scd30.tc¶
SCD30 CO2/Temperature/Humidity Sensor Driver
// SCD30 CO2/Temperature/Humidity Sensor Driver
// I2C address: 0x61
// Measures: CO2 (ppm), Temperature (C), Humidity (%RH)
// Data as IEEE754 floats, CRC8 poly 0x31, init 0xFF (Sensirion)
// Continuous measurement mode, interval 2s
int scd_addr = 0;
int scd_bus = 0;
int scd_ok = 0;
int scd_retry = 0;
float scd_co2 = 0.0;
float scd_temp = 0.0;
float scd_humi = 0.0;
float scd_dewp = 0.0;
float scd_absh = 0.0;
char scd_buf[20];
char scd_lbl[32];
// CRC8 Sensirion: poly 0x31, init 0xFF, over 2 data bytes
int scd_crc(int b1, int b2) {
int crc = 0xFF ^ b1;
int j = 0;
while (j < 8) {
if (crc & 0x80) crc = ((crc << 1) ^ 0x31) & 0xFF;
else crc = (crc << 1) & 0xFF;
j++;
}
crc = crc ^ b2;
j = 0;
while (j < 8) {
if (crc & 0x80) crc = ((crc << 1) ^ 0x31) & 0xFF;
else crc = (crc << 1) & 0xFF;
j++;
}
return crc;
}
// Send 16-bit command (no data)
void scd_cmd(int cmd) {
scd_buf[0] = cmd & 0xFF;
i2cWrite(scd_addr, cmd >> 8, scd_buf, 1, scd_bus);
}
// Send 16-bit command with 16-bit argument + CRC
void scd_cmd_arg(int cmd, int arg) {
scd_buf[0] = cmd & 0xFF;
scd_buf[1] = arg >> 8;
scd_buf[2] = arg & 0xFF;
scd_buf[3] = scd_crc(scd_buf[1], scd_buf[2]);
i2cWrite(scd_addr, cmd >> 8, scd_buf, 4, scd_bus);
}
// Reconstruct IEEE754 float from 6-byte response (2 words + 2 CRCs)
// Layout: [hi_MSB, hi_LSB, CRC, lo_MSB, lo_LSB, CRC]
// Returns float via intBitsToFloat, or 0.0 on CRC error
float scd_get_float(int offset) {
int hi_msb = scd_buf[offset];
int hi_lsb = scd_buf[offset + 1];
int lo_msb = scd_buf[offset + 3];
int lo_lsb = scd_buf[offset + 4];
// Validate both CRCs
if (scd_crc(hi_msb, hi_lsb) != scd_buf[offset + 2]) return 0.0;
if (scd_crc(lo_msb, lo_lsb) != scd_buf[offset + 5]) return 0.0;
int bits = (hi_msb << 24) | (hi_lsb << 16) | (lo_msb << 8) | lo_lsb;
return intBitsToFloat(bits);
}
// Dewpoint (Magnus formula), returns °C
float scd_calc_dewpoint(float t, float h) {
if (h <= 0.0) return 0.0;
float gamma = (17.271 * t) / (237.7 + t) + log(h / 100.0);
return (237.7 * gamma) / (17.271 - gamma);
}
// Absolute humidity in g/m³
float scd_calc_abshumi(float t, float h) {
float ah = 6.112 * exp((17.67 * t) / (t + 243.5)) * h * 2.1674;
return ah / (273.15 + t);
}
int scd_scan() {
int bus = 0;
while (bus < 2) {
if (i2cSetDevice(0x61, bus)) {
scd_addr = 0x61;
scd_bus = bus;
// Get firmware version to verify sensor (cmd 0xD100)
scd_cmd(0xD100);
delay(3);
if (i2cRead0(scd_addr, scd_buf, 3, scd_bus)) {
if (scd_crc(scd_buf[0], scd_buf[1]) == scd_buf[2]) {
// Set measurement interval to 2 seconds (cmd 0x4600, arg 2)
scd_cmd_arg(0x4600, 2);
delay(3);
// Start continuous measurement, no pressure compensation (cmd 0x0010, arg 0)
scd_cmd_arg(0x0010, 0);
delay(3);
i2cSetActiveFound(scd_addr, "SCD30", scd_bus);
return 1;
}
}
scd_addr = 0;
}
bus++;
}
return 0;
}
void EverySecond() {
if (!scd_addr) {
// Retry scan every 5 seconds if sensor wasn't found at boot
scd_retry++;
if (scd_retry >= 5) {
scd_retry = 0;
if (scd_scan()) {
char buf[48];
sprintf(buf, "SCD30 found at 0x%x on bus %d (retry)", scd_addr, scd_bus);
addLog(buf);
}
}
return;
}
// Check data ready (cmd 0x0202)
scd_cmd(0x0202);
delay(3);
if (!i2cRead0(scd_addr, scd_buf, 3, scd_bus)) return;
if (scd_crc(scd_buf[0], scd_buf[1]) != scd_buf[2]) return;
int ready = (scd_buf[0] << 8) | scd_buf[1];
if (!ready) return;
// Read measurement (cmd 0x0300) — 18 bytes: 3 floats × 6 bytes each
scd_cmd(0x0300);
delay(3);
if (!i2cRead0(scd_addr, scd_buf, 18, scd_bus)) return;
float co2 = scd_get_float(0);
float temp = scd_get_float(6);
float humi = scd_get_float(12);
// Sanity check CO2 (valid range ~400-10000 ppm)
if (co2 > 0.0) {
scd_co2 = co2;
scd_temp = temp;
scd_humi = humi;
scd_dewp = scd_calc_dewpoint(temp, humi);
scd_absh = scd_calc_abshumi(temp, humi);
scd_ok = 1;
}
}
void scd_web_label(int idx) {
char vt[32];
LGetString(idx, scd_lbl);
strcpy(vt, "{s}SCD30 ");
strcat(vt, scd_lbl);
strcat(vt, "{m}");
webSend(vt);
}
void WebCall() {
char vt[32];
if (scd_ok) {
scd_web_label(4);
sprintf(vt, "%.0f ppm{e}", scd_co2);
webSend(vt);
scd_web_label(0);
sprintf(vt, "%.1f °C{e}", scd_temp);
webSend(vt);
scd_web_label(1);
sprintf(vt, "%.1f %{e}", scd_humi);
webSend(vt);
scd_web_label(3);
sprintf(vt, "%.1f °C{e}", scd_dewp);
webSend(vt);
scd_web_label(20);
sprintf(vt, "%.1f g/m³{e}", scd_absh);
webSend(vt);
} else {
webSend("{s}SCD30{m}not ready{e}");
}
}
void JsonCall() {
if (!scd_ok) return;
char buf[64];
sprintf(buf, ",\"SCD30\":{\"CarbonDioxide\":%.0f", scd_co2);
responseAppend(buf);
sprintf(buf, ",\"Temperature\":%.1f", scd_temp);
responseAppend(buf);
sprintf(buf, ",\"Humidity\":%.1f", scd_humi);
responseAppend(buf);
sprintf(buf, ",\"DewPoint\":%.1f", scd_dewp);
responseAppend(buf);
sprintf(buf, ",\"AbsHumidity\":%.1f}", scd_absh);
responseAppend(buf);
}
void OnExit() {
if (scd_addr) {
// Stop continuous measurement (cmd 0x0104)
scd_cmd(0x0104);
delay(3);
I2cResetActive(scd_addr, scd_bus);
}
}
int main() {
scd_ok = 0;
scd_addr = 0;
if (scd_scan()) {
char buf[48];
sprintf(buf, "SCD30 found at 0x%x on bus %d", scd_addr, scd_bus);
addLog(buf);
} else {
addLog("SCD30 not found");
}
return 0;
}