Data Sheet

Bosch Sensortec | BME280 Data sheet
25 | 55
Modifications reserved | Data subject to change without notice
Document number: BST-BME280-DS002-15
Revision_1.6_092018
0xE7
dig_H6
signed char
4.2.3 Compensation formulas
Please note that it is strongly advised to use the API available from Bosch Sensortec to perform
readout and compensation. If this is not wanted, the code below can be applied at the user’s risk. Both
pressure and temperature values are expected to be received in 20 bit format, positive, stored in a 32
bit signed integer. Humidity is expected to be received in 16 bit format, positive, stored in a 32 bit
signed integer.
The variable t_fine (signed 32 bit) carries a fine resolution temperature value over to the pressure and
humidity compensation formula and could be implemented as a global variable.
The data type “BME280_S32_t” should define a 32 bit signed integer variable type and can usually be
defined as “long signed int”.
The data type “BME280_U32_t” should define a 32 bit unsigned integer variable type and can usually
be defined as “long unsigned int”.
For best possible calculation accuracy in pressure, 64 bit integer support is needed. If this is not
possible on your platform, please see appendix 8.2 for a 32 bit alternative.
The data type “BME280_S64_t” should define a 64 bit signed integer variable type, which on most
supporting platforms can be defined as “long long signed int”. The revision of the code is rev.1.1.
// Returns temperature in DegC, resolution is 0.01 DegC. Output value of “5123” equals 51.23
DegC.
// t_fine carries fine temperature as global value
BME280_S32_t t_fine;
BME280_S32_t BME280_compensate_T_int32(BME280_S32_t adc_T)
{
BME280_S32_t var1, var2, T;
var1 = ((((adc_T>>3) – ((BME280_S32_t)dig_T1<<1))) * ((BME280_S32_t)dig_T2)) >> 11;
var2 = (((((adc_T>>4) – ((BME280_S32_t)dig_T1)) * ((adc_T>>4) – ((BME280_S32_t)dig_T1)))
>> 12) *
((BME280_S32_t)dig_T3)) >> 14;
t_fine = var1 + var2;
T = (t_fine * 5 + 128) >> 8;
return T;
}
// Returns pressure in Pa as unsigned 32 bit integer in Q24.8 format (24 integer bits and 8
fractional bits).
// Output value of “24674867” represents 24674867/256 = 96386.2 Pa = 963.862 hPa
BME280_U32_t BME280_compensate_P_int64(BME280_S32_t adc_P)
{
BME280_S64_t var1, var2, p;
var1 = ((BME280_S64_t)t_fine) – 128000;
var2 = var1 * var1 * (BME280_S64_t)dig_P6;
var2 = var2 + ((var1*(BME280_S64_t)dig_P5)<<17);
var2 = var2 + (((BME280_S64_t)dig_P4)<<35);
var1 = ((var1 * var1 * (BME280_S64_t)dig_P3)>>8) + ((var1 * (BME280_S64_t)dig_P2)<<12);
var1 = (((((BME280_S64_t)1)<<47)+var1))*((BME280_S64_t)dig_P1)>>33;
if (var1 == 0)
{
return 0; // avoid exception caused by division by zero
}
p = 1048576-adc_P;
p = (((p<<31)-var2)*3125)/var1;
var1 = (((BME280_S64_t)dig_P9) * (p>>13) * (p>>13)) >> 25;
var2 = (((BME280_S64_t)dig_P8) * p) >> 19;
p = ((p + var1 + var2) >> 8) + (((BME280_S64_t)dig_P7)<<4);
return (BME280_U32_t)p;
}
// Returns humidity in %RH as unsigned 32 bit integer in Q22.10 format (22 integer and 10
fractional bits).
// Output value of “47445” represents 47445/1024 = 46.333 %RH
BME280_U32_t bme280_compensate_H_int32(BME280_S32_t adc_H)
{
BME280_S32_t v_x1_u32r;
v_x1_u32r = (t_fine – ((BME280_S32_t)76800));