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[env:sht41]
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platform = nordicnrf52
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board = adafruit_feather_nrf52840
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framework = arduino
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/*
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BTHome v2 temperature / humidity beacon – ultra-low-power
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------------------------------------------------------------
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Board : Pro Micro nRF52840 (nice!nano v2 compatible), powered by CR2032
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Sensor : Adafruit SHT41 (I2C addr 0x44), power-gated from a GPIO
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Core : Adafruit nRF52 Arduino core
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Tools > Board > "Adafruit Feather nRF52840 Express"
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(same UF2 bootloader + S140 SoftDevice as the nice!nano)
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Behaviour
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- Measures T/RH + battery voltage every 60 s
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- Sensor is powered ONLY during the measurement (~15 ms)
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- Non-connectable BTHome v2 advertisement every 10 s
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- DC/DC enabled on REG0 (VDDH->VDD) and REG1 (VDD->1.3 V core)
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- CPU sleeps (FreeRTOS tickless idle -> sd_app_evt_wait) the rest of the time
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All pins below are RAW nRF GPIO numbers (port * 32 + pin), driven directly
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through registers, so the Feather pin mapping does not matter.
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*/
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#include <bluefruit.h>
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// ---------------- user settings ----------------
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#define SENS_PWR_PIN 17 // P0.17 (Pro Micro "D2" / "017") -> SHT41 VIN
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#define SENS_SDA_PIN 20 // P0.20 (Pro Micro "D3" / "020") -> SHT41 SDA
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#define SENS_SCL_PIN 22 // P0.22 (Pro Micro "D4" / "022") -> SHT41 SCL
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#define MEAS_INTERVAL_MS 60000UL // measurement period
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#define ADV_INTERVAL 16000 // 10000 ms / 0.625 ms
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#define TX_POWER_DBM 0 // -40,-20,-16,-12,-8,-4,0,2..8 (lower = less power, less range)
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#define DEVICE_NAME "SHT41"
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#define SHT41_ADDR 0x44
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#define SHT41_CMD_MEAS_HP 0xFD // high precision, no heater (max 8.3 ms)
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// ---------------- state ----------------
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static int16_t g_temp10 = 0; // °C * 10 (BTHome 0x45)
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static uint8_t g_rh = 0; // %RH, 1 % (BTHome 0x2E)
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static uint16_t g_battMv = 0; // mV (BTHome 0x0C)
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static uint8_t g_i2cBuf[6]; // EasyDMA buffer (must be RAM): TX = cmd byte, RX = 6 data bytes
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// =====================================================================
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// Helpers
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// =====================================================================
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#define PIN_CNF(dir, in, pull, drive) \
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((GPIO_PIN_CNF_DIR_##dir << GPIO_PIN_CNF_DIR_Pos) | \
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(GPIO_PIN_CNF_INPUT_##in << GPIO_PIN_CNF_INPUT_Pos) | \
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(GPIO_PIN_CNF_PULL_##pull << GPIO_PIN_CNF_PULL_Pos) | \
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(GPIO_PIN_CNF_DRIVE_##drive << GPIO_PIN_CNF_DRIVE_Pos))
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#define CNF_HIGHZ PIN_CNF(Input, Disconnect, Disabled, S0S1) // no input buffer, no pull: ~0 leakage
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#define CNF_I2C PIN_CNF(Input, Connect, Pullup, S0D1) // open-drain + internal pull-up (backup)
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#define CNF_PWR PIN_CNF(Output, Disconnect, Disabled, H0H1) // high-drive push-pull
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static NRF_GPIO_Type* port(uint32_t pin) { return (pin & 32) ? NRF_P1 : NRF_P0; }
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static void pinCfg(uint32_t pin, uint32_t cnf) { port(pin)->PIN_CNF[pin & 31] = cnf; }
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static void busPins(uint32_t cnf) { pinCfg(SENS_SDA_PIN, cnf); pinCfg(SENS_SCL_PIN, cnf); }
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static void sensorPower(bool on) { // set level first, then make it an output (no glitch)
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NRF_GPIO_Type* p = port(SENS_PWR_PIN);
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(on ? p->OUTSET : p->OUTCLR) = 1UL << (SENS_PWR_PIN & 31);
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pinCfg(SENS_PWR_PIN, CNF_PWR);
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}
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static void sensorOff() { // bus high-Z BEFORE removing power (no back-feeding)
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busPins(CNF_HIGHZ);
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sensorPower(false); // held low = sensor fully off
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}
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// Clear event, fire task, wait (with timeout) for the event
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static void trigger(volatile uint32_t& task, volatile uint32_t& evt, uint32_t timeoutMs) {
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evt = 0;
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task = 1;
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for (uint32_t t0 = millis(); !evt && millis() - t0 < timeoutMs; ) {}
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}
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// =====================================================================
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// Minimal TWIM (I2C master) – enabled only during a measurement
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// =====================================================================
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static bool twimRun(volatile uint32_t& task, uint32_t shorts) {
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NRF_TWIM0->EVENTS_STOPPED = 0;
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NRF_TWIM0->EVENTS_ERROR = 0;
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NRF_TWIM0->SHORTS = shorts;
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task = 1;
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uint32_t t0 = millis();
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while (!NRF_TWIM0->EVENTS_STOPPED && !NRF_TWIM0->EVENTS_ERROR && millis() - t0 <= 20) {}
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bool ok = NRF_TWIM0->EVENTS_STOPPED && !NRF_TWIM0->EVENTS_ERROR;
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if (!ok) { // NACK / bus error / timeout: force STOP
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trigger(NRF_TWIM0->TASKS_STOP, NRF_TWIM0->EVENTS_STOPPED, 5);
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NRF_TWIM0->ERRORSRC = NRF_TWIM0->ERRORSRC; // write-1-to-clear
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NRF_TWIM0->EVENTS_ERROR = 0;
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}
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NRF_TWIM0->EVENTS_STOPPED = 0;
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return ok;
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}
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static uint8_t shtCrc(const uint8_t* d) { // CRC-8, poly 0x31, init 0xFF
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uint8_t crc = 0xFF;
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for (int i = 0; i < 2; i++) {
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crc ^= d[i];
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for (int b = 0; b < 8; b++) crc = (crc & 0x80) ? (crc << 1) ^ 0x31 : (crc << 1);
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}
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return crc;
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}
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// =====================================================================
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// SHT41 measurement with power gating (updates g_temp10 / g_rh on success)
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// =====================================================================
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static bool readSHT41() {
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uint8_t* b = g_i2cBuf;
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sensorPower(true);
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busPins(CNF_I2C);
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delay(3); // SHT41 power-up <1 ms + breakout caps
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NRF_TWIM0->ENABLE = TWIM_ENABLE_ENABLE_Enabled;
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b[0] = SHT41_CMD_MEAS_HP;
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bool ok = twimRun(NRF_TWIM0->TASKS_STARTTX, TWIM_SHORTS_LASTTX_STOP_Msk);
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if (ok) {
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delay(10); // CPU sleeps during conversion
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ok = twimRun(NRF_TWIM0->TASKS_STARTRX, TWIM_SHORTS_LASTRX_STOP_Msk) &&
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shtCrc(b) == b[2] && shtCrc(b + 3) == b[5];
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}
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NRF_TWIM0->ENABLE = TWIM_ENABLE_ENABLE_Disabled;
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sensorOff(); // power down before doing the math
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if (!ok) return false;
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// T = -45 + 175*raw/65535 (0.1 °C units), RH = -6 + 125*raw/65535 (1 %), one rounding step
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uint32_t rawT = (b[0] << 8) | b[1];
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uint32_t rawRH = (b[3] << 8) | b[4];
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g_temp10 = -450 + (int32_t)((1750UL * rawT + 32767UL) / 65535UL);
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int32_t rh = -6 + (int32_t)((125UL * rawRH + 32767UL) / 65535UL);
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g_rh = constrain(rh, 0, 100);
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return true;
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}
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// =====================================================================
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// Battery voltage: VDDH/5 via SAADC (no external divider needed)
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// =====================================================================
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static uint16_t readBatteryMv() {
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static volatile int16_t result;
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static bool calibrated = false;
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NRF_SAADC->RESOLUTION = SAADC_RESOLUTION_VAL_12bit;
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NRF_SAADC->OVERSAMPLE = SAADC_OVERSAMPLE_OVERSAMPLE_Over8x;
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NRF_SAADC->CH[0].CONFIG = // + internal 0.6 V ref, single-ended, no R (all 0)
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(SAADC_CH_CONFIG_GAIN_Gain1_2 << SAADC_CH_CONFIG_GAIN_Pos) | // range 0..1.2 V
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(SAADC_CH_CONFIG_TACQ_10us << SAADC_CH_CONFIG_TACQ_Pos) |
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(SAADC_CH_CONFIG_BURST_Enabled << SAADC_CH_CONFIG_BURST_Pos);
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NRF_SAADC->CH[0].PSELP = SAADC_CH_PSELP_PSELP_VDDHDIV5;
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NRF_SAADC->CH[0].PSELN = SAADC_CH_PSELN_PSELN_NC;
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NRF_SAADC->RESULT.PTR = (uint32_t)&result;
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NRF_SAADC->RESULT.MAXCNT = 1;
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NRF_SAADC->ENABLE = SAADC_ENABLE_ENABLE_Enabled;
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if (!calibrated) {
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trigger(NRF_SAADC->TASKS_CALIBRATEOFFSET, NRF_SAADC->EVENTS_CALIBRATEDONE, 10);
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calibrated = true;
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}
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trigger(NRF_SAADC->TASKS_START, NRF_SAADC->EVENTS_STARTED, 5);
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trigger(NRF_SAADC->TASKS_SAMPLE, NRF_SAADC->EVENTS_END, 5);
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trigger(NRF_SAADC->TASKS_STOP, NRF_SAADC->EVENTS_STOPPED, 5);
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NRF_SAADC->ENABLE = SAADC_ENABLE_ENABLE_Disabled; // ADC off between readings
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int32_t raw = result;
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return raw < 0 ? 0 : raw * 6000L / 4096L; // raw/4096 * 1.2 V * 5
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}
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// =====================================================================
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// BTHome v2 advertisement (objects in ascending ID order: 0x0C, 0x2E, 0x45)
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// =====================================================================
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static void updateAdvertising() {
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const uint8_t sd[] = {
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0xD2, 0xFC, // UUID 0xFCD2 (little endian)
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0x40, // BTHome v2, unencrypted, regular interval
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0x0C, uint8_t(g_battMv), uint8_t(g_battMv >> 8), // voltage, 1 mV
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0x2E, g_rh, // humidity, 1 %, uint8
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0x45, uint8_t(g_temp10), uint8_t(uint16_t(g_temp10) >> 8) // temperature, 0.1 °C, sint16
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};
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Bluefruit.Advertising.stop();
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Bluefruit.Advertising.clearData();
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Bluefruit.Advertising.addFlags(BLE_GAP_ADV_FLAGS_LE_ONLY_GENERAL_DISC_MODE);
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Bluefruit.Advertising.addData(BLE_GAP_AD_TYPE_SERVICE_DATA, sd, sizeof(sd));
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Bluefruit.Advertising.addName();
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Bluefruit.Advertising.start(0); // advertise forever
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}
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// force = publish even if the measurement failed (used once at boot)
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static void measureAndPublish(bool force = false) {
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if (readSHT41()) {
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g_battMv = readBatteryMv(); // read right after sensor load
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} else if (!force) {
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return; // keep advertising last good values
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}
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updateAdvertising();
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}
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// =====================================================================
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void setup() {
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// No Serial.begin(): USB stays unused (it is unpowered on battery anyway)
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// DC/DC on both regulators – write directly while SoftDevice is still off
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uint8_t sdOn = 0;
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sd_softdevice_is_enabled(&sdOn);
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if (!sdOn) {
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NRF_POWER->DCDCEN0 = 1; // REG0: VDDH -> VDD
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NRF_POWER->DCDCEN = 1; // REG1: VDD -> core
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}
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sensorOff();
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// TWIM settings are retained while disabled; pins are only taken over when ENABLE is set
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NRF_TWIM0->PSEL.SCL = SENS_SCL_PIN;
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NRF_TWIM0->PSEL.SDA = SENS_SDA_PIN;
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NRF_TWIM0->FREQUENCY = TWIM_FREQUENCY_FREQUENCY_K400;
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NRF_TWIM0->ADDRESS = SHT41_ADDR;
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NRF_TWIM0->TXD.PTR = (uint32_t)g_i2cBuf;
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NRF_TWIM0->TXD.MAXCNT = 1;
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NRF_TWIM0->RXD.PTR = (uint32_t)g_i2cBuf;
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NRF_TWIM0->RXD.MAXCNT = 6;
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Bluefruit.autoConnLed(false); // no blinking LED while advertising
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Bluefruit.begin();
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sd_power_dcdc_mode_set(NRF_POWER_DCDC_ENABLE); // keep REG1 DC/DC on under SoftDevice
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Bluefruit.setTxPower(TX_POWER_DBM);
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Bluefruit.setName(DEVICE_NAME);
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Bluefruit.Advertising.setType(BLE_GAP_ADV_TYPE_NONCONNECTABLE_NONSCANNABLE_UNDIRECTED);
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Bluefruit.Advertising.setInterval(ADV_INTERVAL, ADV_INTERVAL);
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Bluefruit.Advertising.restartOnDisconnect(false);
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measureAndPublish(true); // start advertising even if first read fails
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}
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void loop() {
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delay(MEAS_INTERVAL_MS); // tickless idle -> System ON sleep
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measureAndPublish();
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}
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