SI7021-A20 Datasheet, PDF (19/36 Pages) Silicon Laboratories – IC HUMIDITY AND TEMPERATURE SENSOR

Si7021-A20
5.1. Issuing a Measurement Command
The measurement commands instruct the Si7021 to perform one of two possible measurements; Relative Humidity
or Temperature. The procedure to issue any one of these commands is identical. While the measurement is in
progress, the option of either clock stretching (Hold Master Mode) or Not Acknowledging read requests (No Hold
Master Mode) is available to indicate to the master that the measurement is in progress; the chosen command
code determines which mode is used.
Optionally, a checksum byte can be returned from the slave for use in checking for transmission errors. The
checksum byte will follow the least significant measurement byte if it is acknowledged by the master. The
checksum byte is not returned if the master “not acknowledges” the least significant measurement byte. The
checksum byte is calculated using a CRC generator polynomial of x⁸ + x⁵ + x⁴ + 1, with an initialization of 0x00.
The checksum byte is optional after initiating an RH or temperature measurement with commands 0xE5, 0xF5,
0xE3, and 0xF3. The checksum byte is required for reading the electronic ID with commands 0xFA 0x0F and 0xFC
0xC9. For all other commands, the checksum byte is not supported.
Table 12. I²C Bit Descriptions
Name
Symbol
Description
START
S SDA goes low while SCL high.
STOP
P SDA goes high while SCL high.
Repeated START
READ
Sr SDA goes low while SCL high. It is allowable to generate a STOP before the
repeated start. SDA can transition to high before or after SCL goes high in
preparation for generating the START.
R Read bit = 1
WRITE
W Write bit = 0
All other bits
— SDA value must remain high or low during the entire time SCL is high (this is
the set up and hold time in Figure 1).
In the I²C sequence diagrams in the following sections, bits produced by the master and slave are color coded as
shown:
Master
Slave
Rev. 1.1
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