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OPERATION — TRANSMITTING / RECEIVING DATA

Data sent and received from the 1-Wire Master passes through the transmit/receive buffer location. The 1-

Wire Master is actually double buffered with separate transmit and receive buffers. Writing to this

location connects the Transmit Buffer to the data bus, while reading connects the Receive Buffer to the

data bus.

Transmit Buffer (Write) / Receive Buffer (Read)

Addr. 01h

Data7

Data6

Data5

Data4

Data3

Data2

Data1

Data0

MSB

LSB

Writing a Byte

To send a byte on the 1-Wire bus, the user writes the desired data to the Transmit Buffer. The data is then

moved to the Transmit Shift Register where it is shifted serially onto the bus LSB first. A new byte of

data can then be written to the Transmit Buffer. As soon as the Transmit Shift Register is empty, the data

will be transferred from the Transmit Buffer and the process repeats. Each of these registers has a flag

that may be used as an interrupt source. The Transmit Buffer Empty (TBE) flag is set when the Transmit

Buffer is empty and ready to accept a new byte. As soon as a byte is written into the Transmit Buffer,

TBE is cleared. The Transmit Shift Register Empty (TEMT) flag is set when the shift register has no data

in it and is ready to accept a new byte. As soon as a byte of data is transferred from the Transmit Buffer,

TEMT is cleared and TBE is set. Remember that proper 1-Wire protocol requires a reset before any bus

communication.

Reading a byte

To read data from a slave device, the device must first be ready to transmit data depending on commands

already received from the CPU. Data is retrieved from the bus in a similar fashion to a write operation.

The host initiates a read by writing to the Transmit Buffer. The data that is then shifted into the Receive

Shift Register is the wired-AND of the written data and the data from the slave device. Therefore, in order

to read a byte from a slave device the host must write 0xFFh. When the Receive Shift Register is full the

data is transferred to the Receive Buffer where it can be accessed by the host. Additional bytes can now

be read by sending 0xFFh again. If the slave device is not ready to transmit, the data received will be

identical to that which was transmitted. The Receive Buffer Register can also generate interrupts. The

Receive Buffer flag (RBF) is set when data is transferred from the Receive Shift Register and cleared

when the host reads the register. If RBF is set, no further transmissions should be made on the 1-Wire bus

or else data may be lost, as the byte in the Receive Buffer will be overwritten by the next received byte.

See the timing diagrams for details of the byte reception operation. Generating a 1-Wire reset on the bus

is covered under Command Operations. Interrupt flags are explained in further detail under Interrupt

Operations. Write and read operations are detailed in the timing diagrams.