Bosch Sensortec | BME680 Datasheet

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Modifications reserved |Data subject not change without notice | Printed in Germany

Document number: BST-BME680-DS001-09 Revision_1.9_022024

3.3.4 IIR filter

The environmental pressure is subject to many short-term changes, caused external disturbances. To suppress disturbances

(e.g. slamming of door or wind blowing into the sensor) in the output data without causing additional interface traffic and

processor work load, the BME680 features an internal IIR filter (see Section 5.3.2.4). It effectively reduces the bandwidth of

the temperature and pressure output signals and increases the resolution of the output data to 20 bit, noting that the humidity

and gas values inside the sensor does not fluctuate rapidly and does not require low pass filtering. The output of a next

measurement step is filtered using the following formula:

������������������������������[������] =

������������������������������[������ − 1] ∙ (������ − 1) + ������������������������

������

������������������������������[������ − 1] is the data coming from the current filter memory, and ������������������������ is the data coming from current ADC acquisition.

������������������������������[������] denotes the new value of filter memory and the value that will be sent to the output registers.

The IIR filter can be configured to different filter coefficients, which slows down the response to the sensor inputs. Note that

the response time with enabled IIR filter depends on the number of samples generated, which means that the data output

rate must be known to calculate the actual response time.

When writing to the register filter, the filter is reset. The next ADC values will pass through the filter unchanged and become

the initial memory values for the filter. If temperature or pressure measurements are skipped, the corresponding filter memory

will be kept unchanged even though the output registers are set to 0x80000. When the previously skipped measurement is

re-enabled, the output will be filtered using the filter memory from the last time when the measurement was not skipped. If

this is not desired, please write to the filter register in order to re-initialize the filter.

3.3.5 Gas sensor heating and measurement

The operation of the gas sensing part of BME680 involves two steps:

1. Heating the gas sensor hot plate to a target temperature (typically between 200 °C and 400 °C) and keep that

temperature for a certain duration of time.

2. Measuring the resistance of the gas sensitive layer.

Up to 10 different hot plate temperature set points can be configured by setting the registers res_heat_x<7:0>, where x =

0…9 .The internal heater control loop operates on the resistance of the heater structure. Hence, the user first needs to

convert the target temperature into a device specific target resistance before writing the resulting register code into the sensor

memory map.

Using the variables listed Table 14: Variable names and register addresses for res_heat_x calculation, the following code

will calculate register code that to be written to res_heat_x<7:0>. It is, however, recommended to use the sensor API

available on github (Chapter 4) for a friendlier user experience.

Floating point:

var1 = ((double)par_g1 / 16.0) + 49.0;

var2 = (((double)par_g2 / 32768.0) * 0.0005) + 0.00235;

var3 = (double)par_g3 / 1024.0;

var4 = var1 * (1.0 + (var2 * (double) target_temp));

var5 = var4 + (var3 * (double)amb_temp);

res_heat_x = (uint8_t)(3.4 * ((var5 * (4.0 / (4.0 + (double)res_heat_range)) * (1.0/(1.0 +

((double)res_heat_val * 0.002)))) - 25));

Integer:

var1 = (((int32_t)amb_temp * par_g3) / 10) << 8;

var2 = (par_g1 + 784) * (((((par_g2 + 154009) * target_temp * 5) / 100) + 3276800) / 10);

var3 = var1 + (var2 >> 1);

var4 = (var3 / (res_heat_range + 4));

var5 = (131 * res_heat_val) + 65536;

res_heat_x100 = (int32_t)(((var4 / var5) - 250) * 34);

res_heat_x = (uint8_t)((res_heat_x100 + 50) / 100);