General 2-Wire Information

● All data is transmitted MSb first over the 2-wire bus.

● One bit of data is transmitted on the 2-wire bus each

SCL period.

● A pullup resistor is required on the SDA line and, when

the bus is idle, both SDA and SCL must remain in a

logic-high state.

● All bus communication must be initiated with a START

condition and terminated with a STOP condition.

During a START or STOP is the only time SDA is

allowed to change states while SCL is high. At all other

times, changes on the SDA line can only occur when

SCL is low: SDA must remain stable when SCL is high.

● After every 8-bit (1-byte) transfer, the receiving device

must answer with an ACK (or NACK), which takes one

SCL period. Therefore, nine clocks are required for

every one-byte data transfer.

Writing to the DS75

To write to the DS75, the master must generate a START

followed by an address byte containing the DS75 bus

address. The value of the R/W bit must be a 0, which

indicates that a write is about to take place. The DS75

will respond with an ACK after receiving the address byte.

This must be followed by a pointer byte from the master,

which tells the DS75 which register is being written to. The

DS75 will again respond with an ACK after receiving the

pointer byte. Following this ACK the master device must

immediately begin transmitting data to the DS75. When

writing to the configuration register, the master must send

one byte of data (see Figure 9a), and when writing to the

of data (see Figure 9b). After receiving each data byte,

the DS75 will respond with an ACK, and the transaction is

finished with a STOP from the master.

Reading from the DS75

When reading from the DS75, if the pointer was already

pointed to the desired register during a previous trans-

action, the read can be performed immediately without

changing the pointer setting. In this case the master

sends a START followed by an address byte containing

the DS75 bus address. The R/W bit must be a 1, which

tells the DS75 that a read is being performed. After the

DS75 sends an ACK in response to the address byte,

the DS75 will begin transmitting the requested data on

the next clock cycle. When reading from the configura-

tion register, the DS75 will transmit one byte of data, after

which the master must respond with a NACK followed by

mit two bytes of data, and the master must respond to the

first data byte with an ACK and to the second byte with

a NACK followed by a STOP (see Figure 9d). If only the

most significant byte of data is needed, the master can

issue a NACK followed by a STOP after reading the first

data byte in which case the transaction will be the same

as for a read from the configuration register.

If the pointer is not already pointing to the desired register,

which shows a pointer update followed by a single-byte

read. The value of the R/W bit in the initial address byte

is a 0 (“write”) since the master is going to write a pointer

byte to the DS75. After the DS75 to the address byte with

an ACK, the master sends a pointer byte that corresponds

to the desired register. The master must then perform a

repeated start followed by a standard one or two byte

read sequence (with R/W =1) as described in the previ-

ous paragraph.

DS75

Digital Thermometer and Thermostat

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