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1-100694-01 Arkusz danych(PDF) 8 Page - Sensitron |
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8 / 12 page Datasheet SHT21 www.sensirion.com Version 1.0 – January 2010 8/12 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 I2C address + write Command (see Table 6) 19 20 21 22 23 24 25 26 27 Measurement S 1 0 0 0 0 0 0 1 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 0 1 1 P Checksum Figure 14 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 13 and Figure 14 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. Resolution RH typ RH max T typ T max Units 14 bit 66 85 ms 13 bit 33 43 ms 12 Bit 22 29 17 22 ms 11 bit 12 15 9 11 ms 10 bit 7 9 ms 8 bit 3 4 ms Table 7 Measurement times for RH and T measurements at different resolutions. Typical values are recommended for calculating energy consumption while maximum values shall be applied for calculating waiting times in communication. Please note: I2C communication allows for repeated Start conditions (S) without closing prior sequence with Stop condition (P) – compare Figures 13, 14 and 16. Still, any sequence with adjacent Start condition may alternatively be closed with a Stop condition. 5.5 Soft Reset This command (see Table 6) is used for rebooting the sensor system without switching the power off and on again. Upon reception of this command, the sensor system reinitializes and starts operation according to the default settings. The soft reset takes less than 15 ms. 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 1 1 1 0 P I2C address + write Soft Reset Figure 15 Soft Reset – grey blocks are controlled by SHT2x. 5.6 User Register The content of user register is described in Table 8. Please note that reserved bits must not be changed. Bit # Bits Description / Coding Default 7, 0 2 Measurement resolution RH T ‘00’ 12 bit 14 bit ‘01’ 8 bit 12 bit ‘10’ 10 bit 13 bit ‘11’ 11 bit 11 bit ‘00’ 6 1 Status: End of battery16 ‘0’: VDD > 2.25 V ‘1’: VDD < 2.25 V ‘0’ 3, 4, 5 3 Reserved 2 1 Enable on-chip heater ‘0’ 1 1 Disable OTP reload ‘1’ Table 8 User Register. Cut-off value for End of Battery signal may vary by ±0.05V. Reserved bits must not be changed. “OTP reload” = ‘0’ loads default settings after each time a measurement command is issued. The End of Battery alert is activated when the battery power falls below 2.25V. The heater is intended to be used for functionality diagnosis – relative humidity drops upon rising temperature. The heater consumes about 5.5mW and provides a temperature increase of about 0.5 – 1.5°C. OTP Reload is a safety feature and loads the entire OTP settings to the register before every measurement. This feature is disabled per default and is not recommended for use. Please use Soft Reset instead – it contains OTP Reload. An example for I2C communication reading and writing the User Register is given in Figure 16. It is important that first the content of the register is read in order getting the default values. Hereafter, the resolution is changed by configuring register. Please note that the reserved user- register bits must not be changed. 16 This status bit is updated after each measurement |
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