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SHT21 Datasheet(PDF) 8 Page - List of Unclassifed Manufacturers |
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SHT21 Datasheet(HTML) 8 Page - List of Unclassifed Manufacturers |
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8 / 14 page  Datasheet SHT21 www.sensirion.com Version 4 – May 2014 8/14 Command Comment Code Trigger T measurement hold master 1110’0011 Trigger RH measurement hold master 1110’0101 Trigger T measurement no hold master 1111’0011 Trigger RH measurement no hold master 1111’0101 Write user register 1110’0110 Read user register 1110’0111 Soft reset 1111’1110 Table 6 Basic command set, RH stands for relative humidity, and T stands for temperature Hold master or no hold master modes are explained in next Section. 5.4 Hold / No Hold Master Mode There are two different operation modes to communicate with the sensor: Hold Master mode or No Hold Master mode. In the first case the SCL line is blocked (controlled by sensor) during measurement process while in the latter case the SCL line remains open for other communication while the sensor is processing the measurement. No hold master mode allows for processing other I2C communication tasks on a bus while the sensor is measuring. A communication sequence of the two modes is displayed in Figure 15 and Figure 16, respectively. In the hold master mode, the SHT2x pulls down the SCL line while measuring to force the master into a wait state. By releasing the SCL line the sensor indicates that internal processing is terminated and that transmission may be continued. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 S 1 0 0 0 0 0 0 0 1 1 1 0 0 1 0 1 I2C address + write Command (see Table 6) 19 20 21 22 23 24 25 26 27 S 1 0 0 0 0 0 0 1 Measurement I2C address + read Hold during measurement 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 0 1 1 0 0 0 1 1 0 1 0 1 0 0 1 0 Data (MSB) Data (LSB) Stat. 46 47 48 49 50 51 52 53 54 0 1 1 0 0 1 0 0 P Checksum Figure 15 Hold master communication sequence – grey blocks are controlled by SHT2x. Bit 45 may be changed to NACK followed by Stop condition (P) to omit checksum transmission. In no hold master mode, the MCU has to poll for the termination of the internal processing of the sensor. This is done by sending a Start condition followed by the I2C header (1000’0001) as shown in Figure 16. If the internal processing is finished, the sensor acknowledges the poll of the MCU and data can be read by the MCU. If the measurement processing is not finished the sensor answers no ACK bit and the Start condition must be issued once more. When using the no hold master mode it is recommended to include a wait period of 20 µs after the reception of the sensor’s ACK bit (bit 18 in Figure 16) and before the Stop condition. For both modes, since the maximum resolution of a measurement is 14 bit, the two last least significant bits (LSBs, bits 43 and 44) are used for transmitting status information. Bit 1 of the two LSBs indicates the measurement type (‘0’: temperature, ‘1’ humidity). Bit 0 is currently not assigned. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 S 1 0 0 0 0 0 0 0 1 1 1 1 0 1 0 1 wait P I2C address + write Command (see Table 6) 20µs 19 20 21 22 23 24 25 26 27 Measurement S 1 0 0 0 0 0 0 1 P measuring I2C address + read 19 20 21 22 23 24 25 26 27 Measurement S 1 0 0 0 0 0 0 1 continue measuring I2C address + read 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 0 1 1 0 0 0 1 1 0 1 0 1 0 0 1 0 Data (MSB) Data (LSB) Stat. 46 47 48 49 50 51 52 53 54 0 1 1 0 0 1 0 0 P Checksum Figure 16 No Hold master communication sequence – grey blocks are controlled by SHT2x. If measurement is not completed upon “read” command, sensor does not provide ACK on bit 27 (more of these iterations are possible). If bit 45 is changed to NACK followed by Stop condition (P) checksum transmission is omitted. In the examples given in Figure 15 and Figure 16 the sensor output is SRH = ‘0110’0011’0101’0000’. For the calculation of physical values Status Bits must be set to ‘0’ – see Chapter 6. The maximum duration for measurements depends on the type of measurement and resolution chosen – values are displayed in Table 7. Maximum values shall be chosen for the communication planning of the MCU. |
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