Added temperature alert configuration functions

[samspencer5991]

Description

This PR adds temperature alert configuration functionality to the MCP9808 library, enabling users to configure and use the built-in alert output feature of the MCP9808 sensor.

New Features

Alert Configuration Functions

• setTempAlert(temp, activeHigh, alertMode, hysteresis) - Convenient one-function alert setup
• setUpperTemp(temp) / getUpperTemp() - Upper temperature boundary
• setLowerTemp(temp) / getLowerTemp() - Lower temperature boundary
• setCriticalTemp(temp) / getCriticalTemp() - Critical temperature threshold
• setHysteresis(hyst) / getHysteresis() - Temperature hysteresis (0°C, 1.5°C, 3°C, 6°C)
• setAlertPolarity(activeHigh) - Configure active-high or active-low output
• setAlertMode(interruptMode) - Choose comparator or interrupt mode
• setAlertSelectCriticalOnly(critOnly) - Monitor critical temp only or all boundaries
• enableAlert(enable) - Enable/disable alert output
• getAlertStatus() - Read current alert status

Supported Use Cases

• Comparator mode: Auto-clearing alerts for thermostat applications (fans, cooling systems)
• Interrupt mode: Latching alerts for microcontroller-based event logging
• Temperature windows: Monitor both upper and lower boundaries
• Critical-only: Simple over-temperature protection

Implementation Details

• Added temperature conversion helpers (tempToReg(), regToTemp()) for proper register formatting
• All functions validate input ranges (-40°C to +125°C)
• Hysteresis applies when temperature decreases below threshold
• Alert Select bit configures critical-only vs. window monitoring
• Compatible with existing library functions

Testing

Tested on a custom RP2040 board with:

• Comparator mode with various hysteresis settings
• Interrupt mode with ISR handling
• Temperature crossing thresholds in both directions
• Multiple heating/cooling cycles

Example Usage

#include <Adafruit_MCP9808.h>

Adafruit_MCP9808 tempsensor = Adafruit_MCP9808();

void setup() {
  tempsensor.begin(0x18);
  
  // Simple critical temperature alert at 30°C
  // Active-low, comparator mode, 1.5°C hysteresis using the critical temperature only
  tempsensor.setTempAlert(30.0, false, false, MCP9808_HYST_1_5C, true);
  
  // Or configure individual settings:
  tempsensor.setCriticalTemp(30.0);
  tempsensor.setAlertPolarity(false);  // Active-low
  tempsensor.setAlertMode(false);      // Comparator mode
  tempsensor.setHysteresis(MCP9808_HYST_1_5C);
  tempsensor.setAlertSelectCriticalOnly(true);
  tempsensor.enableAlert(true);
}

void loop() {
  float temp = tempsensor.readTempC();
  bool alert = tempsensor.getAlertStatus();
  
  Serial.print("Temp: ");
  Serial.print(temp);
  Serial.print("°C | Alert: ");
  Serial.println(alert ? "ACTIVE" : "Inactive");
  
  delay(500);
}

Backwards Compatibility

All existing library functions remain unchanged. This PR only adds new functionality.

Checklist

• Code compiles without warnings
• Functions are documented with doxygen-style comments
• Tested on hardware
• Follows existing library code style

Related Documentation

MCP9808 Datasheet sections referenced:

• Section 5.1.1: Configuration Register (Register 5-2)
• Section 5.1.2: Temperature Limit Registers (Register 5-3)
• Section 5.2.2: Temperature Hysteresis
• Section 5.2.3: Alert Output Configuration