DS1855

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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 in Figure 2

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 in Figure 2 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 DS1855 into a low-power mode. Refer to the timing diagram in Figure 2 for further

details.

Acknowledge

All address and data bytes are transmitted via a serial protocol. The DS1855 pulls the SDA line low

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

Standby Mode

The DS1855 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 DS1855:

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 DS1855 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 DS1855 MSB to LSB. The

address word consists of Ah (1010) followed by A2, A1, and A0 then the read/write (R/W) 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

DS1855. Upon a match of written and hard-wired addresses, the DS1855 will output a zero for one clock

cycle as an acknowledge. If the address does not match, the DS1855 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 the reception of this byte, the DS1855 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 DS1855 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 operation with a

All inputs are disabled during this byte write cycle.