DS1803

3 of 11

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. Figure 2 details how

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

ninth bit.

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

are defined. The DS1803 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.

1. Data transfer from a master transmitter to a slave receiver: The first byte transmitted by the

master is the control byte (slave address). 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 first byte (the slave address) is

transmitted by the master. 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’is returned.

The master device generates all of the 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.

The DS1803 may operate in the following two modes:

1. Slave receiver mode: Serial data and clock are received through SDA and SCL. After each byte is

received, an acknowledge bit is transmitted. START and STOP conditions are recognized as the

beginning and end of a serial transfer. Address recognition is performed by hardware after reception

of the slave address and direction bit.