Developing with HTU21DF Sensor part 4.

In previous posts we have detailed

In this post after laying ground work, let us start development and write code to capture Humidity and Temperature from Sensor. Code is written in C# using “Universal Windows Platform”

Am developing each module at a time and sharing it on GITHUB @ For those we need code (without github write to me,, I will share them code).

under hood of HTU21DF sensor part 3

Cyclic Redundancy Check:

To ensure any detection of raw data modification (corruption) of sensor(HTU21DF) measurement, Cyclic Redundancy Check is used. Polynomial used for CRC is


Above polynomial can be written as


taking coefficients of all terms our binary values are 1 0 0 1 1 0 0 0 1. This binary value or polynomial generator is used a divisor for CRC computations.

Compute checksum: (Best refer to Wikipedia link Smile) or there are readymade tools. But if you are interested to know below are steps..

  • Step 1: Take input bytes from Sensor (MSB + LSB as detailed in previous post). They are 16 bits longs (2 bytes). Append to them 0’s 8 to right (should be equal to length of polynomial generator). <Update> As we are stuffing 0 to right of data bytes, effectively “Left Shift” operation occurs.
  • Start from left most bit that has bit value of 1 and perform an XOR bitwise operation with polynomial generator. Skip any 0s in middle and perform operations if bit left most is 1.
  • Repeat these operations moving Polynomial generators to right, where bit value is 0 till 16 bits.
  • In Step 1, due to polynomial movement, initially stuffed 0’s will have changed to different state and that result is output of CRC computation.

As an example, in previous example measured humidity is 31827 when converted to binary is input. Using polynomial generator in an iterative manner compute CRC. Sample computations are below.


In this manner, receiving application can compute CRC for MSB and LSB compare with CRC byte to check if data received is not corrupted.

Post measure and performing CRC checks, formula need to applied to raw measurements to arrive at accurate values as provided in manual.

Relative Humidity Conversion:


Where S(RH) is Humidity Signal Output that is measured from Sensor.

Relative Humidity Conversion:


With this HTU21DF sensor internals are complete… As continuation to this series of blog, follow up blog will detail how to programmatically get data out Sensor using RASPBERRY PI 2 MODEL B and SPARKFUN WEATHERSHIELD that includes HTU21DF sensor…

Until next post…


under hood of HTU21DF sensor part 2

Schematic view of sensor with Pins:


  • GND: Ground
  • VDD: Voltage for Sensor between 3.6 to 5 Volts DC
  • SCK: Serial Clock Input is used to synchronize communication between master(microcontroller) and HTU21DF sensor.
  • Data: Serial data pin is used to transfer data in and out of sensor and device.

Communication Sequence with Sensors:

  • Start Sequence: A single bit. To initiate transmission start bit has to be issued by microcontroller.
  • Stop Sequence: A single bit again to indicate completion of transmission of data (along with CRC) and if required release holding of SCK.
  • Sending Command: After start bit to indicate initiation transmission, next is to send command to sensor identified by sensor address (similar to MAC address)
    • Device Address of HTU21DF address in hex 0x40 (in Decimal system = 64)
    • Device address is 7 bits wide with last bit capturing type of operation (Read / Write)
    • Thus controller will send initiate communication to HTU21DF Sensor as below for both read and write operations

As I2C protocol specifies, Start –> Address of Sensor (With Read / Write intent)

Read Operation:


Write Operation:


HTU21DF sensor responds (as its address is 0x40) with ACK (Acknowledgement). Then controller sends command to HTU21DF sensor. Commands their Hex and Binaries are below.


In comment section above there are “Hold Master” and “No Hold Master”. As detailed in previous post, I2C protocol provides options where sensor may choose to control serial clock (SCK).

Hold Master ensures control of SCK is with Sensor and controller can not perform any other actions with other sensors if available but is kept in waiting state till sensor measurement is complete. On the other hand, No Hold Master implies, control is SCK is with master (controller) and controller is free to communicate with other sensor. Additionally a polling is done with read command to check if measurement is done.

HOLD MASTER is analogous to synchronous programming and NO HOLD MASTER is equal to Asynchronous programming but with polling. May be NO HOLD MASTER is good for situations where there are more sensors on board.

Post measurement, data is sent by Sensor 2 bytes MSB(Most Significant Byte) followed by LSB (Least Significant Byte) and a checksum.

Additionally on LSB, last 2 significant bits are reserved for transmitting status information. In LSB (Bit 1 is used to indicate temperature / Humidity). 0 Temperature and 1 Humidity.

Communication Flow:

Step 1: Start (Initiate) session to Sensor (Slave in this case HTU21DF)

Step 2: Issue Write intent to sensor indicated by address


Step 3: Sensor sends ACK to master

Step 4: Issue “Measure Humidity with HOLD” command


Step 5: Sensor sends ACK

Step 6: Issue read intent to sensor with address of sensor.


Step 7: Sensor takes control of SCK line and makes master (microcontroller) wait till measurement is complete.

Step 8: Send Data. Post measurement sensor uses 14 bits (of data) + 2 bits (status) to send measured data using SDA line as output from device. Data is divided into 2 sections Most Significant Byte (MSB) and Least Significant Byte (LSB). First sent is MSB followed by LSB.


In above example, relative humidity is measured as 31827 (add numbers on right column (except status bits). Status column 1 indicates measurement is humidity (0 implies temperature).

Step 9: HTU21DF sensor provides CRC8 checksum. is computed by sensor and sent as an additionally redundant byte for data verification.

In this manner communication takes place between master (microcontroller) and I2C compliant sensor like HTU21DF.

Next section we will understand formulae for RH (relative humidity), Temperature and also verify data integrity from device using checksum provided in checksum Byte.

Appreciate if readers review and correct mistakes

Till then….


under hood of HTU21DF sensor part 1

There is so much to learn and so many moving parts in technology now a days, am always reminded of what I do not know, rather than what I know. Hope that clarifies my very basic knowledge and would be glad if someone corrects mistakes in this post. In this post I am trying to learn Sensors to do a side project in my company. So this is a idiots guide to sensor specifically HTU21DF sensor.

HTU21DF is a digital sensor for capturing “Humidity” and “Temperature”. Sensor provides signal in I2C (I Squared C)  format. I2C is a serial communication protocol that communicates over Two Wire Interface (TWI). In this post, let us learn about I2C / TWI Protocol.

I2C / TWI Protocol:

  • A Micro-controller will have limited number of pins.
  • But sensor that a micro-controller can serve are many but limited by pins.
  • A two wire interface bus is built around 2 lines
    • 1 SCL – Clock (Synchronizes data transfer between master & sensors)
    • 1 SDA – Data (Carries data)
  • These two wires / bus helps connect multitude of sensors to Microcontroller.

Schematically I2C bus along with sensors is something as below, in which each sensor is Slave is connected to microcontroller using two wires (bus) one for Signal Clock and other for Signal Data


With 7 Bit address, 128 devices (sensors+ Masters) can be added to controller and with 10 bit addressing 1024 devices can be used using serial bus. Every sensor has unique address on bus, that master uses to communicate to that sensor.

Data Protocol:

Chunk 1: 

  • S= Start Bit.
  • A0 to A6 are address bits for Sensors for data transfers
  • A7 type of operation (Read / Write) that master will perform on slave.
  • Followed by Acknowledge bit used by slave to acknowledge receipt of previous sequence of bits from master.


After ACK, master device hands over control of SDA link to sensor with address in Address bits.

Chunk 2:


Post ACK, are bits for internal registers for sensors. For example temperature  / humidity sensors will have internal registers to capture either address  / data. When detailing HTU21DF we will deal again with these internal registers.

Chunk 3:

Data being transmitted either from Master to Slave to Slave to Master depending upon RW bit set in Chunk 1 and finally ends with a Stop bit to indicate communication between Slave and Master ends.


Above outlines how I2C protocol works on a Two Way Interface using Serial Bus Architecture.

Next up is understanding specific sensor, HTU21DF in terms of “Address”, “Internal Registers” and “Data Format” packet.

Immediately following up with next blog that utilizes these concepts with sensor..