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SDP511 Datasheet(PDF) 5 Page - List of Unclassifed Manufacturers |
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SDP511 Datasheet(HTML) 5 Page - List of Unclassifed Manufacturers |
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5 / 10 page  www.sensirion.com Version 1.9 – July 2015 5/10 3.4 Data transfer format Data is transferred in byte packets in the I2C protocol, which means in 8-bit frames. Each byte is followed by an acknowledge bit. Data is transferred with the most significant bit (MSB) first. A data transfer sequence is initiated by the master generating the Start condition (S) and sending a header byte. The I2C header consists of the 7-bit I2C device address and the data direction bit (R/_W). The value of the R/_W bit in the header determines the data direction for the rest of the data transfer sequence. If R/_W = 0 (WRITE) the direction remains master-to-slave, while if R/_W = 1 (READ) the direction changes to slave- to-master after the header byte. 4. Command Set and Data Transfer Sequences A command is represented by an 8-bit command code. The data direction may not change after the command byte, since the R/_W bit of the preceding I2C header has already determined the direction to be master-to-slave. In order to execute commands in Read mode using I2C, the following principle is used. On successful (acknowledged) receipt of a command byte, the sensor stores the command nibble internally. The Read mode of this command is then invoked by initiating an I2C data transfer sequence with R/_W = 1. If a correctly addressed sensor recognizes a valid command and access to this command is granted, it responds by pulling down the SDA line during the subsequent SCL pulse for the acknowledge signal (ACK). Otherwise it leaves the SDA line unasserted (NACK). The two most important commands are described in this data sheet, and the data transfer sequences are specified. Contact Sensirion for advanced sensor options. 4.1 Measurement triggering Each individual measurement is triggered by a separate read operation. Note that two transfer sequences are needed to perform a measurement. First write command byte hF1 (trigger measurement) to the sensor, and then execute a read operation to trigger the measurement and retrieve the flow or differential pressure information. On receipt of a header with R/_W=1, the sensor generates the Hold Master condition on the bus until the first measurement is completed. After the Hold Master condition is released, the master can read the result as two consecutive bytes. A CRC byte follows if the master continues clocking the SCL line after the second result byte. The sensor checks whether the master sends an acknowledge after each byte and aborts the transmission if it does not. I2C Measurement 8-bit command code: hF1 Command: Trigger differential pressure measurement 13 12 18 19 22 21 27 MSByte MeasData LSByte MeasData 11 14 15 16 17 20 23 24 25 26 28 31 30 36 Check Byte 29 32 33 34 35 1 2 3 4 5 6 7 8 9 18 S I2CAdr S W 1 2 3 4 5 6 7 8 9 10 Hold Master P 10 11 12 13 14 15 16 17 1 1 1 1 0 0 0 1 Command 1 0 0 0 0 0 0 R 1 0 0 0 0 0 0 I2CAdr 0 1 Hatched areas indicate that the sensor controls the SDA line. Note that the first measurement result after reset is not valid. 4.2 Soft reset This command forces a sensor reset without switching the power off and on again. On receipt of this command, the sensor reinitializes the control/status register contents from the EEPROM and starts operating according to these settings. I2C Soft Reset 8-bit command code: hFE Command: Soft reset 1 2 3 4 5 6 7 8 S 9 18 system reboot 10 11 12 13 14 15 16 17 I2CAdr W 1 0 0 0 0 0 0 1 1 1 1 Command 1 1 1 0 0 4.3 CRC-8 Redundant Data Transmission Cyclic redundancy checking (CRC) is a popular technique used for error detection in data transmission. The transmitter appends an n-bit checksum to the actual data sequence. The checksum holds redundant information about the data sequence and allows the receiver to detect transmission errors. The computed checksum can be regarded as the remainder of a polynomial division, where the dividend is the binary polynomial defined by the data sequence and the divisor is a “generator polynomial”. The sensor implements the CRC-8 standard based on the generator polynomial x8 + x5 + x4 +1. Note that CRC protection is only used for date transmitted from the slave to the master. For details regarding cyclic redundancy checking, please refer to the relevant literature. |
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