DS1775 Datasheet, PDF (8/14 Pages) Dallas Semiconductor – SOT23-5 Digital Thermometer and Thermostat

DS1775
2-WIRE SERIAL DATA BUS
The DS1775 supports a bidirectional 2-wire bus and data transmission protocol. A device that sends data
onto the bus is defined as a transmitter, and a device receiving data as a receiver. The device that controls
the message is called a “master”. The devices that are controlled by the master are “slaves”. The bus must
be controlled by a master device which generates the serial clock (SCL), controls the bus access, and
generates the START and STOP conditions. The DS1775 operates as a slave on the 2-wire bus.
Connections to the bus are made via the open-drain I/O lines SDA and SCL.
The following bus protocol has been defined (see Figure 3):
• Data transfer may be initiated only when the bus is not busy.
• During data transfer, the data line must remain stable whenever the clock line is HIGH. Changes
in the data line while the clock line is high are interpreted as control signals.
Accordingly, the following bus conditions have been defined:
Bus not busy: Both data and clock lines remain HIGH.
Start data transfer: A change in the state of the data line, from HIGH to LOW, while the clock is
HIGH, defines a START condition.
Stop data transfer: A change in the state of the data line, from LOW to HIGH, while the clock line
is HIGH, defines the STOP condition.
Data valid: The state of the data line represents valid data when, after a START condition, the data
line is stable for the duration of the HIGH period of the clock signal. The data on the line must be
changed during the LOW period of the clock signal. There is one clock pulse per bit of data.
Each data transfer is initiated with a START condition and terminated with a STOP condition. The
number of data bytes transferred between START and STOP conditions is not limited, and is
determined by the master device. The information is transferred byte-wise and each receiver
acknowledges with a ninth bit.
Within the bus specifications a standard mode (100kHz clock rate) and a fast mode (400kHz clock
rate) are defined. The DS1775 works in both modes.
Acknowledge: Each receiving device, when addressed, is obliged to generate an acknowledge after
the reception of each byte. The master device must generate an extra clock pulse which is associated
with this acknowledge bit.
A device that acknowledges must pull down the SDA line during the acknowledge clock pulse in such
a way that the SDA line is stable LOW during the HIGH period of the acknowledge related clock
pulse. Of course, setup and hold times must be taken into account. A master must signal an end of
data to the slave by not generating an acknowledge bit on the last byte that has been clocked out of
the slave. In this case, the slave must leave the data line HIGH to enable the master to generate the
STOP condition.
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