circuit, and thermal cutoffs ensure safe Li-ion operation during continuous BT + haptic load cycles.

Control Logic & Implementation

The system follows a state-based event loop with atomic transitions:

1. Input Capture → Decode → State Transition: SPDT switch interrupts trigger timer captures. Dot/dash classification uses adaptive timing windows.
2. Morse Segmentation: Character boundaries are detected via inter-character silence thresholds. Word boundaries use extended silence windows.
3. Serial Communication: UART handles bidirectional data transfer between Pi and MCU. Ring buffers prevent overflow during burst encoding.
4. Output Scheduling: PWM duty cycle maps to LRA resonance frequency. Audio buzzer provides fallback/confirmation tones.

// ATmega328P Interrupt-Driven Morse Decoder & State Machine
volatile uint32_t press_start = 0;
volatile uint8_t morse_char = 0;
volatile enum { IDLE, DOT, DASH, CHAR_COMPLETE } state = IDLE;

ISR(INT0_vect) {
    if (bit_is_clear(PIND, PD2)) { // Switch pressed
        press_start = micros();
        state = IDLE;
    } else { // Switch released
        uint32_t duration = micros() - press_start;
        if (duration < 120) morse_char = 'D'; // Dot
        else if (duration < 240) morse_char = 'H'; // Dash
        else state = CHAR_COMPLETE; // Character gap
        
        if (state != CHAR_COMPLETE) {
            // Buffer character for UART transmission
            uart_tx_buffer_push(morse_char);
        }
    }
}

Processing Pipelines

• Speech → Text → Morse → Bluetooth → Haptic/Audio
• Haptic Input → Timing Decode → Text → Speech Full-duplex operation is maintained via independent TX/RX ring buffers and priority queuing on the Pi side.

Pitfall Guide

1. OS-Level Timing Jitter in Monolithic Designs: Running sensor polling and STT/TTS on a single general-purpose OS introduces scheduling latency that breaks real-time tactile feedback. Always isolate time-critical I/O to a bare-metal MCU.
2. Fixed Morse Timing Windows: Hardcoding dot/dash thresholds fails across users with different input speeds. Implement adaptive calibration or user-configurable timing windows (e.g., 60ms/180ms baseline with ±20% tolerance).
3. UART/Bluetooth Buffer Overflow: HC-05 modules lack hardware flow control. Without software ring-buffering and backpressure handling, burst Morse encoding will drop characters. Implement circular buffers with atomic read/write pointers.
4. PWM Frequency Mismatch for LRAs: Linear Resonant Actuators require excitation near their mechanical resonance (~150-250Hz). Standard 50Hz/1kHz PWM causes weak or distorted haptic feedback. Use timer-configured PWM at the LRA's resonant frequency.
5. State Machine Race Conditions: Concurrent input decoding and output scheduling without atomic state transitions cause dropped characters or duplicate transmissions. Use volatile flags and disable interrupts briefly during critical state updates.
6. Battery Voltage Sag Under Load: Li-ion cells experience voltage droop during simultaneous BT transmission and haptic actuation, potentially triggering MCU brown-out resets. Implement hardware protection circuits and firmware brown-out detection (BOD) thresholds.
7. Cross-Modal Latency Accumulation: Chaining STT → Morse Encoding → BT → TTS compounds delay. Use pipeline buffering, prioritize control packets over data, and implement predictive pre-fetching on the Pi to mask network/processing latency.

Deliverables

• 📘 Architecture Blueprint: Complete dual-layer system diagram, signal flow specifications, timing window calibration charts, and BOM with sourcing notes. Includes UART/HC-05 wiring schematics and PWM/LRA driver circuit.
• ✅ Implementation Checklist: Step-by-step validation protocol covering hardware assembly, firmware flashing, timing calibration, Bluetooth pairing, STT/TTS pipeline verification, and stress testing under continuous full-duplex operation.
• ⚙️ Configuration Templates: Ready-to-deploy config.h headers for Morse timing thresholds, UART baud rates, PWM frequency/duty mappings, Bluetooth pairing profiles, and Pi-side pipeline priority queues. Includes calibration scripts for adaptive user-speed tuning.