DS1845B-050 Datasheet DS1845B-050, DS1845E-010, DS1845E-100 | P4-P6

DS1845

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

steps reset the DS1845.

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 DS1845 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 DS1845

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. 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 DS1845. Upon a match of written and hard-wired addresses, the

DS1845 will output a zero for one clock cycle as an acknowledge. If the address does not match the

DS1845 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 DS1845

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 DS1845 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 stop condition. The DS1845 then enters an internally timed write process t

to the

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

The DS1845 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 1

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.

The master must terminate the write cycle with a stop condition or the data clocked into the DS1845 will

not be latched into permanent memory.

Acknowledge Polling: Once the internally-timed write has started and the DS1845 inputs are disabled,

acknowledge polling can be initiated. The process involves transmitting a start condition followed by the

device address. The R/W bit signifies the type of operation that is desired. The read or write sequence

will only be allowed to proceed if the internal write cycle has completed and the DS1845 responds with a

zero.

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

into the read mode of operation. There are three read operations: current address read, random read and

sequential address read.

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CURRENT ADDRESS READ

The DS1845 has an internal address register that maintains the address used during the last read or write

operation, incremented by one. This data is maintained as long as V

is valid. If the most recent

address was the last byte in memory, then the register resets to the first address. This address stays valid

between operations as long as power is available.

Once the device address is clocked in and acknowledged by the DS1845 with the R/W bit set to high, the

current address data word is clocked out. The master does not respond with a zero, but does generate a

stop condition afterwards.

RANDOM READ

A random read requires a dummy byte write sequence to load in the data word address. Once the device

and data address bytes are clocked in by the master, and acknowledged by the DS1845, the master must

generate another start condition. The master now initiates a current address read by sending the device

address with the read/write bit set high. The DS1845 will acknowledge the device address and serially

clocks out the data byte.

SEQUENTIAL ADDRESS READ

Sequential reads are initiated by either a current address read or a random address read. After the master

receives the first data byte, the master responds with an acknowledge. As long as the DS1845 receives

this acknowledge after a byte is read, the master may clock out additional data words from the DS1845.

After reaching address FFh, it resets to address 00h.

The sequential read operation is terminated when the master initiates a stop condition. The master does

not respond with a zero.

For a more detailed description of 2-wire theory of operation, refer to the following section.

2-WIRE SERIAL PORT OPERATION

The 2-wire serial port interface supports a bi-directional data transmission protocol with device

addressing. A device that sends data on the bus is defined as a transmitter, and a device receiving data as

a receiver. The device that controls the message is called a “master.” The devices that are controlled by

the master are “slaves.” The bus must be controlled by a master device that generates the serial clock

(SCL), controls the bus access, and generates the START and STOP conditions. The DS1845 operates as

a slave on the two-wire bus. Connections to the bus are made via the open-drain I/O lines SDA and SCL.

The following I/O terminals control the 2-wire serial port: SDA, SCL, A0, A1, A2. Timing diagrams for

the 2-wire serial port can be found in Figures 2 and 3. Timing information for the 2-wire serial port is

provided in the AC Electrical Characteristics table for 2-wire serial communications.

The following bus protocol has been defined:

- Data transfer may be initiated only when the bus is not busy.

- During data transfer, the data line must remain stable whenever the clock line is HIGH. Changes in

the data line while the clock line is HIGH will be interpreted as control signals.

Accordingly, the following bus conditions have been defined:

Bus not busy: Both data and clock lines remain HIGH.

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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 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

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

byte transmitted by the master is

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

byte (the

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.

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DS1845B-050

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DS1845B-050

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