IoT & Embedded Systems Development

Detailed Technical Deep Dive: IoT & Embedded Systems

Summary: This document outlines Jishnu Mahanta's architectural approach to engineering battery-powered sensor networks, writing clean, firmware layers, and developing physical enclosures.

1. Power Management & Sleep Cycle Calibration

In field deployments, battery life is the ultimate constraint. A standard ESP32 drawing 80mA to 240mA during active operation will drain a 1200mAh LiPo battery in a matter of hours. Achieving a multi-year battery life requires shifting from active polling to event-driven execution and strict sleep-cycle profiles.

To optimize power draw, the hardware design must isolate high-power sensors (such as gas sensors or GPS receivers) using P-channel MOSFET power gates. Below is the standard firmware pattern I implement in C++ using ESP-IDF to coordinate deep sleep intervals and interrupt-driven wake cycles:

#include "esp_sleep.h"
#include "driver/rtc_io.h"

#define SENSOR_POWER_PIN GPIO_NUM_4
#define WAKE_UP_PIN GPIO_NUM_12

void setup() {
  // Initialize RTC variables
  esp_sleep_wakeup_cause_t wakeup_reason = esp_sleep_get_wakeup_cause();
  
  if (wakeup_reason == ESP_SLEEP_WAKEUP_EXT0) {
    // Handle sensor trigger interrupt (e.g. rain sensor or tilt sensor)
    processSensorInterrupt();
  } else {
    // Regular scheduled telemetry upload
    performTelemetryRoutine();
  }
  
  // Power down peripheral rail before sleep
  pinMode(SENSOR_POWER_PIN, OUTPUT);
  digitalWrite(SENSOR_POWER_PIN, LOW); // Cut power to MOSFET gate
  
  // Configure external pin wake trigger (active low)
  esp_sleep_enable_ext0_wakeup(WAKE_UP_PIN, 0); 
  
  // Configure timer wake (e.g. 1 hour = 3600 seconds)
  esp_sleep_enable_timer_wakeup(3600ULL * 1000000ULL);
  
  // Start deep sleep
  esp_deep_sleep_start();
}

2. Physical Layout Design & High-Speed Routing Rules

When routing 2-to-4 layer PCBs in KiCad, high-frequency signals (such as USB-C data lanes or high-speed SPI busses) must be laid out with impedance matching and proper return paths.

Impedance Rules for High-Speed Data Lines:

• Ground Return: Always place a solid, continuous copper ground plane directly beneath signal tracks on Layer 2.
• Differential Pairs: USB data lines must be routed as a differential pair with matched trace lengths (within 0.15mm) and a constant spacing to maintain a 90-ohm differential impedance.
• Power Decoupling: Position 0.1µF and 10µF ceramic capacitors within 2mm of the VCC input pins of microcontrollers to filter high-frequency switching noise.

3. Firmware Architecture & FreeRTOS Decoupling

Modern IoT edge nodes handle multiple concurrent processes: updating a display, measuring sensors, listening to voice commands, and communicating over MQTT. Running these inside a single blocking loop leads to system lag. I build modular firmware structures by utilizing FreeRTOS queues and task prioritization:

  +--------------------+      Queue      +---------------------+
  | Sensor Polling     | ---------------> | MQTT Telemetry Sync |
  | (Priority 1, 10Hz) |                  | (Priority 2, Event) |
  +--------------------+                  +---------------------+
            |                                        |
            v                                        v
  Reads ADC / I2C / SPI                     Pipes packets over TLS

This decoupling ensures that if network latency increases during telemetry transfer, sensor measurements continue to run at stable intervals. If a task crashes, a dedicated supervisor task detects the failure and resets only the affected subsystem rather than restarting the entire board.