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SHT21 Datasheet(PDF) 7 Page - Sensitron |
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SHT21 Datasheet(HTML) 7 Page - Sensitron |
7 / 12 page Datasheet SHT21 www.sensirion.com Version 1.0 – January 2010 7/12 commands from the master (MCU). Current consumption during start up is 350µA maximum. 5.2 Start / Stop Sequence Each transmission sequence begins with Start condition (S) and ends with Stop condition (P) as displayed in Figure 11 and Figure 12. Figure 11 Transmission Start Condition (S) - a high to low transition on the SDA line while SCL is high. The Start condition is a unique state on the bus created by the master, indicating to the slaves the beginning of a transmission sequence (bus is considered busy after a Start). Figure 12 Transmission Stop Condition (P) - a low to high transition on the SDA line while SCL is high. The Stop condition is a unique state on the bus created by the master, indicating to the slaves the end of a transmission sequence (bus is considered free after a Stop). 5.3 Sending a Command After sending the Start condition, the subsequent I2C header consists of the 7-bit I2C device address ‘1000’000’ and an SDA direction bit (Read R: ‘1’, 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. After the issue of a measurement command (‘1110’0011’ for temperature, ‘1110’0101’ for relative humidity’), the MCU must wait for the measurement to complete. The basic commands are summarized in Table 6. Hold master or no hold master modes are explained in next Section. 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 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 13 and Figure 14, 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 0 1 1 P Checksum Figure 13 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 14. 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. For both modes, since the maximum resolution of a measurement is 14 bit, the two last 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. SDA SCL 70% 30% 70% 30% SDA SCL 70% 30% 70% 30% |
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