DS1847

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2-WIRE OPERATION

Clock and Data Transitions: The SDA pin is normally pulled high with an external resistor or device.

Data on the SDA pin may only change during SCL low time periods. Data changes during SCL high

periods will indicate a start or stop conditions depending on the conditions discussed below. Refer to the

timing diagram (Figure 4) for further details.

Start Condition: A high-to-low transition of SDA with SCL high is a start condition that must precede

any other command. Refer to the timing diagram (Figure 4) for further details.

Stop Condition: A low-to-high transition of SDA with SCL high is a stop condition. After a read

sequence, the stop command places the DS1847 into a low-power mode. Refer to the timing diagram

(Figure 4) for further details.

Acknowledge Bit: All address and data byte are transmitted via a serial protocol. The DS1847 pulls the

SDA line low during the ninth clock pulse to acknowledge that it has received each word.

Standby Mode: The DS1847 features a low-power mode that is automatically enabled after power-on,

after a stop command, and after the completion of all internal operations.

2-Wire Interface Reset: After any interruption in protocol, power loss, or system reset, the following

steps reset the DS1847:

1. Clock up to nine cycles.

2. Look for SDA high in each cycle while SCL is high.

3. Create a START condition while SDA is high.

Device Addressing: The DS1847 must receive an 8-bit device address word following a START

condition to enable a specific device for a read or write operation. The address word is clocked into the

DS1847 MSB to LSB. The address word consists of Ah (1010) followed by A2, A1, and A0 then the

R/W (READ/WRITE) bit. If the R/W bit is high, a read operation is initiated. If the R/W is low, a write

operation is initiated. For a device to become active, the values of A2, A1 and A0 must be the same as the

hard-wired address pins on the DS1847. Upon a match of written and hard-wired addresses, the DS1847

will output a zero for one clock cycle as an acknowledge. If the address does not match, the DS1847

returns to a low-power mode.

Write Operations: After receiving a matching address byte with the R/W bit set low, the device goes

into the write mode of operation. The master must transmit an 8-bit EEPROM memory address to the

device to define the address where the data is to be written. After this byte has been received, the DS1847

will transmit a zero for one clock cycle to acknowledge the receipt of the address. The master must then

transmit an 8-bit data word to be written into this address. The DS1847 will again transmit a zero for one

clock cycle to acknowledge the receipt of the data. At this point, the master must terminate the write

EEPROM memory. All inputs are disabled during this byte write cycle.

The DS1847 is capable of an 8-byte page write. A page write is initiated the same way as a byte write, but

the master does not send a STOP condition after the first byte. Instead, after the slave acknowledges

receipt of the data byte, the master can send up to seven more bytes using the same nine-clock sequence.