SHTC1 datasheet(7/14 Pages) ETC2

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Version 3 – May 2014

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5.3 Starting a Measurement

A measurement communication sequence consists of a

START condition followed by the I2C header with the 7-bit

I2C device address and a write bit (write W: ‘0’). The

sensor indicates the proper reception of a byte by pulling

the SDA pin low (ACK bit) after the falling edge of the 8th

SCL clock. Then the sensor is ready to receive a 16-bit

measurement command. Again, the SHTC1 acknowledges

the proper reception of each byte with ACK condition. A

complete measurement cycle is presented in Figure 7.

With the acknowledgement of the measurement

command, the SHTC1 starts measuring humidity and

temperature.

5.4 Sensor Behavior during Measurement and

Clock Stretching

In general, the sensor does not respond to any I2C activity

during measurement, i.e. I2C read and write headers are

not acknowledged (NACK). However, when clock

stretching has been enabled by using a corresponding

measurement command, the sensor responds to a read

header with an ACK and subsequently pulls down the SCL

line until the measurement is complete. As soon as the

measurement is complete, the sensor starts sending the

measurement results.

During measurement, the sensor has a current

consumption according to Table 3.

For best possible repeatability of humidity and temperature

measurements, it is recommended to avoid any

communication on the I2C bus while the SHTC1 is

measuring. For more information, see application note

“SHTC1 Optimization of Repeatibility”.

5.5 Readout of Measurement Results

After a measurement command has been issued and the

sensor has completed the measurement, the master can

read the measurement results by sending a START

condition followed by an I2C read header. The sensor will

acknowledge the reception of the read header and send

two bytes of data followed by one byte CRC checksum

and another two bytes of data followed by one byte CRC

checksum. Each byte must be acknowledged by the

microcontroller with an ACK condition for the sensor to

continue sending data. If the SHTC1 does not receive an

ACK from the master after any byte of data, it will not

continue sending data.

Whether the sensor sends out humidity or temperature

data first depends on the measurement command that

was sent to the sensor to initiate the measurement (see

Table 9).

The I2C master can abort the read transfer with a NACK

condition after any data byte if it is not interested in

subsequent data, e.g. the CRC byte or the second

measurement result, in order to save time.

In case the user needs humidity and temperature data but

does not want to process CRC data, it is recommended to

read the first two bytes of data with the CRC byte (without

processing the CRC data) and abort the read transfer after

reading the second two data bytes with a NACK. This

procedure is more time efficient than starting two different

measurements and aborting the read transfer after the first

two data bytes each time.

5.6 Soft Reset

The SHTC1 provides a soft reset mechanism that forces

the system into a well-defined state without removing the

power supply. If the system is in idle state (i.e. if no

measurement is in progress) the soft reset command can

be sent to SHTC1 according to Figure 8. This triggers the

sensor to reset all internal state machines and reload

calibration data from the memory.

Command

Hex. Code

Bin. Code

Software reset

0x805D

1000’0000’0101’1101

Table 10 Soft reset command.

5.7 Read-out of ID Register

The SHTC1 has an ID register which contains an SHTC1-

specific product code. The read-out of the ID register can

be used to verify the presence of the sensor and proper

communication. The command to read the ID register is

shown in Table 11.

Command

Hex. Code

Bin. Code

Read ID register

0xEFC8

1110’1111’1100’1000

Table 11 Read-out command of ID register.

It needs to be sent to the SHTC1 after an I2C write header.

After the SHTC1 has acknowledged the proper reception

of the command, the master can send an I2C read header

and the SHTC1 will submit the 16-bit ID followed by 8 bits

of CRC. The structure of the ID is described in Table 12.

16-bit ID

xxxx'xxxx’xx 00’0111

bits 5 to 0: SHTC1-specific product code

bits 15 to 6: unspecified information

Table 12 Structure of the 16-bit ID. Bits 15:6 of the ID contain

unspecified information (marked as “x”), which may vary from

sensor to sensor, while bits 5:0 contain the SHTC1-specific

product code.

5.8 Checksum Calculation

The 8-bit CRC checksum transmitted after each data word

is generated by a CRC algorithm with the properties

displayed in Table 13. The CRC covers the contents of the

two previously transmitted data bytes.

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