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BME 688
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BOSCH BME 688 Datasheet

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BME688 – Datasheet
Document revision
1.0
Document release date
April, 2021
Document number
BST-BME688-DS000-00
Technical reference code(s)
0 273 017 016
Notes
Data and descriptions in this document are subject to change without notice. Product photos and pictures are for illustration purposes only and may differ from the real product appearance.
BME688
Digital low power gas, pressure, temperature & humidity sensor with AI
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BME688
Digital low power gas, pressure, temperature & humidity sensor with AI
The BME688 is the first gas sensor with Artificial Intelligence (AI) and integrated high-linearity and high-accuracy pressure, humidity and temperature sensors. It is housed in a robust yet compact 3.0 x 3.0 x 0.9 mm³ package and especially developed for mobile & connected applications where size and low power consumption are critical requirements. The gas sensor can detect Volatile Organic Compounds (VOCs), volatile sulfur compounds (VSCs) and other gases such as carbon monoxide and hydrogen in the part per billion (ppb) range.
Additionally to all features of the BME680, the BME688 has a gas scanner function. In standard configuration, the presence of VSCs is being detected as indicator for e.g. bacteria growth. And the gas scanner can be customized with respect to sensitivity, selectivity, data rate and power consumption as well. The BME AI-Studio tool enables customers to train the BME688 gas scanner on their specific application, like in home appliances, IoT products or Smart Home.
Typical applications
Indoor and outdoor air quality measurement Bad breath or spoiled food detection based on the measurement of volatile sulfur compounds, which are
an indicator of bacteria growth
Detection of unusual gases and smells, which might indicate for instance a leakage or fire Diaper state detection, e.g. for baby care Early detection of odors & bad smells Wildfire detection
Target Devices
Handsets such as mobile phones, tablet PCs, GPS devices Wearables e.g. smart watches or connected sensors Mobile / care devices like toothbrushes or breath checkers Smart home devices, e.g. smart speakers, connected sensors or lamps Room climate sensors & weather stations Home appliances Gaming, e.g. flying toys Connected devices, for instance for asset tracking or surveillance Sensor networks / sensor nodes
Videos, further documents & software is available online:
BME688: https://www.bosch-sensortec.com/products/environmental-sensors/gas-sensors/bme688/ Software: https://www.bosch-sensortec.com/software-tools/software/bme688-software/
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Key features
Package 3.0 mm x 3.0 mm x 0.93 mm metal lid LGA Digital interface I²C (up to 3.4 MHz) and SPI (3 and 4 wire, up to 10 MHz) Supply voltage V
DD
main supply voltage range: 1.71 V to 3.6 V
V
DDIO
interface voltage range: 1.2 V to 3.6 V
Current consumption 2.1 µA at 1 Hz humidity and temperature
3.1 µA at 1 Hz pressure and temperature
3.7 µA at 1 Hz humidity, pressure and temperature 90 µA at ULP mode for p/h/T & air quality 0,9 mA at LP mode for p/h/T & air quality 3,9 mA in standard gas scan mode
0.09‒12 mA for p/h/T/gas in customized operation modes
0.15 µA in sleep mode
Operating range -40‒+85 °C, 0‒100% r.H., 300‒1100 hPa The product is RoHS compliant, halogen-free, MSL1
Key parameters for gas sensor
F1 score for H
2
S scanning 0.92
Standard scan speed 10.8 s / scan Electric charge for standard scan 0.18 mAh (5 scans ~ 1 min) Power consumption < 0.1 mA in ultra-low power mode Output data processing Major direct outputs: Index for Air Quality (IAQ), bVOC- & CO
2
-equivalents (ppm),
Gas scan result (%) & Intensity level (all listed in Table 20: BSEC outputs)
Key parameters for humidity sensor
Response time 󰇛

󰇜 ~8 s
Accuracy tolerance ±3% r.H. Hysteresis ±1.5% r.H.
Key parameters for pressure sensor
RMS Noise 0.12 Pa, equiv. to 1.7 cm Offset temperature coefficient ±1.3 Pa/K, equiv. to ±10.9 cm at 1 °C temperature change
Key hardware & software tools for the BME688
BME688 Development KitBME AI-Studio software for use-case testing & development BSEC 2.x library for system µC Sensor API (driver) w/ code examples
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Table of contents
1. Specification 7
2. Absolute maximum ratings 15
3. Sensor usage 16
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4. Software 30
5. Global memory map and register description 32
6. Digital interfaces 45
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7. Pin-out and connection diagram 51
8. Legal disclaimer 58
9. Document history and modifications 59
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1. Specification
If not stated otherwise,
all values are valid over tbhe full voltage range, all minimum/maximum values are given for the full accuracy temperature range minimum/maximum values of drifts, offsets and temperature coefficients are ±3 values over lifetime, typical values of currents and state machine timings are determined at 25 °C, minimum/maximum values of currents are determined using corner lots over complete temperature range , and minimum/maximum values of state-machine timings are determined using corner lots over 0‒+65 °C temperature
range.
Besides the general electrical specifications, the following tables are separated for the gas, pressure, humidity and temperature functions of the BME688.
1.1 General Electrical Specification
Table 1: Electrical parameter specification
OPERATING CONDITIONS BME688
Parameter
Symbol
Condition
Min
Typ
Max
Unit
Supply Voltage
Internal Domains1
VDD
ripple max. 50 mVpp
1.71
1.8
3.6
V
Supply Voltage
I/O Domain
V
DDIO
1.2
1.6
3.6
V
Sleep current
I
DDSL
0.15
1
µA
Standby current
(inactive period of
normal mode)
I
DDSB
0.29
0.8
µA
Current during
humidity
measurement
I
DDH
Max value at 85 °C
340
450
µA
Current during
pressure
measurement
I
DDP
Max value at -40 °C
714
849
µA
Current during
temperature
measurement
I
DDT
Max value at 85 °C
350
µA
Start-up time
t
startup
Time to first communication after both VDD > 1.58 V and
V
DDIO
> 0.65 V
2
ms
Power supply
rejection ratio (DC)
PSRR
full VDD range
±0.01
±5
%r.H./V
Pa/V
Standby time
accuracy
Δt
standby
±5
±25
%
1
The power efficiency, performance and heat dissipation scales with the applied supply voltage. The BME688 is optimized for 1.8 V.
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1.2 Gas sensor specification
The tables 2-5 list the gas sensor specification. All the parameters are deduced from lab measurements under controlled environmental conditions, which are chosen according to the ISO16000-29 standard “Test methods for VOC detectors”. The detailed procedure to measure the gas sensor is available in the application notes AN012 (“Measurement Instructions for Lab Environment”) or AN016 (“handbook”).
Referring to chapter 4, a software solution (BSEC: Bosch Software Environmental Cluster) is available for the BME688. The software is carefully engineered to seamlessly work with the 4 integrated sensors inside the BME688. Based on intelligent algorithms, the BSEC provides numerous useful outputs (see Table 20), e.g. an Index for Air Quality (IAQ), bVOC- & CO2­equivalents (ppm) and Gas scan results (%). The IAQ output is an index that can have values between 0 and 500 with a resolution of 1 to indicate or quantify the quality of the air available in the surrounding. Table 3 lists the IAQ system specification. The detailed classification and colour coding of the IAQ index is described in Table 6. Furthermore, the BSEC solution supports different operation modes (see Table 19) for the gas sensor to address the necessary power budget and update rate requirements of the end-application. Unless mentioned otherwise, the specifications are deduced from new sensors that have been operated for at least five days mainly in ambient air and consequently have the same history (i.e. same power mode and exposed to the same environment).
The sensors are tested especially with hydrogen sulfide (H2S), ethanol (“EtOH”), carbon monoxide (CO) and breath-VOCs (“b-VOC”). The b-VOC mixture, as listed in Table 7, represents the most important compounds in an exhaled breath of healthy humans. The values are derived from several publications on breath analysis studies. The composition does not contain species which would chemically react to ensure that the mixture is stable for at least 6 months. Furthermore, the composition is also limited to species which can be manufactured in one mixture.
Table 2: Gas sensor parameter specification
Parameter
Symbol
Condition
Min
Typ
Max
Unit
Operational range1
-40 85
°C
10 95
% r.H.
Supply Current during
heater operation
IDD
Heater target temperature
320 °C, constant operation
(VDD ≤ 1.8 V, 25°C)
9
12
13
mA
Peak Supply Current
I
Peak
occurs within first ms of
switching on the hotplate
15
17
18
mA
Average Supply
Current
(VDD ≤ 1.8 V, 25°C)
I
DD,IAQ
Ultra-low power mode
0.09
mA
Low power mode
0.9
mA
Continuous mode
12 mA
Response time2
(brand-new sensors)
τ
33-63%
Ultra-low power mode
92 s
τ
33-63%
Low power mode
1.4 s
τ
33-63%
Continuous mode
0.75 s
Resolution of gas sensor resistance
measurement
0.05
0.08
0.11
%
Noise in gas sensor
resistance (RMS)
NR
1.5 %
1
The sensors are electrically operable within this range. Actual performance may vary.
2
Response time of unsoldered, brand-new sensors extracted from the sensors’ resistance change in response to a 0.6–60 ppm step of EtOH and a 0.5–15 ppm step of b-VOC at 20 °C, 50%
r.H. and atmospheric pressure.
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Table 3: IAQ system parameter specification3
Parameter
Symbol
Condition
Min
Typ
Max
Unit
Accuracy status4
AX
IAQ
Android compatible
0 3 IAQ Resolution
IAQrs 1
IAQ Range
IAQrg 0 500
Sensor-to-sensor
deviation5
IAQ
S2S
All operation modes
±15%
±15
Durability to
siloxanes
6,7,8
IAQ
S2S
Sensor-to-sensor deviation
±15%
±15
IAQ
drift
Drift at low & high
concentrations
±1%
±4
3
IAQ parameters only apply for the combination of BME688 together with the Bosch Software Environmental Cluster (BSEC) solution (available for download, see chapter 4)
4
The accuracy status is equal to zero during the power-on stabilization times of the sensor and is equal to 3 when the sensor achieves best performance
5
Tested with 0.6–90 ppm of EtOH at 5–40 °C, 20–80% r.H. and atmospheric pressure. Condition is valid after the calibration period of the BSEC algorithm.
6
Siloxanes are present in a typical indoor environment and can in principle perturb the metal-oxide-based gas sensor performance.
7
220 hours of 700 mg/m3 of octamethylcyclotetrasiloxane (D4) in ambient conditions (i.e. 20 °C and 50% r.H.) simulates the amount of siloxanes in a typical indoor environment over
more than 10 years.
8
Tested with 0.5–15 ppm of b-VOC at 20 °C and 50% r.H. using continuous operation mode for 220 hours of 700 mg/m3 of octamethylcyclotetrasiloxane (D4).
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Table 4: Standard gas scanner mode for H2S selectivity9
Parameter
Symbol
Condition
Min
Typ
Max
Unit
Average Supply
Current
(VDD ≤ 1.8 V, 25°C)
I
DD,IAQ
Gas scanner standard mode
consists of
5 scan cycles and 10
sleeping cycles
3.96
mA
Gas scanning power
consumption in ampere
hours
Ah
scan
Electric charge for one
scanning phase (5 scan
cycles in standard gas
scanner mode)
0.18
mAh
Duty cycle of standard gas
scanner mode
t
standard scan
Duration of gas scanning
phase
54 s
f
standard scan
ODR during scanning phase
1/10.8
1/s
t
standard sleep
Duration of sleeping phase
108
s
f
standard sleep
ODR during sleeping phase
0
1/s
Gas scan cycle
t
standard cycle
Duration of gas scan cycle
10.8
s
Selectivity to H2S
P
H2S
Probability of H2S and non
H2S class
0 1
Resolution of probability
0.01
S
H2S
Sensitivity (recall) of H2S
classification
0.90
F1
H2S
F1 score of H2S
classification
0.92
D
S/F1
Average long-term drift10 of
Sensitivity / F1 score
0.012
/year
IAQ
IAQrg 0 500
9
The classification performance for VSCs was developed based on comprehensive lab tests with H2S concentrations between 100 and 500 ppb (i.e. 0.1 – 0.5 ppm) as representative for sulfur compounds and tested at 5–40 °C, 20–80% r.H. and atmospheric pressure. The selectivity was approved by testing against the presence of bVOCs up to 15 ppm, Ethanol up to 15 pm and Carbon monoxide up to 50 ppm. The specification is based on BSEC 2.0.6.1.
10
Typical value for operation in normal room air temperature/humidity range derived from JEDEC standard JESD22-A108D. Software-compensated on sensor component level by considering the performance of aged sensors. Other environment conditions or contaminations (e.g. outgassing, adhesives or packaging materials) might lead to different values. Please see the Handling, Soldering and Mounting Instructions (HSMI) for details.
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Table 5: Customization of gas scanner mode11
Parameter
Symbol
Condition
Min
Typ
Max
Unit
Classification
Number of Classes
2 4
Number of temperature
steps
10
Duration of gas scan
cycle
10 to 30
sec
Table 6: Index for Air Quality (IAQ) classification and color-coding12
IAQ Index
Air Quality
Impact (long-term exposure)
Suggested action
0 – 50
Excellent
Pure air; best for well-being
No measures needed
51 – 100
Good
No irritation or impact on well-being
No measures needed
101 – 150
Lightly polluted
Reduction of well-being possible
Ventilation suggested
151 – 200
Moderately polluted
More significant irritation possible
Increase ventilation with clean air
201 – 2509
Heavily polluted
Exposition might lead to effects like headache depending on type of VOCs
optimize ventilation
251 – 350
Severely polluted
More severe health issue possible if harmful VOC present
Contamination should be identified if level is reached even w/o presence of people; maximize ventilation & reduce attendance
> 351
Extremely polluted
Headaches, additional neurotoxic effects possible
Contamination needs to be identified; avoid presence in room and maximize ventilation
Table 7: bVOC mixture with Nitrogen as carrier gas
Molar fraction
Compound
Production tolerance
Certified accuracy
5 ppm
Ethane
20 %
5 %
10 ppm
Isoprene /2-methyl-1,3 Butadiene
20 %
5 %
10 ppm
Ethanol
20 %
5 %
50 ppm
Acetone
20 %
5 %
15 ppm
Carbon Monoxide
10 %
2 %
11
For customization we recommend to use the BME AI-Studio software with comprehensive documentation (see Table 18: Software components).
12
According to the guidelines issued by the German Federal Environmental Agency, exceeding 25 mg/m3 of total VOC leads to headaches and further neurotoxic impact on health. The BSEC
software auto-calibrates the low and high concentrations applied during testing to IAQ of 25 and 250, respectively.
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1.3 Humidity sensor specification
Table 8: Humidity parameter specification
Parameter
Symbol
Condition
Min
Typ
Max
Unit
Operating Range13
-40
25
85
°C
0 100
% r.H.
Full accuracy range
0 65
°C
10 90
% r.H.
Supply Current
I
DD,H
1 Hz forced mode,
temperature and humidity
measurement
2.1
2.8
µA
Absolute Accuracy
AH
20‒80 % r.H., 25 °C,
including hysteresis
±3
% r.H.
Hysteresis14
HH
10→90→10 % r.H., 25°C
±1.5
% r.H.
Nonlinearity15
NLH
10→90 % r.H., 25°C
1.7
% r.H.
Response time to
complete 63% of step16
τ
0-63%
N2 (dry) → 90 % r.H., 25°C
8
s
Resolution
R
H
0.008
% r.H.
Noise in humidity
(RMS)
NH
Highest oversampling
0.01
% r.H.
Long-term stability
∆H
stab
10‒90 % r.H., 25°C
0.5
% r.H./
year
13
When exceeding the operating range (e.g. for soldering), humidity sensing performance is temporarily degraded and reconditioning is recommended as described in Section 7.7. Operating
range only for non-condensing environment.
14
For hysteresis measurement the sequence 0103050709070503010 % r.H. is used. The hysteresis is defined as the maximum difference between measurements at of
the same humidity up / down branch and the averaged curve of both branches.
15
Non-linear contributions to the sensor data are corrected during the calculation of the relative humidity by the compensation formulas described in Section 3.5 and following.
16
The air-flow in direction to the vent-hole of the device has to be dimensioned in a way that a sufficient air exchange inside to outside will be possible. To observe effects on the response
time-scale of the device an air-flow velocity of approximately 1 m/s is needed.
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1.4 Pressure sensor specification
Table 9: Pressure parameter specification
Parameter
Symbol
Condition
Min
Typ
Max
Unit
Operating temperature
range
TA
operational
-40
25
85
°C
full accuracy
0 65
Operating pressure
range
P
full accuracy
300
1100
hPa
Supply current
I
DD,LP
1 Hz forced mode,
pressure and temperature,
lowest power
3.1
4.2
µA
Temperature coefficient
of offset17
TCOP
25‒40 °C, 900 hPa
±1.3
Pa/K
±10.9
cm/K
Absolute accuracy
pressure
A
p, full
300‒1100 hPa
0‒65°C
±0.6
hPa
Relative accuracy
pressure
A
rel
700‒900hPa,
25‒40 °C, at constant
humidity
±0.12
hPa
A
rel
900‒1100hPa
25‒40 °C, at constant
humidity
±0.12
hPa
Resolution of
pressure output data
RP
Highest oversampling
0.18 Pa
Noise in pressure
N
P,fullBW
Full bandwidth, highest
oversampling
1.4 Pa
11 cm
Reduced bandwidth, highest
oversampling
0.2 Pa
1.7 cm
Solder drift
Minimum solder height 50µm
-0.5
1.2
+2.0
hPa
Long-term stability18
P
stab
per year
±1.0
hPa
Possible sampling rate
f
sample_P
Lowest oversampling,
see chapter 3.6.2
157
182 Hz
17
When changing temperature from 25 °C to 40 °C at constant pressure / altitude, the measured pressure / altitude will change by (15×TCOP).
18
Long-term stability is specified in the full accuracy operating pressure range 0‒65 °C
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1.5 Temperature sensor specification
Table 10: Temperature parameter specification
Parameter
Symbol
Condition
Min
Typ
Max
Unit
Operating temperature
range
TA
operational
-40
25
85
°C
Supply current
I
DD,T
1 Hz forced mode,
temperature measurement
only
1.0 µA
Absolute accuracy
temperature19
A
T,25
25 °C
±0.5 °C
A
T,full
0‒65 °C
±1.0 °C
Output resolution
RT
API output resolution
0.01 °C
RMS noise
NT
Lowest oversampling
0.005
°C
19
Temperature measured by the internal temperature sensor. This temperature value depends on the PCB temperature, sensor element self-heating and ambient temperature and is typically
above ambient temperature.
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2. Absolute maximum ratings
The absolute maximum ratings are determined over the complete temperature range using corner lots. The values are provided in Table 11.
Table 11: Absolute maximum ratings
Parameter
Condition
Min
Max
Unit
Voltage at any supply pin
VDD and V
DDIO
pin
-0.3
4.25
V
Voltage at any interface pin
-0.3
V
DDIO
+ 0.3
V
Storage temperature
≤ 65% r.H.
-45
+85
°C
Pressure
0
20 000
hPa
ESD
HBM, at any pin
±2
kV
Machine model
±200
V
Condensation
No power supplied
Allowed
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3. Sensor usage
3.1 Use cases
The key features of the BME688 hardware-software system are:
Selectivity to target gas classes Provision for custom usecase develpment Calculation of index for air quality (IAQ) level outside of the device Calculation of ambient air temperature outside of the device (e.g. phone) Calculation of ambient relative humidity outside of the device
3.2 Selectivity
Selectivity to target gas classes will enable wide variety of use cases for customers. From a gas sensing perspective selectivity is the ability to classify different gas compositions (with different combinations and concentrations of gases) that occur in different situations. Operating the metal oxide layer of the gas sensor at different temperatures enable measuring with different sensitivities and can create a unique fingerprint for numerous gas compositions. The standard heater profile is as shown below figure is executed in sensor ASIC which consists of 10 heater temperature steps. Features interpreted by AI based software brings selectivity feature.
Figure 1: Standard Heater profile
3.3 Indoor-air-quality
The BME688 is a metal oxide-based sensor that detects gases by adsorption (and subsequent oxidation/reduction) on its sensitive layer. The intensity of the signal typically scales with the chemical reactivity of the gases. The BME688 reacts to most volatile compounds as well as many other gases polluting indoor air (exceptions are very few gases which are chemically pretty inert, e.g. N2 and CO2). In contrast to sensors selective for one specific component, the BME688 is capable of measuring the sum of VOCs/contaminants in the surrounding air. This enables the BME688 to detect e.g. outgasing from paint, furniture and/or garbage, high VOC levels due to cooking, food consumption, exhaled breath and/or sweating.
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As a raw signal, the BME688 will output the gas sensor resistance values and its changes due to varying VOC concentrations (the higher the concentration of reducing VOCs, the lower the resistance and vice versa). Since this raw signal is influenced by parameters other than VOC concentration (e.g. humidity level), the raw values are transformed to an index for air quality (IAQ) by smart algorithms inside BSEC.
The IAQ scale ranges from 0 (clean air) to 500 (heavily polluted air). During operation, the algorithms automatically calibrate and adapt themselves to the typical environments where the sensor is operated (e.g., home, workplace, inside a car, etc.). This automatic background calibration ensures that users experience consistent IAQ performance. The calibration process considers the recent measurement history (typ. up to four days, configurable) to ensure that IAQ ~ 25 corresponds to “typical good” air and IAQ ~ 250 indicates “typical polluted” air.
3.4 Sensor modes
The sensor supports low-level power modes: sleep, parallel and forced mode. These modes can be selected using the mode<1:0> control register (see Section 5.3.1.3). The key differences between the modes are summarized in Table 12. Measurement cycle is abbreviated as TPHG (Temperature, Pressure, Humidity and Gas) in the following descriptions.
After a power-up sequence, the sensor automatically starts in sleep mode. If the device is currently performing a measurement, execution of mode switching commands is delayed until the end of the currently running measurement period. It is important to note that, further mode change commands or other write commands to the control registers are ignored until the mode change command has been executed. All control registers should be set to the desired values before writing to the mode register.
Table 12: Low-level operation modes
Operation mode
mode<1:0>
Key features
Sleep
00
No measurements are performed Minimal power consumption
Forced mode
01
Single TPHG cycle is performed Sensor automatically returns to sleep mode afterwards Gas sensor heater only operates during gas measurement
Parallel mode
10
Multiple TPHG cycles are performed Sensor will not automatically returns to sleep mode Gas sensor heater operates in parallel to TPH measurement
In forced mode, temperature, pressure, humidity and gas conversion are performed sequentially for 1 heater temperature step and heating duration. In parallel mode, TPH measurements and gas conversion are performed parallelly. Hence in parallel mode at least one of T,P,H is expected to be enabled. Up to 10 temperature steps and heating durations for the gas sensor hot plate can be stored in the sensor registers and can be executed as heater profile. Heater profiles can be repeated without host support. In the Figure 1, heater steps and the corresponding measurements are identified as G0 – G9.
Figure 2 illustrates the handling of these measurement sequences and the gas sensor hot plate is heated for the forced mode and parallel mode.
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T P H
G0
T P H
G0 T P H G1 T P H G2
Legend
Forced mode
Sleep mode
Heater operation
Parallel mode
...
Heating G0
Heating G1
Heating G2
ADC Operation
Figure 2: Sequence of ADC and gas sensor heater operation
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3.5 Sensor configuration
Quick start – Forced mode
The sensor is configured by writing to a set of control registers (see Chapter 5 for a detailed list of all available registers and their descriptions). This section illustrates, with the help of a basic step-by-step example, how to configure the sensor for simple forced mode measurements with a single heater step. For a more detailed description of the measurement flow, please refer to Section 3.6.
In this example, the sensor will be configured to use 2x oversampling for its temperature measurements, 16x oversampling for the pressure signal, and 1x oversampling for humidity. Moreover, the gas sensor hot plate will be configured to be heated for 100 ms at 300 °C before the gas measurement is performed.
First, the user must configure the oversampling settings for temperature, pressure and humidity by setting the control registers osrs_t<2:0> and osrs_h<2:0>, respectively. Supported settings range from 16x oversampling down to 0x, which is equivalent to skipping the corresponding sub-measurement. See Section 5.3.3 for further details.
1. Set humidity oversampling to 1x by writing 0b001 to osrs_h<2:0>
2. Set temperature oversampling to 2x by writing 0b010 to osrs_t<2:0>
3. Set pressure oversampling to 16x by writing 0b101 to osrs_p<2:0>
It is highly recommended to set first osrs_h<2:0> followed by osrs_t<2:0> and osrs_p<2:0> in one write command (see Section 3.6).
Next, the user shall set at least one gas sensor hot plate temperature step and heating duration. Up to 10 heating duration can be configured through the control registers gas_wait_x<7:0>, where x ranges from 0 to 9. See Section 5.3.4 for definition of register content. The corresponding heater step are stored in the registers res_heat_x<7:0>. Section 3.6.5 explains how to convert the target heater temperature, e.g. 300 °C, into a register code. For forced mode operation, the used heater step is selected by setting the control register nb_conv<3:0> to the heater profile to be used, e.g. to use gas_wait_0<7:0> and res_heat_0<7:0>, nb_conv<3:0> shall be set to 0x0. Finally, gas functionality shall be enabled by setting the run_gas_l bit to
1.
4. Set gas_wait_0<7:0> to 0x59 to select 100 ms heat up duration
5. Set the corresponding heater step by writing the target heater resistance to res_heat_0<7:0>
6. Set nb_conv<3:0> to 0x0 to select the previously defined heater settings
7. Set run_gas_l to 1 to enable gas measurements
Now, a single forced mode measurement with the above settings can be triggered by writing 0b01 to mode<1:0>. For more details on data readout, please see Section 5.3.1.3.
8. Set mode<1:0> to 0b01 to trigger a single measurement.
Please find abstracted example code using BME68x-Sensor-API in https://github.com/BoschSensortec/BME68x-Sensor-
API/blob/master/examples/forced_mode/forced_mode.c
Sensor configuration flow – Forced mode
Figure 3 illustrates which control registers must be set. For details on the individual control registers, please refer to Chapter 5. Moreover, details on the measurement flow for the individual modes can be found in Section 3.6.
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Figure 3: Sensor configuration flow in Forced Mode
Quick start – Parallel mode
Please find abstracted example code using BME68x-Sensor-API in https://github.com/BoschSensortec/BME68x-Sensor-
API/tree/master/examples/parallel_mode
Sensor configuration flow – Parallel mode
Figure 3 illustrates which control registers must be set. For details on the individual control registers, please refer to Chapter 5. Moreover, details on the measurement flow for the individual modes can be found in Section 3.6.
Forced Mode
Select oversampling for T, P and H
• Set osrs_x<2:0>
Select IIR filter for temperature sensor
• Set filter<2:0>
Enable gas coversion
• Set run_gas to 1
Select index of heater step
• Set nb_conv <3:0> (from 0 to 9)
Define heater-on time
• Convert duration to register code
• Set gas_wait_x<7:0> (time base unit is ms)
Set heater temperature
• Convert temperature to register code
• Set res_heat_x<7:0>
Set mode to forced mode
• Set mode<1:0> to 0b01
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Figure 4: Sensor configuration flow in Parallel Mode
Parallel Mode
Select oversampling for T, P and H
• Set osrs_x<2:0>
Select IIR filter for temperature sensor
• Set filter<2:0>
Enable gas coversion
• Set run_gas to 1
Select number of steps in heater profile
• Set nb_conv <3:0> (from 1 to 10)
Set heater-on time of each step
• Calculate required wait time before each TPHG measurement and set gas_wait_shared<7:0>
• Calculate number of TPHG repitions needed and set
gas_wait_x<7:0> (up to 255)
Set heater temperature of each step
• Convert heater temperature to register code
• Set res_heat_x<7:0>
Set mode to parallel mode
• Set mode<1:0> to 0b10
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Figure 5: Illustration of parallel measurement mode, an example with 3 step heater profile
3.6 Measurement flow
Referring to Figure 2, the BME688 measurement period consists of a temperature, pressure and humidity measurement with selectable oversampling. Moreover, it contains a heating phase for the gas sensor hot plate as well as a measurement of the gas sensor resistance. In parallel mode there are 3 data registers naming Field 0, Field 1 and Field 2 which will be filled in first in first out order.
After the measurement period, the pressure and temperature data can be passed through an optional IIR filter, which removes short-term fluctuations. For humidity and gas, such a filter is not needed and has not been implemented.
Temperature measurement
Temperature measurement can be enabled or skipped. Skipping the measurement is typically not recommended since temperature information is used to compensate temperature influences in the other parameters. When enabled, several oversampling options exist. The temperature measurement is controlled by the osrs_t<2:0> setting which is detailed in Section 5.3.3.2. For the temperature measurement, oversampling is possible to reduce the noise. The resolution of the temperature data depends on the IIR filter (see Section 5.3.3.4) and the oversampling setting:
When the IIR filter is enabled, the temperature resolution is 20 bit When the IIR filter is disabled, the temperature resolution is 16 + (osrs_t – 1) bit, e.g. 18 bit when osrs_t is set to ‘3’
Using the variables listed in Table 13, the following code will calculate the compensated temperature value (in degrees Celsius). It is, however, recommended to use the sensor API available on github (Chapter 0) for a friendlier user experience.
measuring
gas measuring
new data flag
gas meas index sub meas index meas sequence wait T P H G0 wait T P H G wait T P H G1 wait T P H G2 Heater profile
gas_wait_shared
Number of steps
Heating step 0
Heating step 1
Heating step 2
0112012
3
gas_wait_0=1
gas_wait_1=2
gas_wait_2=1
Data read by host
Data read by host
nb_conv = 3
Field 0 data updated
Field 0 status updated
Field 1 data updated
Field 1 status updated
Field 2 data updated
Field 2 status updated
Field 0 data updated
Field 0 status updated
Dummy gas conversion
Write '10' to mode register to start
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Floating point:
var1 = (((double)temp_adc / 16384.0) - ((double)par_t1 / 1024.0)) * (double)par_t2; var2 = ((((double)temp_adc / 131072.0) - ((double)par_t1 / 8192.0)) * (((double)temp_adc / 131072.0) - ((double)par_t1 / 8192.0))) * ((double)par_t3 * 16.0); t_fine = var1 + var2; temp_comp = t_fine / 5120.0;
Integer:
var1 = ((int32_t)temp_adc >> 3) - ((int32_t)par_t1 << 1); var2 = (var1 * (int32_t)par_t2) >> 11; var3 = ((((var1 >> 1) * (var1 >> 1)) >> 12) * ((int32_t)par_t3 << 4)) >> 14; t_fine = var2 + var3; temp_comp = ((t_fine * 5) + 128) >> 8;
where
par_t1, par_t2 and par_t3 are calibration parameters, temp_adc is the raw temperature output data, see 5.3.5.2
temp_comp is the compensated temperature output data in degrees Celsius.
Table 13: Variable names and register addresses for temp_comp calculation
Variable name
Register address (LSB / MSB)
par_t1
0xE9 / 0xEA
par_t2
0x8A / 0x8B
par_t3
0x8C
temp_adc
Field 0 - 0x24<7:4> / 0x23 / 0x22 Field 1 - 0x35<7:4> / 0x34 / 0x33 Field 2 - 0x46<7:4> / 0x45 / 0x44
Pressure measurement
Pressure measurement can be enabled or skipped. When enabled, several oversampling options exist. The pressure measurement is controlled by the osrs_p<2:0> setting which is detailed in Section 5.3.3. For the pressure measurement, oversampling is possible to reduce noise. The resolution of the pressure data depends on the IIR filter (see Section 5.3.3.4) and the oversampling setting:
When the IIR filter is enabled, the pressure resolution is 20 bit When the IIR filter is disabled, the pressure resolution is 16 + (osrs_p – 1) bit, e.g. 18 bit when osrs_p is set to ‘3’
Using the variables listed in Table 14 the following code will calculate the compensated pressure value (in Pascal). It is, however, recommended to use the sensor API available on github for a friendlier user experience.
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Floating point:
var1 = ((double)t_fine / 2.0) - 64000.0; var2 = var1 * var1 * ((double)par_p6 / 131072.0); var2 = var2 + (var1 * (double)par_p5 * 2.0); var2 = (var2 / 4.0) + ((double)par_p4 * 65536.0); var1 = ((((double)par_p3 * var1_p * var1) / 16384.0) + ((double)par_p2 * var1_p)) / 524288.0; var1 = (1.0 + (var1 / 32768.0)) * (double)par_p1; press_comp = 1048576.0 - (double)press_adc; press_comp = ((press_comp - (var2_p / 4096.0)) * 6250.0) / var1; var1 = ((double)par_p9 * press_comp * press_comp) / 2147483648.0; var2 = press_comp * ((double)par_p8 / 32768.0); var3 = (press_comp / 256.0) * (press_comp / 256.0) * (press_comp / 256.0) * (par_p10 / 131072.0); press_comp = press_comp + (var1_p + var2_p + var3_p + ((double)par_p7 * 128.0)) / 16.0;
Integer:
var1 = ((int32_t)t_fine >> 1) - 64000; var2 = ((((var1 >> 2) * (var1 >> 2)) >> 11) * (int32_t)par_p6) >> 2; var2 = var2 + ((var1 * (int32_t)par_p5) << 1); var2 = (var2 >> 2) + ((int32_t)par_p4 << 16); var1 = (((((var1 >> 2) * (var1 >> 2)) >> 13) * ((int32_t)par_p3 << 5)) >> 3) + (((int32_t)par_p2 * var1) >> 1);
var1 = var1 >> 18; var1 = ((32768 + var1) * (int32_t)par_p1) >> 15; press_comp = 1048576 - press_raw; press_comp = (uint32_t)((press_comp - (var2 >> 12)) * ((uint32_t)3125)); if (press_comp >= (1 << 30)) press_comp = ((press_comp / (uint32_t)var1) << 1); else press_comp = ((press_comp << 1) / (uint32_t)var1); var1 = ((int32_t)par_p9 * (int32_t)(((press_comp >> 3) * (press_comp >> 3)) >> 13)) >> 12; var2 = ((int32_t)(press_comp >> 2) * (int32_t)par_p8) >> 13; var3 = ((int32_t)(press_comp >> 8) * (int32_t)(press_comp >> 8) * (int32_t)(press_comp >> 8) * (int32_t)par_p10) >> 17; press_comp = (int32_t)(press_comp) + ((var1 + var2 + var3 + ((int32_t)par_p7 << 7)) >> 4);
where
par_p1, par_p2, …, par_p10 are calibration parameters, press_adc is the raw pressure output data, see 5.3.5.1 press_comp is the compensated pressure output data in Pascal.
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Table 14: Variable names and register addresses for press_comp calculation
Variable name
Register address (LSB / MSB)
par_p1
0x8E / 0x8F
par_p2
0x90 / 0x91
par_p3
0x92
par_p4
0x94 / 0x95
par_p5
0x96 / 0x97
par_p6
0x99
par_p7
0x98
par_p8
0x9C / 0x9D
par_p9
0x9E / 0x9F
par_p10
0xA0
press_adc
Field 0 - 0x21<7:4> / 0x20 / 0x1F Field 1 - 0x32<7:4> / 0x31 / 0x30 Field 2 - 0x43<7:4> / 0x42 / 0x41
Humidity measurement
The humidity measurement can be enabled or skipped. When enabled, several oversampling options exist. The humidity measurement is controlled by the osrs_h<2:0> setting, which is described in detail in Section 5.3.3.1. For the humidity measurement, oversampling is possible to reduce noise. The resolution of the humidity measurement is fixed at 16 bit ADC output.
Using the variables listed in Table 15, the following code will calculate the compensated humidity value (in percent). It is, however, recommended to use the sensor API available on github for a friendlier user experience.
Floating point:
var1 = hum_adc - (((double)par_h1 * 16.0) + (((double)par_h3 / 2.0) * temp_comp)); var2 = var1 * (((double)par_h2 / 262144.0) * (1.0 + (((double)par_h4 / 16384.0) * temp_comp) + (((double)par_h5 / 1048576.0) * temp_comp * temp_comp))); var3 = (double)par_h6 / 16384.0; var4 = (double)par_h7 / 2097152.0; hum_comp = var2 + ((var3 + (var4 * temp_comp)) * var2 * var2);
Integer:
temp_scaled = (int32_t)temp_comp; var1 = (int32_t)hum_adc - (int32_t)((int32_t)par_h1 << 4) – (((temp_scaled * (int32_t)par_h3) / ((int32_t)100)) >> 1); var2 = ((int32_t)par_h2 * (((temp_scaled * (int32_t)par_h4) / ((int32_t)100)) + (((temp_scaled * ((temp_scaled * (int32_t)par_h5) / ((int32_t)100))) >> 6) / ((int32_t)100)) + ((int32_t)(1 << 14)))) >> 10;
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var3 = var1 * var2; var4 = (((int32_t)par_h6 << 7) + ((temp_scaled * (int32_t)par_h7) / ((int32_t)100))) >> 4; var5 = ((var3 >> 14) * (var3 >> 14)) >> 10; var6 = (var4 * var5) >> 1; hum_comp = (var3 + var6) >> 12; hum_comp = (((var3 + var6) >> 10) * ((int32_t) 1000)) >> 12;
where
par_h1, par_h2, …, par_h7 are calibration parameters, hum_adc is the raw humidity output data, see 5.3.5.3 hum_comp is the compensated humidity output data in percent.
Table 15: Variable names and register addresses for hum_comp calculation
Variable name
Register address (LSB / MSB)
par_h1
0xE2<3:0> / 0xE3
par_h2
0xE2<7:4> / 0xE1
par_h3
0xE4
par_h4
0xE5
par_h5
0xE6
par_h6
0xE7
par_h7
0xE8
hum_adc
Field 0 - 0x26 / 0x25 Field 1 - 0x37 / 0x36 Field 2 - 0x48 / 0x47
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 BME688 features an internal IIR filter (see Section 5.3.3.4). 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:

󰇟󰇠 

󰇟
  󰇠󰇛  󰇜 


󰇟  󰇠 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.
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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.
Gas sensor heating and measurement
The operation of the gas sensing part of BME688 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 in Table 16, 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 0) 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);
where
par_g1, par_g2, and par_g3 are calibration parameters, target_temp is the target heater temperature in degree Celsius, amb_temp is the ambient temperature (hardcoded or read from temperature sensor), var5 is the target heater resistance in Ohm, res_heat_x is the decimal value that needs to be stored in register, where ‘x’ corresponds to the temperature profile
number between 0 and 9,
res_heat_range is the heater range stored in register address 0x02 <5:4>, and res_heat_val is the heater resistance correction factor stored in register address 0x00 (signed, value from -128 to
127).
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Table 16: Variable names and register addresses for res_heat_x calculation
Variable name
Register address (LSB / MSB)
par_g1
0xED
par_g2
0xEB/0xEC
par_g3
0xEE
res_heat_range
0x02 <5:4>
res_heat_val
0x00
For each of the 10 temperature steps, the heating duration must be specified. Referring to Figure 2, the heating phase starts after the temperature, pressure and humidity measurements are complete. This means there is no heating in parallel to these measurements, which is desirable to minimize undesired cross-influences between the various sensor components.
The heating duration is specified by writing to the corresponding gas_wait_x<7:0> control register. Heating durations between 1 ms and 4032 ms can be configured. In practice, approximately 20–30 ms are necessary for the heater to reach the intended target temperature.
3.7 Data readout
The procedure goes as follows, the new_data bit (see Section 5.3.6.1) can be checked to see if a new data is generated. If gas measurements are performed the gas_valid_r (see Section 5.3.6.5) and heat_stab_r (see Section 5.3.6.6) status bits of the respectively field should be checked to ensure that the gas measurement was successful. If heat_stab_r is zero, it indicates that either the heating time was not enough to allow the sensor to reach to configured target temperature or that the target temperature was too high for the sensor to reach.
After the uncompensated values of temperature, pressure and humidity have been read, the actual humidity, pressure and temperature need to be calculated using the compensation parameters stored in the device. Please refer to the BME68x-
Sensor-API for more details.
Gas resistance readout
Readout of gas resistance ADC value and calculation of gas resistance consists of 2 steps
1. Read gas ADC value (gas_adc) and gas ADC range (gas_range) (see Section 5.3.5)
2. Convert ADC value into gas resistance in ohm
Using the variables listed in Table 17, the conversion is done as follows:
Floating point:
uint32_t var1 = UINT32_C(262144) >> gas_range; int32_t var2 = (int32_t) gas_adc - INT32_C(512); var2 *= INT32_C(3); var2 = INT32_C(4096) + var2; gas_res = 1000000.0f * (float)var1 / (float)var2;
Integer:
uint32_t var1 = UINT32_C(262144) >> gas_range; int32_t var2 = (int32_t) gas_adc - INT32_C(512); var2 *= INT32_C(3); var2 = INT32_C(4096) + var2; /* multiplying 10000 then dividing then multiplying by 100 instead of multiplying by 1000000 to prevent overflow */
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calc_gas_res = (UINT32_C(10000) * var1) / (uint32_t)var2; gas_res = calc_gas_res * 100;
where
gas_adc is the raw gas resistance output data (i.e. ADC value), see 0 gas_range is the ADC range of the measured gas resistance, see 5.3.5.5 gas_res is the compensated gas resistance output data in Ohms.
Table 17: Variable names and register addresses for gas_res calculation
Variable name
Register address (LSB / MSB)
gas_adc
Field 0 - 0x2D<7:6> / 0x2C Field 1 - 0x3E<7:6> / 0x3D
Field 2 - 0x4F<7:6> / 0x4E
gas_range
Field 0 - 0x2D<3:0> Field 1 - 0x3E<3:0> Field 2 - 0x4F<3:0>
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4. Software
4.1 BME software components
Table 18: Software components
Software component
Type
Function
Link
BME68x sensor API
C code
Provide software interfaces to get compensated
raw data from sensor via SPI/I2C interface
BME68x-Sensor-API
BSEC
C static library
Provides fused sensor outputs and AI interpreter
for classifcation of gas classes
BME688-Software
BME AI Studio
PC application
AI toolchain to develop, verify and deploy custom
gas classification use cases
4.2 BME software library - BSEC software
BME688 sensor is intended to be used together with Bosch Software Environmental Cluster (BSEC) solution and BME6xy sensor API to unlock its full potential. The BSEC software features intelligent algorithms which enable use cases such as indoor-air-quality monitoring using the BME688. Moreover, the software algorithms handle humidity compensation, baseline as well as long-term drift correction of the gas sensor signal.
Bosch Sensortec BSEC software is available as a closed source binary which will be made available via a Software License Agreement (SLA) on the Bosch Sensortec website.
Different power modes for the gas sensor and corresponding data rates are supported by the software solution:
Gas Scan mode (scan) is designed for interactive applications where selectivity of traget gases are needed in
addition in indoor airquality monitoring. The standard heater profile has an update rate of 10.8 s and can be finetuned for use-cases using BME AI Studio.
Ultra low power (ULP) mode is designed for battery-powered and/or frequency-coupled devices over extended
periods of time. This mode features an update rate of 300 seconds and an average current consumption of <0.1 mA
Quick Ultra-low power (q-ULP) has a 3 s data rate for Temprature, pressure and humidity w/o significantly
increasing the power consumption compared to ULP.
Low power (LP) mode that is designed for interactive applications where the air quality is tracked and observed at
a higher update rate of 3 seconds with a current consumption of <1 mA
Continuous (CONT) mode provides an update rate of 1 Hz and shall only be used short-term for use cases that
incorporate very fast events or stimulus.
Table 19: BSEC gas sensor power-modes
BSEC power mode
Update rate
Average current consumption
Standard gas scan mode (scan)
1/10.8 s
3.96 mA
Ultra-low power mode (ULP)
3.3 mHz
0.09 mA
quick Ultra-low power mode (q-ULP)
0.33 Hz (T, p, h) / 3.3 mHz (IAQ)
0.1 mA
Low power mode (LP)
0.33 Hz
0.9 mA
Continuous mode (for testing purposes
only)
1 Hz
12 mA
The following table describes the available outputs of BSEC. Full descriptions of the outputs and the available interfaces are available in the integration guide shipped together with the BSEC software.
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Table 20: BSEC outputs
Output
Description
Raw pressure
Raw data from sensor API bypassed to BSEC output
Raw temperature
Raw data from sensor API bypassed to BSEC output
Raw relative humidity
Raw data from sensor API bypassed to BSEC output
Raw gas resistance
Raw data from sensor API bypassed to BSEC output
Sensor-compensated
temperature (°C)
Temperature which is compensated for internal cross-
influences caused by the BME sensor
Sensor-compensated relative
humidity (%)
Relative humidity which is compensated for internal cross-
influences caused by the BME sensor
Sensor-compensated gas
resistance (Ohm)
Raw gas resistance compensated by temperature and
humidity influences.
Ambient temperature (°C)
Ambient temperature after compensating the influence of
device (where BME688 is integrated in) heatsources
Ambient relative humidity (%)
Ambient relative humidity after compensating influence of
device (where BME688 is integrated in) heatsources
IAQ (0-500)
Index for Air Quality, especially recommended for mobile
devices, since the auto-trim algorithm automatically adopts to
different environments.
Static IAQ (“s-IAQ”)
“Static” Index for Air Quality, especially recommended for
stationary devices (w/ o auto-trimming algorithm)
CO2 equivalents (ppm)
Estimation of the CO2 level in ppm. The sensor does not
directly measure CO2, but derives this from the average correlation between VOCs and CO2 in human’s exhaled
breath.
b-VOC equivalents (ppm)
Conversion into breath-VOC equivalents in ppm concentration.
The scaling is derived from lab tests with the b-VOC gas
mixture described in Table 7.
Accuracy status (0-3)
Accuracy status of IAQ
Stabilization time status
Indicates if the sensor is undergoing initial stabilization during
its first use after production
Run in status
Indicates when the sensor is ready after after switch-on
Gas (%)
Alternative indicator for air pollution which rates the current raw
gas resistance value based on the individual sensor history:
0% = “lowest air pollution ever measured”
100% = “highest air pollution level ever measured”
Gas scan result (%)
The gas scan result is given in % for each of the used classes.
In standard scan mode, the probability of H2S and non H2S
class is provided by the variables GAS_ESTIMATE_1 &
GAS_ESTIMATE_2 respectively. A maximum of 4 classes can
be used by configuring using BME AI-Studio.
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5. Global memory map and register description
5.1 General remarks
Communication with the device is performed by reading from and writing to registers. Registers have a width of 8 bits. If I2C is used, 8-bit addressing is utilized. If SPI is used, 7-bit address is only available for register access. For details on the interface, consult Chapter 0.
In SPI mode complete memory map is accessed using page 0 and page 1. Register spi_mem_page is used for page selection. After power-on, spi_mem_page is in its reset state and page 0 (0x80 to 0xFF) will be active. Page 1 (0x00 to 0x7F) will be active on setting spi_mem_page to 1.
Global memory map consists of calibration registers, control registers, data registers, status registers and reserved registers. There are, however, several registers which are reserved. Accordingly, they should not be written to and no specific value is guaranteed when they are read.
Table 21: Memory map page selection
Digital Interface
Register address range
Register spi_mem_page
Memory Page
I2C
0x00 to 0xFF
Not Applicable
Not Applicable
SPI
0x80 to 0xFF
0 (default; power on state)
Page 0
SPI
0x00 to 0x7F
1
Page 1
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5.2 Memory map
The memory map is given in Table 22, noting that not all reserved registers are depicted.
Table 22: Memory map
Register name
I2C
SPI
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit2
Bit 1
Bit 0
Reset state
Adr
Adr
Pg
Status 73h
73h 1
spi_mem_page
01h
Variant_Id F0h
70h
0
variant_id<7:0>
01h
Reset E0h
60h
0
reset<7:0>
00h
Chip_Id D0h
50h
0
chip_id<7:0>
61h
Config 75h
75h
1
filter<2:0>
spi_3w_en
00h
Ctrl_meas 74h
74h
1
osrs_t<2:0>
osrs_p<2:0>
mode<1:0>
00h
Ctrl_hum 72h
72h 1
spi_3w_int_en osrs_h<2:0>
00h
Ctrl_gas_1 71h
71h 1
run_gas
nb_conv<3:0>
00h
Ctrl_gas_0 70h
70h 1 heat_off
00h
Gas_wait_shared 6Eh
6Eh
1
gas_wait_shared<7:0>
00h
Gas_wait_x
6Dh…64h
6Dh…64h
1
gas_wait_9<7:0> downto gas_wait_0<7:0>
00h
Res_heat_x
63h…5Ah
63h…5Ah
1
res_heat_9<7:0> downto res_heat_0<7:0>
00h
Idac_heat_x
59h…50h
59h…50h
1
idac_heat_9<7:0> downto idac_heat_0<7:0>
00h
gas_r_lsb [2] 4Fh
4Fh 1 gas_r<1:0>
gas_valid_r
heat_stab_r
gas_range_r
00h
gas_r_msb [2] 4Eh
4Eh
1
gas_r<9:2>
00h
hum_lsb [2] 48h
48h
1
hum_lsb<7:0>
00h
hum_msb [2] 47h
47h
1
hum_msb<7:0>
80h
temp_xlsb [2] 46h
46h
1
temp_xlsb<7:4>
0 0 0
0
00h
temp_lsb [2] 45h
45h
1
temp_lsb<7:0>
00h
temp_msb [2] 44h
44h
1
temp_msb<7:0>
80h
press_xlsb [2] 43h
43h
1
press_xlsb<7:4>
0 0 0
0
00h
press_lsb [2] 42h
42h
1
press_lsb<7:0>
00h
press_msb [2] 41h
41h
1
press_msb<7:0>
80h
sub_meas_index [2] 40h
40h
1
sub_meas_index_2<7:0>
00h
meas_status [2] 3Fh
3Fh 1 new_data
gas_measuring
measuring gas_meas_index_2<3:0>
00h
gas_r_lsb [1] 3Eh
3Eh 1 gas_r<1:0>
gas_valid_r
heat_stab_r
gas_range_r
00h
gas_r_msb [1] 3Dh
3Dh
1
gas_r<9:2>
00h
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hum_lsb [1] 37h
37h
1
hum_lsb<7:0>
00h
hum_msb [1] 36h
36h
1
hum_msb<7:0>
80h
temp_xlsb [1] 35h
35h
1
temp_xlsb<7:4>
0 0 0
0
00h
temp_lsb [1] 34h
34h
1
temp_lsb<7:0>
00h
temp_msb [1] 33h
33h
1
temp_msb<7:0>
80h
press_xlsb [1] 32h
32h
1
press_xlsb<7:4>
0 0 0
0
00h
press_lsb [1] 31h
31h
1
press_lsb<7:0>
00h
press_msb [1] 30h
30h
1
press_msb<7:0>
80h
sub_meas_index [1] 2Fh
2Fh
1
sub_meas_index_1<7:0>
00h
meas_status [1] 2Eh
2Eh 1 new_data
gas_measuring
measuring gas_meas_index_1<3:0>
00h
gas_r_lsb [2] 2Dh
2Dh 1 gas_r<1:0>
gas_valid_r
heat_stab_r
gas_range_r
00h
gas_r_msb [0] 2Ch
2Ch
1
gas_r<9:2>
00h
hum_lsb [0] 26h
26h
1
hum_lsb<7:0>
00h
hum_msb [0] 25h
25h
1
hum_msb<7:0>
80h
temp_xlsb [0] 24h
24h
1
temp_xlsb<7:4>
0 0 0
0
00h
temp_lsb [0] 23h
23h
1
temp_lsb<7:0>
00h
temp_msb [0] 22h
22h
1
temp_msb<7:0>
80h
press_xlsb [0] 21h
21h
1
press_xlsb<7:4>
0 0 0
0
00h
press_lsb [0] 20h
20h
1
press_lsb<7:0>
00h
press_msb [0] 1Fh
1Fh
1
press_msb<7:0>
80h
sub_meas_index [0] 1Eh
1Eh
1
sub_meas_index_0<7:0>
00h
meas_status [0] 1Dh
1Dh 1 new_data
gas_measuring
measuring gas_meas_index_0<3:0>
00h
Registers
Color/Type
Reserved
Do not change
Status register
Read only
Data register
Read only
Control register
Read/write
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5.3 Register description General control registers
5.3.1.1 SPI 3 wire interrupt enable – spi_3w_int_en
Register Name
Address
Content<bit position>
Description
ctrl_hum
0x72
spi_3w_int_en <6>
New data interrupt can be enabled if the device is in SPI 3 wire mode and pi_3w_int_en=1. The new data interrupt is then indicated on the SDO pad.
5.3.1.2 SPI 3 wire enable – spi_3w_en
Register Name
Address
Content<bit position>
Description
config
0x75
spi_3w_en<0>
Enable SPI 3 wire mode
5.3.1.3 Mode Selection – mode
The operation modes of the sensor can be controlled by the register mode as specified below.
Register Name
Address
Content<bit position>
Description
ctrl_meas
0x74
mode<1:0>
Select sensor power mode as shown in the following table
mode<1:0>
Mode
00
Sleep mode
01
Forced mode
10
Parallel mode
5.3.1.4 SPI memory map page selection – spi_mem_page
In SPI mode complete memory page is accessed using page 0 & page 1. Register spi_mem_page is used for page selection. After power-on, spi_mem_page is in its reset state and page 0(0x00 to 0x7F) will be active. Page1 (0x7F to 0xFF) will be active on setting spi_mem_page. Please refer Table 21 for better understanding.
Register Name
Address
Content<bit position>
Description
status
0x73(Page 0/1)
spi_mem_page <4>
Selects memory map page in SPI mode
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5.3.1.5 Reset – reset
Writing 0xB6 to this register initiates a soft-reset procedure, which has the same effect like power-on reset. The default value stored in this register is 0x00.
Register Name
Address
Content<bit position>
Description
reset
0x60 (Page 0 in SPI mode)
0xE0 in I2C
reset<7:0>
Resets the device
Sensor identification registers
5.3.2.1 Variant Id – Variant_Id
Register Name
Address
Content<bit position>
Description
Variant_Id
0x70(Page 0 in SPI mode)
0xF0 in I2C
variant_id<7:0>
Variant Id of the sensor
For BME688 - 0x01h
5.3.2.2 Chip id – chip_id
Register Name
Address
Content<bit position>
Description
Chip_Id
0x50(Page 0 in SPI mode)
0xD0 in I2C
chip_id<7:0>
Chip id of the device
Temperature, pressure and relative humidity control registers
5.3.3.1 Humidity sensor over sampling control – osrs_h
Register Name
Address
Content<bit position>
Description
ctrl_hum
0x72
osrs_h<2:0>
Controls over sampling setting of humidity
sensor as described in the following table
osrs_h<2:0>
Humidity oversampling
000
Skipped (output set to 0x8000)
001
oversampling ×1
010
oversampling ×2
011
oversampling ×4
100
oversampling ×8
101, Others
oversampling ×16
5.3.3.2 Over sampling setting – Temperature data – osrs_t
Register Name
Address
Content<bit position>
Description
ctrl_meas
0x74
osrs_t<7:5>
Temperature oversampling settings as shown
in the following table
osrs_t<2:0>
Temperature oversampling
000
Skipped (output set to 0x8000)
001
oversampling ×1
010
oversampling ×2
011
oversampling ×4
100
oversampling ×8
101, Others
oversampling ×16
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5.3.3.3 Over sampling setting – Pressure data – osrs_p
Register Name
Address
Content<bit position>
Description
ctrl_meas
0x74
osrs_p<4:2>
Pressure oversampling settings as shown in
the following table
osrs_p<2:0>
Pressure oversampling
000
Skipped (output set to 0x8000)
001
oversampling ×1
010
oversampling ×2
011
oversampling ×4
100
oversampling ×8
101, Others
oversampling ×16
5.3.3.4 IIR filter control – filter
IIR filter applies to temperature and pressure data but not to humidity and gas data. The data coming from the ADC are filtered and then loaded into the data registers. The temperature and pressure result registers are updated together at the same time at the end of the measurement. IIR filter output resolution is 20 bits. The result registers are reset to value 0x80000 when the temperature and/or pressure measurements have been skipped (osrs_x=”000‟). The appropriate filter memory is kept unchanged (the value from the last measurement is kept). When the appropriate OSRS register is set back to nonzero, then the first value stored to the result registers are filtered.
Register Name
Address
Content<bit position>
Description
config
0x75
filter<4:2>
IIR filter settings as shown in the following
table
filter<2:0>
Filter coefficient
000
0
001
1
010
3
011
7
100
15
101
31
110
63
111
127
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Gas control registers
The sensor can have 10 programmable gas sensor heater steps. Each heater step consists of a target heater resistance corresponding to heater temperature, heater-on time and optionally an initial heater current.
5.3.4.1 Heater current - idac_heat_x
BME688 contains a heater control block that will inject enough current into the heater resistance to achieve the requested heater temperature. There is a control loop which periodically measures heater resistance value and adapts the value of current injected from a DAC.
The heater operation could be speeded up by setting an initial heater current for a target heater temperature by using register idac_heat_x<7:0>. This step is optional since the control loop will find the current after a few iterations anyway. The current injected to the heater in mA can be calculated by: (idac_heat_7_1 + 1) / 8, where idac_heat_7_1 is the decimal value stored in idac_heat<7:1> (unsigned, value from 0 to 127).
Heater steps
Register name
Address
Content
Description
0...9
idac_heat_x
x is from 0 to 9
0x50…0x59
idac_heat_x<7:0>
x is from 0 to 9
idac_heat of particular heater set point
5.3.4.2 Target heater resistance - res_heat_x
Target heater resistance is programmed by user through res_heat_x<7:0> registers. The definition of res_heat_x is given in Section 3.6.5.
Heater step
Register name
Address
Content
Description
0...9
res_wait_x
x is from 0 to 9
0x5A…0x63
res_heat_x<7:0>
x is from 0 to 9
Decimal value that needs to be stored for achieving target heater resistance
5.3.4.3 Gas Sensor heater-on time in Forced mode - gas_wait_x
Referring to Figure 2, the time between the beginning of the heat phase and the start of gas sensor resistance measurement depends on gas_wait_x setting as mentioned below.
Heater step
Register name
Address
Content
Description
0...9
gas_wait_x x is from 0 to 9
0x64…0x6D
gas_wait_x<5:0> x is from 0 to 9
64 timer values with 1 ms step sizes, all zeros means no wait
0...9
gas_wait_x x is from 0 to 9
0x64…0x6D
gas_wait_x<7:6> x is from 0 to 9
Timer multiplication factor as shown in below table
gas_wait_x<7:6>
Gas sensor wait time multiplication factor
00
1
01 4 10
16
11
64
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5.3.4.4 Gas Sensor heater-on time in parallel mode -- gas_wait_x & gas_wait_shared
Referring to 3.5.4, total heater-on time depends on number of TPHG sequence, time taken for TPHG measurement and wait phase of each TPHG sequence. gas_wait_x<7:0> sets number of times TPHG sequence needs to be repeated. The number of TPHG sub-measurement sequences within the one Gas conversion for one target temperature resistance is defined by gas_wait_X(7:0) settings. The duration of measurement is calculated as follows: Duration = gas_wait_X × (wait time defined by gas_wait_shared + TTPHG_duration)
Heater step
Register name
Address
Content
Description
0...9
gas_wait_x x is from 0 to 9
0x64…0x6D
gas_wait_x<7:0> x is from 0 to 9
255 values with 1 LSB corresponds to 1 TPHG sequence, all zeros means gas_wait_shared is fully skipped and one TPHG sequence is executed.
gas_wait_shared<7:0> sets wait time before each TPHG measurement sequence with duration as configured below. This
is common for the whole gas measurement sequence in parallel mode.
Register Name
Address
Content<bit position>
Description
gas_wait_shared
0x6E
gas_wait_shared<5:0>
64 timer values with 0.477ms step sizes, all zeros means no wait. gas_wait_shared=0x00 is treated as if gas_wait_shared=0x01.
gas_wait_shared
<7:6>
Gas sensor wait time multiplication factor
00 1 01 4 10
16
11
64
5.3.4.5 Heater off - heat_off
Register Name
Address
Content<bit
position>
Description
ctrl_gas_0
0x70
heat_off <3>
Turn off current injected to heater by setting bit to one
5.3.4.6 Heater profile selection- nb_conv
nb_conv set heater profile of the sensor.
Register Name
Address
Content<bit position>
Description
ctrl_gas_1
0x71
nb_conv<3:0>
Forced mode: Indicates index of heater step that will be used in forced mode as describe in below table
Parallel mode: Indicates sequence of up to 10 heater steps as describe in below table
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In forced mode:
nb_conv<3:0>
Heater step
0000
0
0001
1
0010
2
0011
3
0100
4
0101
5
0110
6
0111
7
1000
8
1001
9
In parallel mode:
nb_conv<3:0>
Length of heater profile
0000
No conversion
0001
Heater step 0
0010
Heater step 0-1
0011
Heater step 0-2
0100
Heater step 0-3
0101
Heater step 0-4
0110
Heater step 0-5
0111
Heater step 0-6
1000
Heater step 0-7
1001
Heater step 0-8
1010 to 1111
Heater step 0-9
Each heater step in profile has specific heater temperature and heater-on time and gas_meas_index register (see 5.3.6.4) stores index of heater step (up to 10 steps numbered from 0 to 9) corresponding to TPHG data.
5.3.4.7 Run Gas - run_gas
Register Name
Address
Content<bit position>
Description
ctrl_gas_1
0x71
run_gas<4>
The gas conversions are started only in appropriate mode if run_gas = ‘1’
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Data registers
In this section, the data registers for the temperature, pressure, humidity and gas sensors are explained. Shadowing registers are utilized to buffer the data and to ensure stable data in case an update of the data registers occurs simultaneously with the serial interface reading out.
3 fields store TPHG data, status bits and time indexes:
In forced mode only field 0 is used, In parallel mode, fields are filled from 0 to 2 and overwritten if more than 3 heater steps have been selected (5.3.4.6).
5.3.5.1 Pressure data
Register Name
[Field]
Address
Content<bit position>
Description
press_msb [0] press_msb [1] press_msb [2]
0x1F 0x30 0x41
press_msb<7:0>
Contains the MSB part [19:12] of the raw pressure measurement output data.
press_lsb [0] press_lsb [1] press_lsb [2]
0x20 0x31 0x42
press_lsb<7:0>
Contains the LSB part [11:4] of the raw pressure measurement output data
press_xlsb [0] press_xlsb [1] press_xlsb [2]
0x21 0x32 0x43
press_xlsb<7:4>
Contains the XLSB part [3:0] of the raw pressure measurement output data. Contents depend on pressure resolution controlled by oversampling setting.
5.3.5.2 Temp data
Register Name
[Field]
Address
Content<bit position>
Description
temp_msb [0] temp_msb [1] temp_msb [2]
0x22 0x33 0x44
temp_msb<7:0>
Contains the MSB part [19:12] of the raw temperature measurement output data.
temp_lsb [0] temp_lsb [1] temp_lsb [2]
0x23 0x34 0x45
temp_lsb<7:0>
Contains the LSB part [11:4] of the raw temperature measurement output data.
temp_xlsb [0] temp_xlsb [1] temp_xlsb [2]
0x24 0x35 0x46
temp_xlsb<7:4>
Contains the XLSB part [3:0] of the raw temperature measurement output data. Contents depend on temperature resolution controlled by oversampling setting.
5.3.5.3 Humidity data
Register Name
[Field]
Address
Content<bit position>
Description
hum_msb [0] hum_msb [1] hum_msb [2]
0x25 0x36 0x47
hum_msb<7:0>
Contains the MSB part [15:8] of the raw humidity measurement output data.
hum_lsb [0] hum_lsb [1] hum_lsb [2]
0x26 0x37 0x48
hum_lsb<7:0>
Contains the LSB part [7:0] of the raw humidity measurement output data.
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5.3.5.4 Gas resistance data
Register Name
[Field]
Address
Content<bit position>
Description
gas_r_msb [0] gas_r_msb [1] gas_r_msb [2]
0x2C 0x3D 0x4E
gas_r<7:0>
Contains the MSB part gas resistance [9:2] of the raw gas resistance.
gas_r_lsb [0] gas_r_lsb [1] gas_r_lsb [2]
0x2D 0x3E 0x4F
gas_r<7:6>
Contains the LSB part gas resistance [1:0] of the raw gas resistance.
5.3.5.5 Gas resistance range
Register Name
[Field]
Address
Content<bit position>
Description
gas_r_lsb [0] gas_r_lsb [1] gas_r_lsb [2]
0x2D 0x3E 0x4F
gas_range_r<3:0>
Contains ADC range of measured gas resistance.
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Status registers
The measured data are stored into the output data registers at the end of each TPHG conversion phase along with status flags and index of measurement
5.3.6.1 New data status
Register Name
[Field]
Address
Content<bit position>
Description
meas_status [0] meas_status [1] meas_status [2]
0x1D
0x2E
0x3F
new_data<7>
New data flag indicate that data field contains a new data, not already read-out.
5.3.6.2 Gas measuring status
Measuring bit is set to 1 only during gas measurements, goes to 0 as soon as measurement is completed and data transferred to data registers. The registers storing the configuration values for the measurement (gas_wait_shared, gas_wait_x, res_heat_x, idac_heat_x, image registers) should not be changed when the device is measuring.
Register Name
[Field]
Address
Content<bit position>
Description
meas_status [0] meas_status [1] meas_status [2]
0x1D 0x2E 0x3F
gas_measuring<6>
Gas measuring status flag
5.3.6.3 Measuring status
Measuring status will be set to ‘1’ whenever a conversion (temperature, pressure, humidity and gas) is running and back to ‘0’ when the results have been transferred to the data registers.
Register Name
[Field]
Address
Content<bit position>
Description
meas_status [0] meas_status [1] meas_status [2]
0x1D
0x2E
0x3F
measuring<5>
Measuring status flag
5.3.6.4 Gas Measurement Index
User can trigger a heater profile of up to 10 heater steps by setting nb_conv<3:0>(see 5.3.4.6). Each heater step in profile has specific heater temperature and heater-on time and gas_meas_index register stores index of heater step (up to 10 steps numbered from 0 to 9) corresponding to TPHG data
Register Name
[Field]
Address
Content<bit position>
Description
meas_status [0] meas_status [1] meas_status [2]
0x1D 0x2E 0x3F
gas_meas_index_0<3:0>
Gas measurement index
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5.3.6.5 Gas valid status
In each TPHG sequence in parallel mode contains a gas measurement slot, either a real one which result is used or a dummy one to keep a constant sampling rate and predictable device timing. A real gas conversion (i.e., not a dummy one) is indicated by the gas_valid_r status register.
Register Name
[Field]
Address
Content<bit position>
Description
gas_r_lsb [0] gas_r_lsb [1] gas_r_lsb [2]
0x2D 0x3E 0x4F
gas_valid_r<5>
Gas valid bit
5.3.6.6 Heater Stability Status
Heater temperature stability for target heater resistance is indicated heat_stab_x status bits and updated with data
registers.The heater stability flag behavior depends on the current device mode:
Forced mode: Stability is monitored only during the auto ranging sequences for gas measurements. Parallel mode: Stability is monitored from the beginning of the last one TPHG sequence (the on
containing the real gas measurements) to the end of the gas measurement.
Register Name
[Field]
Address
Content<bit position>
Description
gas_r_lsb [0] gas_r_lsb [1] gas_r_lsb [2]
0x2D 0x3E 0x4F
heat_stab_r<4>
Heater stability bit
5.3.6.7 Sub Measurement Index
Indicates current TPHG sequence number on operating sensor on parallel mode, which are incremented for each TPHG measurement. These status bytes form “virtual time sensor” and contain a snapshot of the internal 8 bit conversion counter since the last change of device mode.
Register Name
[Field]
Address
Content<bit position>
Description
sub_meas_index [0] sub_meas_index [1] sub_meas_index [2]
0x1E 0x2F 0x40
sub_meas_index<7:0>
TPHG measurement sequence number
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6. Digital interfaces
The sensor supports the I²C and SPI digital interfaces, where it acts as a slave for both protocols. The I²C interface supports the Standard, Fast and High Speed modes. The SPI interface supports both SPI mode ‘00’ (CPOL = CPHA = ‘0’) and mode ‘11’ (CPOL = CPHA = ‘1’) in 4-wire and 3-wire configuration.
The following transactions are supported:
Single byte write multiple byte write (using pairs of register addresses and register data) single byte read multiple byte read (using a single register address which is auto-incremented)
6.1 Interface selection
Interface selection is done automatically based on CSB (chip select) status. If CSB is connected to V
DDIO
, the I²C interface is active. If CSB is pulled down, the SPI interface is activated. After CSB has been pulled down once (regardless of whether any clock cycle occurred), the I²C interface is disabled until the next power-on-reset. This is done in order to avoid inadvertently decoding SPI traffic to another slave as I²C data. Since the device startup is defe rred until both VDD and V
DDIO
are established, there is no risk of incorrect protocol detection because of the power-up sequence used. However, if I²C is to be used and CSB is not directly connected to V
DDIO
but is instead connected to a programmable pin, it must be ensured that
this pin already outputs the V
DDIO
level during power-on-reset of the device. If this is not the case, the device will be locked
in SPI mode and not respond to I²C commands.
6.2 I²C Interface
For detailed timings, please review Table 24 . All modes (standard, fast, high speed) are supported. SDA and SCL are not pure open-drain. Both pads contain ESD protection diodes to VDDIO and GND. As the devices does not perform clock stretching, the SCL structure is a high-Z input without drain capability.
Picture 6: SDI/SCK ESD schematic
The 7-bit device address is 111011x. The 6 MSB bits are fixed. The last bit is changeable by SDO value and can be changed during operation. Connecting SDO to GND results in slave address 1110110 (0x76); connection it to V
DDIO
results in slave address 1110111 (0x77), which is the same as BMP280’s I²C address. The SDO pin cannot be left floating; if left floating, the I²C address will be undefined.
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The I²C interface uses the following pins:
SCK: serial clock (SCL) SDI: data (SDA) SDO: Slave address LSB (GND = ‘0’, V
DDIO
= ‘1’)
CSB must be connected to V
DDIO
to select I²C interface. SDI is bi-directional with open drain to GND: it must be externally
connected to V
DDIO
via a pull up resistor. Refer to Chapter 7 for connection instructions.
The following abbreviations will be used in the I²C protocol figures:
S Start P Stop ACKS Acknowledge by slave ACKM Acknowledge by master NACKM Not acknowledge by master
I²C write
Writing is done by sending the slave address in write mode (RW = ‘0’), resulting in slave address 111011X0 (‘X’ is determined by state of SDO pin. Then the master sends pairs of register addresses and register data. The transaction is ended by a stop condition. This is depicted in Picture 7.
Start RW ACKS ACKS ACKS
1 1 1 0 1 1 X 0 1 0 1 0 0 0 0 0 bit7 bit6 bit5 bit4 bit3 bit2 bit1 bit0
…
ACKS ACKS Stop
…
1 0 1 0 0 0 0 1 bit7 bit6 bit5 bit4 bit3 bit2 bit1 bit0
Register data - address A0h
Register address (A0h)
Register address (A1h)
S
Slave Address
Control byte
Data byte
Control byte
Data byte
P
Register data - address A1h
Picture 7: I²C multiple byte write (not auto-incremented)
I²C read
To be able to read registers, first the register address must be sent in write mode (slave address 111011X0). Then either a stop or a repeated start condition must be generated. After this the slave is addressed in read mode (RW = ‘1’) at address 111011X1, after which the slave sends out data from auto-incremented register addresses until a NOACKM and stop condition occurs. This is depicted in Picture 8, where register 0xF6 and 0xF7 are read.
Start RW ACKS ACKS
1 1 1 0 1 1 X 0 1 1 1 1 0 1 1 0
Start RW ACKS ACKM NOACKM Stop
1 1 1 0 1 1 X 1 bit7 bit6 bit5 bit4 bit3 bit2 bit1 bit0 bit7 bit6 bit5 bit4 bit3 bit2 bit1 bit0
Control byte
Data byte
Data byte
Register address (F6h)
S
Slave Address
P
S
Slave Address
Register data - address F7h
Register data - address F6h
Picture 8: I²C multiple byte read
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6.3 SPI interface
The SPI interface is compatible with SPI mode ‘00’ (CPOL = CPHA = ‘0’) and mode ‘11’ (CPOL = CPHA = ‘1’). The automatic selection between mode ‘00’ and ‘11’ is determined by the value of SCK after the CSB falling edge.
The SPI interface has two modes, namely 4-wire and 3-wire mode. However, the protocol is the same for both. The 3-wire mode is selected by setting ‘1’ to the register spi3w_en. The pad SDI is used as a data pad in 3-wire mode.
The SPI interface uses the following pins:
CSB: chip select, active low SCK: serial clock SDI: serial data input; data input/output in 3-wire mode SDO: serial data output; hi-Z in 3-wire mode
For more connection instructions, please refer to Chapter 7.
CSB is active low and has an integrated pull-up resistor. Data on SDI is latched by the device at SCK rising edge and SDO is changed at SCK falling edge. Communication starts when CSB goes to low and stops when CSB goes to high; during these transitions on CSB, SCK must be stable. The SPI protocol is shown in It is important to note that Picture 9. For timing details, please review Table 25.
CSB
SCK
SDI
RW
AD6
AD5
AD4
AD3
AD2
AD1
AD0
DI5
DI4
DI3
DI2
DI1
DI0
DI7
DI6
SDO
DO5
DO4
DO3
DO2
DO1
DO0
DO7
DO6
tri-state
Picture 9: SPI protocol (shown for mode ‘11’ in 4-wire configuration)
It is important to note that in the SPI mode, only 7 bits of the register addresses are used; the MSB of register address is not used and replaced by a read/write bit (RW = ‘0’ for write and RW = ‘1’ for read). For example, address 0xF7 is accessed by using SPI register address 0x77. On the one hand, the byte 0x77 is transferred for write access, and on the other hand, the byte 0xF7 is transferred for read access.
SPI write
Writing is done by lowering CSB and sending pairs control bytes and register data. The control bytes consist of the SPI register address (= full register address without bit 7) and the write command (bit7 = RW = ‘0’). Several pairs can be written without raising CSB. The transaction is ended by a raising CSB. The SPI write protocol is depicted in Picture 10.
Start
RW RW Stop
0 1 1 1 0 1 0 0 bit7 bit6 bit5 bit4 bit3 bit2 bit1 bit0 0 1 1 1 0 1 0 1 bit7 bit6 bit5 bit4 bit3 bit2 bit1 bit0
Control byte
CSB
= 1
Data byte
Register address (F5h)
Data register - adress F5h
Register address (F4h)
CSB
= 0
Control byte
Data byte
Data register - address F4h
Picture 10: SPI multiple byte write (not auto-incremented)
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SPI read
Reading is done by lowering CSB and first sending one control byte. The control bytes consist of the SPI register address (= full register address without bit 7) and the read command (bit 7 = RW = ‘1’). After writing the control byte, data is sent out of the SDO pin (SDI in 3-wire mode); the register address is automatically incremented. The SPI read protocol is depicted in Picture 11.
Start
RW Stop
1 1 1 1 0 1 1 0 bit15 bit14 bit13 bit12 bit11 bit10 bit9 bit8 bit7 bit6 bit5 bit4 bit3 bit2 bit1 bit0
CSB
= 1
Data byte
Data register - address F7h
Register address (F6h)
CSB
= 0
Control byte
Data byte
Data register - address F6h
Picture 11: SPI multiple byte read
6.4 Interface parameter specification
General interface parameters
Table 23: Interface parameters
Parameter
Symbol
Condition
Min
Typ
Max
Unit
Input low level
V
il_si
V
DDIO
=1.2 V to 3. 6V
20
%V
DDIO
Input high level
V
ih_si
V
DDIO
=1.2 V to 3.6 V
80
%V
DDIO
Output low level I2C
V
ol_SDI
V
DDIO
=1.62 V, Iol=3 mA
20
%V
DDIO
Output low level I2C
V
ol_SDI_1.2
V
DDIO
=1.20 V, Iol=3 mA
23
%V
DDIO
Output low level SPI
V
ol_SDO
V
DDIO
=1.62 V, Iol=1 mA
20
%V
DDIO
Output low level SPI
V
ol_SDO_1.2
V
DDIO
=1.20 V, Iol=1 mA
23
%V
DDIO
Output high level
Voh
V
DDIO
=1.62 V, Ioh=1 mA (SDO, SDI)
80
%V
DDIO
Output high level
V
oh_1.2
V
DDIO
=1.20 V, Ioh=1 mA (SDO, SDI)
60
%V
DDIO
Pull-up resistor
R
pull
Internal CSB pull-up resistor to V
DDIO
70
120
190
kΩ
I2C bus load capacitor
Cb
On SDI and SCK
400
pF
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I²C timings
For I²C timings, the following abbreviations are used:
“S&F mode” = standard and fast mode “HS mode” = high speed mode C
b
= bus capacitance on SDA line
All other naming refers to I²C specification 2.1 (January 2000).
The I²C timing diagram is in Picture 12. The corresponding values are given in Table 24
t
HDDAT
tf
t
BUF
SDI
SCK
SDI
t
LOW
t
HDSTA
tr
t
SUSTA
t
HIGH
t
SUDAT
t
SUSTO
Picture 12: I2C timing diagram
Table 24: I2C timings
Parameter
Symbol
Condition
Min
Typ
Max
Unit
SDI setup time
t
SU;DAT
S&F Mode
HS mode
160
30
ns
ns
SDI hold time
t
HD;DAT
S&F Mode, Cb≤100 pF S&F Mode, Cb≤400 pF HS mode, Cb≤100 pF HS mode, Cb≤400 pF
80 90 18 24
115 150
ns ns ns ns
SCK low pulse
t
LOW
HS mode, Cb≤100 pF
V
DDIO
= 1.62 V
160
ns
SCK low pulse
t
LOW
HS mode, Cb≤100 pF
V
DDIO
= 1.2 V
210
ns
The above-mentioned I2C specific timings correspond to the following internal added delays:
Input delay between SDI and SCK inputs: SDI is more delayed than SCK by typically 100 ns in Standard and Fast
Modes and by typically 20 ns in High Speed Mode.
Output delay from SCK falling edge to SDI output propagation is typically 140 ns in Standard and Fast Modes and
typically 70 ns in High Speed Mode.
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SPI timings
The SPI timing diagram is in Picture 13, while the corresponding values are given in Table 25. All timings apply both to 4­and 3-wire SPI.
CSB
SCK
T_setup_csb
T_low_sck
T_high_sck
T_hold_csb
SDI
T_setup_sdi
T_hold_sdi
SDO
T_delay_sdo
Picture 13: SPI timing diagram
Table 25: SPI timings
Parameter
Symbol
Condition
Min
Typ
Max
Unit
SPI clock i/p frequency
F_spi
0
10
MHz
SCK low pulse
T_low_sck
20
ns
SCK high pulse
T_high_sck
20
ns
SDI setup time
T_setup_sdi
20
ns
SDI hold time
T_hold_sdi
20
ns
SDO output delay
T_delay_sdo
25 pF load, V
DDIO
=1.6 V min
30
ns
SDO output delay
T_delay_sdo
25 pF load, V
DDIO
=1.2 V min
40
ns
CSB setup time
T_setup_csb
20
ns
CSB hold time
T_hold_csb
20
ns
SPI clock input frequency
F_spi
0
10
MHz
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7. Pin-out and connection diagram
7.1 Pin-out
The pin numbering of BME688 is performed in the untypical clockwise direction when seen in top view and counter-clockwise when seen in bottom view. Picture 14 and Table 26 give a detailed description and illustration of the input/output pins.
Picture 14: Top (left) and bottom (right) views of the chip with input/output pins
Table 26: Pin description
Pin
Name
I/O type
Description Connection
SPI 4W
SPI 3W
I
2
C
1
GND
Supply
Ground
GND
2
CSB
In
Chip select
CSB
CSB
V
DDIO
3
SDI
In/Out
Serial data input
SDI
SDI/SDO
SDA
4
SCK
In
Serial clock input
SCK
SCK
SCL
5
SDO
In/Out
Serial data output
SDO
DNC
GND for default address
6
V
DDIO
Supply
Digital / Interface supply
V
DDIO
7
GND
Supply
Ground
GND
8
VDD
Supply
Analog supply
VDD
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7.2 Connection diagrams
For the I2C connection, it is recommended to use 100 nF for C1 and C2. Moreover, the value for the pull-up resistors R1 and R2 should be based on the interface timing and the bus load; a normal value is 4.7 kΩ. Finally, a direct connection between CSB and V
DDIO
is required. Similarly for the 4- and 3-wire SPI connections, it is suggested to use 100 nF for C1 and C2.
Picture 15: Connection diagrams for (a) I2C, (b) 4-wire SPI, and (c) 3-wire SPI
(a) (b) (c)
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7.3 Package dimensions
Picture 16: Package dimensions for top, bottom and side view
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7.4 Landing pattern recommendation
For the design of the landing pattern, the dimensions shown in Picture 17: Recommended landing pattern (top view; dimensions are in millimeters) are recommended. It is important to note that areas marked in red are exposed PCB metal pads.
In case of a solder mask defined (SMD) PCB process, the land dimensions should be defined by solder mask
openings. The underlying metal pads are larger than these openings.
In case of a non-solder mask defined (NSMD) PCB process, the land dimensions should be defined in the metal
layer. The mask openings are larger than these metal pads.
1
2
4
3
8
7
5
6
Soldier stop Landing pattern
1.50
3.00
0.40
0.60
0.20
0.40
0.60
1.50
3.00
0.10
Picture 17: Recommended landing pattern (top view; dimensions are in millimeters)
Solder stop
Landing pattern
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7.5 Marking
Mass production devices
Table 27: Marking of mass production parts
Marking
Symbol
Description
CCC
Lot counter: 3 alphanumeric digits, variable to generate mass production trace-code
T
Product number: 1 alphanumeric digit, fixed to identify product type. “T” identifies the BME688 w/ the part no. 0 273 017 016
L
Sub-contractor ID: 1 alphanumeric digit, variable to identify sub-contractor (L = “P”)
Note: The product numbers “S” and “E” identify the BME680.
Engineering samples
Table 28: Marking of engineering samples
Marking
Symbol
Description
XX
Sample ID: 2-3 alphanumeric digits, variable to generate trace-code
N
Eng. Sample ID: 1 alphanumeric digit, fixed to identify engineering sample, N = “ * ” or “e” or “E”
CC
Counter ID: 2-3 alphanumeric digits, variable to generate trace-code
Note: Early engineering samples (e.g. w/ marking “688E HR1”) did not yet have a flashed variant_id as described in
chapter 5.3.2.1. Consequently, they are not automatically recognized by the API. In order to ensure correct function, the API needs to be manually modified according to the used variant.
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7.6 Soldering guidelines and reconditioning recommendations
The moisture sensitivity level of the BME688 sensors corresponds to JEDEC Level 1, see also:
IPC/JEDEC J-STD-020C “Joint Industry Standard: Moisture/Reflow Sensitivity Classification for non-hermetic Solid
State Surface Mount Devices”
IPC/JEDEC J-STD-033A “Joint Industry Standard: Handling, Packing, Shipping and Use of Moisture/Reflow
Sensitive Surface Mount Devices”
The sensor fulfils the lead-free soldering requirements of the above-mentioned IPC/JEDEC standard, i.e. reflow soldering with a peak temperature up to 260°C. The minimum height of the solder after reflow shall be at least 50 µm. This is required for good mechanical decoupling between the sensor device and the printed circuit board (PCB).
Picture 18: Soldering profile
7.7 Mounting and assembly recommendations
This HSMI-document provides all the necessary instructions to handle, solder and mount the environmental sensor BME688. Following the reported guidelines is very important to prevent the damage of the sensor and the resultant loss of warranty.
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7.8 Environmental safety
RoHS
The BME688 sensor meets the requirements of the EC restriction of hazardous substances (RoHS) directive, see also:
Directive 2011/65/EU and its amendments, including the amendment 2015/863/EU on the restriction of the use of certain hazardous substances in electrical and electronic equipment.
Halogen content
The BME688 is halogen-free. For more details on the analysis results please contact your Bosch Sensortec representative.
Internal package structure
Within the scope of Bosch Sensortec’s ambition to improve its products and secure the mass product supply, Bosch Sensortec qualifies additional sources (e.g. 2nd source) for the packaging and processing of the BME688.
While Bosch Sensortec took care that all of the technical packages parameters are described above are 100% identical for all sources, there can be differences in the chemical content and the internal structural between the different package sources.
However, as secured by the extensive product qualification process of Bosch Sensortec, this has no impact to the usage or to the quality of the BME688 product.
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8. Legal disclaimer
8.1 Engineering samples
Engineering Samples are marked with an asterisk (*), (E) or (e). Samples may vary from the valid technical specifications of the product series contained in this data sheet. They are therefore not intended or fit for resale to third parties or for use in end products. Their sole purpose is internal client testing. The testing of an engineering sample may in no way replace the testing of a product series. Bosch Sensortec assumes no liability for the use of engineering samples. The Purchaser shall indemnify Bosch Sensortec from all claims arising from the use of engineering samples.
8.2 Product use
Bosch Sensortec products are developed for the consumer goods industry. They may only be used within the parameters of this product data sheet. They are not fit for use in life-sustaining or safety-critical systems. Safety­critical systems are those for which a malfunction is expected to lead to bodily harm, death or severe property damage. In addition, they shall not be used directly or indirectly for military purposes (including but not limited to nuclear, chemical or biological proliferation of weapons or development of missile technology), nuclear power, deep sea or space applications (including but not limited to satellite technology).
Bosch Sensortec products are released on the basis of the legal and normative requirements relevant to the Bosch Sensortec product for use in the following geographical target market: BE, BG, DK, DE, EE, FI, FR, GR, IE, IT, HR, LV, LT, LU, MT, NL, AT, PL, PT, RO, SE, SK, SI, ES, CZ, HU, CY, US, CN, JP, KR, TW. If you need further information or have further requirements, please contact your local sales contact.
The resale and/or use of Bosch Sensortec products are at the purchaser’s own risk and his own responsibility. The examination of fitness for the intended use is the sole responsibility of the purchaser.
The purchaser shall indemnify Bosch Sensortec from all third party claims arising from any product use not covered by the parameters of this product data sheet or not approved by Bosch Sensortec and reimburse Bosch Sensortec for all costs in connection with such claims.
The purchaser accepts the responsibility to monitor the market for the purchased products, particularly with regard to product safety, and to inform Bosch Sensortec without delay of all safety-critical incidents
8.3 Application examples and hints
With respect to any examples or hints given herein, any typical values stated herein and/or any information regarding the application of the device, Bosch Sensortec hereby disclaims any and all warranties and liabilities of any kind, including without limitation warranties of non-infringement of intellectual property rights or copyrights of any third party. The information given in this document shall in no event be regarded as a guarantee of conditions or characteristics. They are provided for illustrative purposes only and no evaluation regarding infringement of intellectual property rights or copyrights or regarding functionality, performance or error has been made.
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9. Document history and modifications
Rev.
Chapter
Description of modifications
Date
1.0 Initial release
April 2021
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Bosch Sensortec GmbH Gerhard-Kindler-Straße 9 72770 Reutlingen / Germany
www.bosch-sensortec.com
Modifications reserved | Printed in Germany Preliminary - specifications subject to change without notice Document number: BST-BME688-DS000-00 Revision_1.0 042021
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