DS1845

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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 can be changed

during the LOW period of the clock signal. There is one clock pulse per bit of data. Figures 2 and 3

detail how data transfer is accomplished on the two-wire bus. Depending upon the state of the R/W bit,

two types of data transfer are possible.

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 9

th

bit.

Within the bus specifications a regular mode (100 kHz clock rate) and a fast mode (400 kHz clock rate)

are defined. The DS1845 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 a 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.

1. Data transfer from a master transmitter to a slave receiver. The 1

the command/control byte. Next follows a number of data bytes. The slave returns an acknowledge bit

after each received byte.

2. Data transfer from a slave transmitter to a master receiver. The master transmits the 1

command/control byte) to the slave. The slave then returns an acknowledge bit. Next follows a

number of data bytes transmitted by the slave to the master. The master returns an acknowledge bit

after all received bytes other than the last byte. At the end of the last received byte, a ‘not

acknowledge’ can be returned.

The master device generates all serial clock pulses and the START and STOP conditions. A transfer is

ended with a STOP condition or with a repeated START condition. Since a repeated START condition is

also the beginning of the next serial transfer, the bus will not be released.