DS1626_07 Datasheet, PDF (12/14 Pages) Dallas Semiconductor – High-Precision 3-Wire Digital Thermometer and Thermostat

DS1626/DS1726
3-WIRE SERIAL DATA BUS
The 3-wire bus consists of three signals: RST (reset—active low), CLK (clock), and DQ (data). 3-wire
communication is controlled by the RST signal, which functions as “chip select” signal. All data is
transferred LSb first over the 3-wire bus. All communication with the DS1626/DS1726 is initiated by
driving RST high. Driving RST low terminates communications and causes DQ to go to a high-
impedance state. Note that RST must be toggled low after every communication sequence to ensure
that subsequent commands are recognized by the DS1626/DS1726.
When writing to the DS1626/DS1726, data must be valid during the rising edge of CLK. During read
operations the DS1626/DS1726 output data on DQ on the falling edge of CLK and the data remains
valid through the following rising edge, at which time the DQ pin becomes high impedance until the
next falling edge.
To communicate with the DS1626/DS1726, the master must first drive RST high and then begin
generating the CLK signal while transmitting the desired DS1626/DS1726 command byte. If the
command is a Start Convert T, Stop Convert T, or Software POR command, the transaction is finished
when the last bit of the command has been sent. Figure 7a illustrates a Start Convert T command
sequence.
When writing to the DS1626/DS1726, the master must begin transmitting data during the clock cycle
immediately following the command byte. The DS1626/DS1726 will save only the number of data bits
needed for the specific transaction. For example, for the Write TH or Write TL commands, after
twelve bits of data have been transmitted by the master, the DS1626/DS1726 will ignore any
subsequent data transmitted before RST goes low. Thus, if data is being transmitted in byte-length
segments, the DS1626/DS1726 will load the first twelve bits into the T_H/T_L register, and the next four
bits will be ignored. On the other hand, it is necessary to transmit at least the required number of bits
for the requested transaction (i.e., 12-bits to T_H/T_L or 8-bits to the configuration register), because the
DS1626/DS1726 will not save data until the expected number of bits have been received. Write TH
and Write TL sequences are illustrated in Figure 7b and a Write Config sequence is shown in Figure
7c. Note that these figures assume byte-wide data transfers.
When reading data from the DS1626/DS1726, the DS1626/DS1726 will begin sending data during the
clock cycle immediately following the command byte. After the last data byte has been sent, the
DS1626/DS1726 will transmit a 0 during each subsequent clock until RST goes low. Figure 7d
illustrates a Read Temperature sequence and a Read Config transaction is shown in Figure 7e. The
sequence for reading the T_H or T_L registers is the same as the Read Temp transaction in Figure 7d
except that the Read TH or Read TL command is used.
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