Touch climate display on an Arduino Uno

// Touch climate display: an AHT20 temperature and humidity sensor and a 16x2
// LCD on the same two I2C wires, and a TTP223 touch pad that switches the
// temperature between Celsius and Fahrenheit.
//
//   A4, A5  SDA and SCL, to the AHT20 (0x38); the LCD backpack (0x27) is on
//           the header's other SDA and SCL, which are the same two wires
//   pin 2   the TTP223's SIG: high while a finger is on the pad
//   power   the AHT20 on 5V, the LCD on IOREF (also 5 V), the TTP223 on 3V3:
//           its 3.3 V high is still a HIGH to a 5 V Uno
//
// Two I2C devices on one pair of wires is what the bus is for: each answers
// only its own address. The AHT20 module and the backpack both carry
// pull-ups, which is normal and harmless.
//
// # The sensor
//
// Adafruit_AHTX0.h asks the AHT20 for a reading and waits the 80 ms the chip
// takes to make one. The sketch then takes the numbers in hundredths
// (getTemperatureCentis and getHumidityCentis), because Fahrenheit is a sum
// and there is no floating point on this board:
//
//   F x 100 = C x 100 x 9 / 5 + 3200
//
// # The touch pad
//
// The TTP223 needs no pull-up and does not bounce: SIG is a real output,
// high while touched. It takes up to 60 ms to decide, and it ignores the pad
// for the first half second after power comes up. The sketch flips the units
// on the rising edge, so holding a finger there flips them once.
//
// Everything is on the display and nothing goes to the serial monitor: the
// sensor library, two I2C drivers and the LCD font fill most of the 8 KB an
// example is allowed.

#include <Adafruit_AHTX0.h>
#include "mokxi_lcd1602_i2c.h"

const int TOUCH = 2;

Adafruit_AHTX0 aht;
Lcd1602I2c lcd(A5, A4);

bool fahrenheit = false;
bool wasTouched = false;

/** Hundredths to tenths, rounded half away from zero. */
int tenthsOf(long centis) {
  return (int)((centis >= 0 ? centis + 5 : centis - 5) / 10);
}


void show() {
  sensors_event_t humidity, temp;
  if (!aht.getEvent(&humidity, &temp)) {
    lcd.setCursor(0, 0);
    lcd.print("sensor?");
    lcd.padLine();
    return;
  }
  long c = aht.getTemperatureCentis();
  int shown = tenthsOf(fahrenheit ? c * 9 / 5 + 3200 : c);
  int rh = (int)((aht.getHumidityCentis() + 50) / 100);

  lcd.setCursor(0, 0);
  lcd.print("Temp  ");
  lcd.printFixed(shown, 1);
  lcd.print(fahrenheit ? " F" : " C");
  lcd.padLine();
  lcd.setCursor(0, 1);
  lcd.print("Humid ");
  lcd.print(rh);
  lcd.print(" %");
  lcd.padLine();
}

void setup() {
  pinMode(TOUCH, INPUT);
  lcd.begin();
  lcd.print("Mokxi climate");
  if (!aht.begin()) {
    lcd.setCursor(0, 1);
    lcd.print("no AHT20");
    while (true) {
      delay(100);
    }
  }
  delay(400);
  lcd.clear();
}

void loop() {
  bool touched = digitalRead(TOUCH) == HIGH;
  bool flipped = touched && !wasTouched;
  wasTouched = touched;
  if (flipped) {
    fahrenheit = !fahrenheit;
  }

  static unsigned long last = 0;
  if (flipped || millis() - last >= 500) {
    last = millis();
    show();
  }
  delay(20);
}