Indoor and outdoor thermometer on two OLEDs, on an Arduino Uno

// Indoor and outdoor weather on two OLEDs, through a TCA9548A.
//
// Two BME280s and two SSD1306 OLEDs, and every one of them has the same
// I2C address as its twin: the BME280s answer at 0x76 and the screens at
// 0x3C. On one bus they would talk over each other. A TCA9548A I2C
// multiplexer (at 0x70) keeps them apart: the indoor sensor and screen are
// on its channel 0, the outdoor pair on channel 1, and the sketch selects a
// channel before it talks to the pair on it.
//
//   TCA9548A  SDA -> SDA (A4)  SCL -> SCL (A5)  VIN -> 5V  GND -> GND
//   channel 0 (SD0, SC0): indoor BME280 and indoor OLED
//   channel 1 (SD1, SC1): outdoor BME280 and outdoor OLED
//
// One display object drives both screens. Its 1 KB frame is redrawn and
// sent for each screen in turn, which leaves room in the Uno's 2 KB of RAM
// that a second frame would take. Each screen still needs its own begin(),
// with its channel selected, to be set up.
//
// Every two seconds each screen shows its own temperature, humidity and
// pressure, with the lowest and highest temperature since power-up, and the
// serial monitor prints both.
//
// Things to try. Drag the outdoor sensor's temperature below zero and watch
// only the outdoor screen change. Change TEMP_IN_F to true for Fahrenheit.

#include <Wire.h>
#include <TCA9548.h>
#include <Adafruit_GFX.h>
#include <Adafruit_SSD1306.h>
#include <Adafruit_BME280.h>

const uint8_t MUX_ADDRESS = 0x70;
const uint8_t BME_ADDRESS = 0x76;
const uint8_t OLED_ADDRESS = 0x3C;
const uint8_t INDOOR = 0;   // the multiplexer channel of the indoor pair
const uint8_t OUTDOOR = 1;  // and of the outdoor pair
const unsigned long INTERVAL_MS = 2000;
const bool TEMP_IN_F = false;

TCA9548 mux(MUX_ADDRESS);
Adafruit_SSD1306 display(128, 64, &Wire, -1);
Adafruit_BME280 bmeIn;
Adafruit_BME280 bmeOut;

struct Reading {
  float temp;
  float humidity;
  float pressure;
  float low;
  float high;
  bool ok;
};

Reading indoor = {0, 0, 0, 1000, -1000, false};
Reading outdoor = {0, 0, 0, 1000, -1000, false};
unsigned long lastRead = 0;

float shown(float c) {
  return TEMP_IN_F ? c * 9.0 / 5.0 + 32.0 : c;
}

// Set up the screen and the sensor on one channel.
bool startPair(uint8_t channel, Adafruit_BME280 &bme, const char *name) {
  mux.selectChannel(channel);
  if (!display.begin(SSD1306_SWITCHCAPVCC, OLED_ADDRESS)) {
    Serial.print(name);
    Serial.println(F(": no OLED at 0x3C"));
  }
  bool found = bme.begin(BME_ADDRESS);
  if (!found) {
    Serial.print(name);
    Serial.println(F(": no BME280 at 0x76"));
  }
  return found;
}

void readPair(uint8_t channel, Adafruit_BME280 &bme, Reading &r) {
  mux.selectChannel(channel);
  r.temp = bme.readTemperature();
  r.humidity = bme.readHumidity();
  r.pressure = bme.readPressure() / 100.0;
  if (r.temp < r.low) r.low = r.temp;
  if (r.temp > r.high) r.high = r.temp;
}

void drawPair(uint8_t channel, const char *title, const Reading &r) {
  mux.selectChannel(channel);
  display.clearDisplay();
  display.setTextColor(SSD1306_WHITE);
  display.setTextSize(1);
  display.setCursor(0, 0);
  display.print(title);
  if (!r.ok) {
    display.setCursor(0, 24);
    display.print(F("No BME280 found"));
    display.display();
    return;
  }
  display.setTextSize(2);
  display.setCursor(0, 14);
  display.print(shown(r.temp), 1);
  display.print(TEMP_IN_F ? F(" F") : F(" C"));
  display.setTextSize(1);
  display.setCursor(0, 36);
  display.print(F("Humidity "));
  display.print(r.humidity, 0);
  display.print(F(" %"));
  display.setCursor(0, 46);
  display.print(r.pressure, 1);
  display.print(F(" hPa"));
  display.setCursor(0, 56);
  display.print(F("Lo "));
  display.print(shown(r.low), 1);
  display.print(F("  Hi "));
  display.print(shown(r.high), 1);
  display.display();
}

void printReading(const char *name, const Reading &r) {
  Serial.print(name);
  if (!r.ok) {
    Serial.print(F(" no sensor"));
    return;
  }
  Serial.print(' ');
  Serial.print(r.temp, 1);
  Serial.print(F(" C "));
  Serial.print(r.humidity, 0);
  Serial.print(F(" % "));
  Serial.print(r.pressure, 0);
  Serial.print(F(" hPa"));
}

void setup() {
  Serial.begin(9600);
  Wire.begin();
  if (!mux.begin()) {
    Serial.println(F("No TCA9548A at 0x70: check SDA, SCL and its power"));
    while (true);
  }
  indoor.ok = startPair(INDOOR, bmeIn, "indoor");
  outdoor.ok = startPair(OUTDOOR, bmeOut, "outdoor");
  Serial.println(F("two screens ready"));
  lastRead = millis() - INTERVAL_MS;
}

void loop() {
  if (millis() - lastRead < INTERVAL_MS) return;
  lastRead = millis();
  if (indoor.ok) readPair(INDOOR, bmeIn, indoor);
  if (outdoor.ok) readPair(OUTDOOR, bmeOut, outdoor);
  drawPair(INDOOR, "INDOOR", indoor);
  drawPair(OUTDOOR, "OUTDOOR", outdoor);
  printReading("indoor", indoor);
  Serial.print(F("  "));
  printReading("outdoor", outdoor);
  Serial.println();
}