User's Manual

UMXXX
Celsius degrees), +/-0.5degC (from 15 to 40 Celsius degrees); data rate 1, 7 or
12.5 Hz. Response time 15 s (time to 63%). Long term drift 0.05degC/year.
HTS221 ACQUISITION CHAIN
All components on the board, the MEMS sensors, the BlueNRG-2 processor, have an
embedded temperature sensor. These temperature sensors have been design to enable
the measurement of the temperature of the silicon, not the ambient temperature. On the
opposite, the temperature sensor embedded in HTS221 has the physical properties and the
accuracy required by the measurement of the ambient temperature.
The raw output of the humidity acquisition chain is stored in two 8bit registers
(HUMIDITY_OUT_H and _L) which must be concatenated to get a 16bit two’s complement
value (H_OUT). The raw output is already temperature-compensated. Calibrations
coefficients to go from raw humidity output to Relative Humidity (RH) are stored in the
device. Factory calibration is performed at two different humidity levels and one
temperature. This is the sequence to compute RH:
1. First true RH during calibration: H0_rH_x2, set RH0 = H0_rH_x2 / 2
2. Second true RH during calibration: H1_rH_x2, set RH1 = H0_rH_x2 / 2
3. First raw H output during calibration: H0_T0_OUT, set H0 = H0_T0_OUT
4. Second raw H output during calib.: H1_T0_OUT, set H1 = H1_T0_OUT
5. Current raw H output: H_OUT, set H = H_OUT
6. Current RH% by linear interpolation: RH = RH0 + (RH1-RH0) * (H-H0)/(H1-H0)
The raw output of the temperature acquisition chain is stored in two 8bit registers
(TEMP_OUT_H and _L) which must be concatenated to get a 16bit two’s complement value
(T_OUT). Calibration coefficients to go from raw temperature output to Temperature in
Celsius degrees are stored in the device. Factory calibration is performed at two different
temperatures. This is the sequence to compute Temperature in Celsius degrees:
1. MSB of true temperatures during calibration: T1T0MSB,
• set MSB1 = T1T0MSB & 0x03, MSB0 = T1T0MSB/4
2. First true T in Celsius deg. (two’s comp): T0_degC_x8,
• set T0degC = (T0_degC_x8 + MSB0*256)/8
3. Second true T in Celsius deg. (two’s comp): T1_degC_x8,
• set T1degC = (T1_degC_x8 + MSB1*256)/8
4. First raw T output during calibration: T0_OUT, set T0 = T0_OUT