SHT31 BASCOM-AVR Code: I²C Reading, CRC-8 and Conversion

What this SHT31 implementation covers

Temperature and relative humidity monitoring is useful in drying equipment, storage cabinets and other environmental-control systems. Among common digital sensors such as DHT22, BME280, SHT31 and SHT35, the SHT31 provides a compact I²C interface and separate integrity checks for temperature and humidity data.

This implementation follows the complete path from an I²C command to a checked and converted value. It separates four stages that are often collapsed into one: starting a measurement, receiving the six-byte data structure, validating data integrity and formatting the result.

The real SHT31 module and bench wiring

The breakout board exposes the primary connections VIN, GND, SCL and SDA/RH. Additional reverse-side pads are labelled AL and AD, with the supporting resistors and passive components already installed on the module.

The module connects to a small controller board through a short four-wire lead, while a ribbon cable connects the controller to the OLED. The wiring follows the labelled power and I²C functions on the breakout board.

BASCOM-AVR configuration and variables

The BASCOM source file is named SHT31 Test mit CRC.bas. It targets an ATmega328P definition, uses a 16 MHz clock and configures serial output at 9600 baud. Hardware TWI is initialized at 400 kHz.

The program stores converted readings in Temperatur and Feuchte, and formats them into Str_temp and Str_rel. Ar(10) is the receive buffer, although only elements 1 through 6 are used for the transaction. Calc holds the combined 16-bit raw word. Crc, Anded and Rep support the bitwise CRC loop.

Initialization and variable declarations

$regfile = "m328pdef.dat"
$crystal = 16000000
$baud = 9600

$lib "i2c_twi.lbx"
Config Scl = PORTC.5
Config Sda = PORTC.4
Config Twi = 400000
I2cinit

Dim Temperatur As Single , Str_temp As String * 10
Dim Feuchte As Single , Str_rel As String * 10
Dim Ar(10) As Byte
Dim Calc As Word
Dim I As Integer
Dim Crc As Byte
Dim Anded As Byte
Dim Rep As Byte

Do
  Str_temp = ""
  Str_rel = ""

How the code starts one SHT31 measurement

Inside Do ... Loop, the program writes the 8-bit address 0x88 followed by 0x24 0x00. Together, 0x2400 requests single-shot, high-repeatability measurement without clock stretching, not just a generic read. The SHT3x-DIS datasheet specifies 12.5 ms typical and 15.5 ms maximum for high-repeatability conversion.

The example uses Wait 1 (one second), a deliberately conservative demo delay rather than the sensor’s required conversion time. The selected 7-bit I²C address is 0x44: (0x44 << 1) | 0 gives 0x88 for writing and (0x44 << 1) | 1 gives 0x89 for reading. If ADDR selects 0x45, the corresponding transmitted bytes are 0x8A/0x8B.

What the six returned bytes contain

The read transaction starts again and sends 0x89. The first five bytes are read with ACK, while the sixth and final byte is read with NACK before the bus is stopped. This ACK/NACK pattern lets the controller accept each continuing byte and then terminate the read cleanly after the known frame length.

Measurement command and six-byte read

I2cstart
I2cwbyte &H88
I2cwbyte &H24
I2cwbyte &H00
I2cstop

Wait 1

I2cstart
I2cwbyte &H89
I2crbyte Ar(1) , Ack
I2crbyte Ar(2) , Ack
I2crbyte Ar(3) , Ack
I2crbyte Ar(4) , Ack
I2crbyte Ar(5) , Ack
I2crbyte Ar(6) , Nack
I2cstop

How the CRC-8 check works

Temperature and humidity are validated independently. For temperature, the loop processes Ar(1) and Ar(2), then compares the result with Ar(3). For humidity, it processes Ar(4) and Ar(5), then compares the result with Ar(6).

1. Initialize Crc to 0xFF.
2. XOR the next data byte into the current CRC.
3. Repeat eight times, testing the top bit with 0x80.
4. Shift left once; when the tested bit was set, XOR with 0x31.
5. Accept the word only when the calculated CRC equals its received CRC byte.

A six-byte read is not proof that the values are valid. Both two-byte measurement words need their own initialized CRC calculation; the previous temperature CRC must not be reused for humidity. The checksum uses CRC-8, polynomial 0x31 (x⁸ + x⁵ + x⁴ + 1), init 0xFF, no reflection, and final XOR 0x00.

Temperature CRC and conversion code

Crc = &HFF
For I = 1 To 2
  Crc = Crc Xor Ar(i)
  For Rep = 1 To 8
    Anded = Crc And &H80
    If Anded <> 0 Then
      Shift Crc , Left , 1
      Crc = Crc Xor &H31
    Else
      Shift Crc , Left , 1
    End If
  Next Rep
Next I

If Crc = Ar(3) Then
  Calc = Makeint(ar(2) , Ar(1))
  Temperatur = Calc * 175
  Temperatur = Temperatur / 65535
  Temperatur = Temperatur - 45
  Str_temp = Fusing(temperatur , "#.#")
  Str_temp = Str_temp + " C "
Else
  Str_temp = "--- C "
End If

SHT31 temperature and humidity conversion formulas

After a successful temperature CRC check, the program combines Ar(2) and Ar(1) with Makeint. The resulting unsigned 16-bit value is scaled across 65535 counts.

The valid temperature is formatted with Fusing(Temperatur, "#.#"), producing one decimal place, and C is appended. If CRC fails, the temperature string becomes --- C.

Humidity follows the same structure after its own CRC check. Ar(5) and Ar(4) are combined, then scaled directly to percent relative humidity.

The valid value is formatted to one decimal place and followed by %. A failed humidity CRC produces --- %. The placeholders are useful because they preserve the output shape without presenting corrupted data as a plausible environmental reading.

Serial output and the final OLED test

After the temperature check, CRC must be reset to 0xFF and recalculated over Ar(4) and Ar(5) before comparison with Ar(6). The original HTML excerpt omitted this intermediate step. The corrected listing below then prints both formatted results.

Corrected humidity CRC, conversion and serial output

Crc = &HFF
For I = 4 To 5
  Crc = Crc Xor Ar(i)
  For Rep = 1 To 8
    Anded = Crc And &H80
    If Anded <> 0 Then
      Shift Crc , Left , 1
      Crc = Crc Xor &H31
    Else
      Shift Crc , Left , 1
    End If
  Next Rep
Next I

If Crc = Ar(6) Then
  Calc = Makeint(ar(5) , Ar(4))
  Feuchte = Calc * 100
  Feuchte = Feuchte / 65535
  Str_rel = Fusing(feuchte , "#.#")
  Str_rel = Str_rel + " % "
Else
  Str_rel = "--- % "
End If

Print Str_temp ; " | " ; Str_rel
Print

Loop
End

The supplied OLED image shows 18.4 °C and 50.9% RH in the depicted setup. However, the BASCOM source presented here only prints sensor data over serial; it does not implement OLED drawing. The image should not be treated as proof that this exact corrected BASCOM listing was compiled and tested on the photographed hardware.

Complete corrected SHT31 BASCOM-AVR example

This single copyable listing combines the code blocks above. It targets the ATmega328P at 16 MHz, uses the original 400 kHz TWI setup and fixes the missing humidity CRC calculation. Code-review example: it has not been compiled in a BASCOM-AVR IDE or independently run on the pictured bench. For a deployed instrument, add explicit address ACK/read-failure handling, recovery and logging rather than relying on CRC alone.

$regfile = "m328pdef.dat"
$crystal = 16000000
$baud = 9600

$lib "i2c_twi.lbx"
Config Scl = PORTC.5
Config Sda = PORTC.4
Config Twi = 400000
I2cinit

Dim Temperatur As Single , Str_temp As String * 10
Dim Feuchte As Single , Str_rel As String * 10
Dim Ar(10) As Byte
Dim Calc As Word
Dim I As Integer
Dim Crc As Byte
Dim Anded As Byte
Dim Rep As Byte

Do
  Str_temp = ""
  Str_rel = ""

I2cstart
I2cwbyte &H88
I2cwbyte &H24
I2cwbyte &H00
I2cstop

Wait 1

I2cstart
I2cwbyte &H89
I2crbyte Ar(1) , Ack
I2crbyte Ar(2) , Ack
I2crbyte Ar(3) , Ack
I2crbyte Ar(4) , Ack
I2crbyte Ar(5) , Ack
I2crbyte Ar(6) , Nack
I2cstop

Crc = &HFF
For I = 1 To 2
  Crc = Crc Xor Ar(i)
  For Rep = 1 To 8
    Anded = Crc And &H80
    If Anded <> 0 Then
      Shift Crc , Left , 1
      Crc = Crc Xor &H31
    Else
      Shift Crc , Left , 1
    End If
  Next Rep
Next I

If Crc = Ar(3) Then
  Calc = Makeint(ar(2) , Ar(1))
  Temperatur = Calc * 175
  Temperatur = Temperatur / 65535
  Temperatur = Temperatur - 45
  Str_temp = Fusing(temperatur , "#.#")
  Str_temp = Str_temp + " C "
Else
  Str_temp = "--- C "
End If

Crc = &HFF
For I = 4 To 5
  Crc = Crc Xor Ar(i)
  For Rep = 1 To 8
    Anded = Crc And &H80
    If Anded <> 0 Then
      Shift Crc , Left , 1
      Crc = Crc Xor &H31
    Else
      Shift Crc , Left , 1
    End If
  Next Rep
Next I

If Crc = Ar(6) Then
  Calc = Makeint(ar(5) , Ar(4))
  Feuchte = Calc * 100
  Feuchte = Feuchte / 65535
  Str_rel = Fusing(feuchte , "#.#")
  Str_rel = Str_rel + " % "
Else
  Str_rel = "--- % "
End If

Print Str_temp ; " | " ; Str_rel
Print

Loop
End

SHT31 I²C NACK, invalid readings and CRC-8 troubleshooting

Check the failure at the earliest observable stage. An address NACK, a measurement-not-ready NACK, a corrupted data frame and a wrong conversion are different faults and should not be diagnosed with the same fix.

What makes this implementation useful

The value of the example is not a large software framework. It keeps every boundary where a sensor reading can fail separate and inspectable: command transfer, conversion delay, data length, byte order, CRC, numeric conversion and output formatting.

• The frame layout is explicit. Temperature and humidity each have their own data pair and CRC byte, which must be calculated and checked separately.
• Integrity is checked before calculation. On a successful six-byte transfer, CRC-mismatched raw words are rejected instead of being converted into realistic-looking numbers.
• CRC failure is visible. --- C and --- % distinguish checksum failures from valid zeroes; I²C transaction failures still need separate handling.
• The formulas are traceable. The displayed number can be followed back to the exact 16-bit word used in the calculation.

For a drying cabinet or another environmental monitor, that traceability matters more than merely producing two numeric readings. It gives the developer a clear place to test the bus, received data, CRC and conversion independently.