LTC2497 datasheet(13/28 Pages) LINER

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LTC2497

2497f

I2C INTERFACE

The LTC2497 communicates through an I2C interface. The

I2C interface is a 2-wire open-drain interface supporting

multiple devices and multiple masters on a single bus. The

connected devices can only pull the data line (SDA) low

and can never drive it high. SDA is required to be externally

connected to the supply through a pull-up resistor. When

the data line is not being driven, it is high. Data on the

I2C bus can be transferred at rates up to 100kbits/s in the

standard mode and up to 400kbits/s in the fast mode.

Each device on the I2C bus is recognized by a unique

address stored in that device and can operate either as a

transmitter or receiver, depending on the function of the

device. In addition to transmitters and receivers, devices

can also be considered as masters or slaves when perform-

ing data transfers. A master is the device which initiates a

data transfer on the bus and generates the clock signals

to permit that transfer. Devices addressed by the master

are considered a slave.

The LTC2497 can only be addressed as a slave. Once ad-

dressed, it can receive channel selection bits or transmit the

last conversion result. The serial clock line, SCL, is always

an input to the LTC2497 and the serial data line SDA is

bidirectional. The device supports the standard mode and

the fast mode for data transfer speeds up to 400kbits/s.

Figure 2 shows the deﬁnition of the I2C timing.

The Start and Stop Conditions

A Start (S) condition is generated by transitioning SDA from

high to low while SCL is high. The bus is considered to be

busy after the Start condition. When the data transfer is

ﬁnished, a Stop (P) condition is generated by transitioning

SDA from low to high while SCL is high. The bus is free

after a Stop is generated. Start and Stop conditions are

always generated by the master.

When the bus is in use, it stays busy if a Repeated Start

(Sr) is generated instead of a Stop condition. The repeated

Start timing is functionally identical to the Start and is

used for writing and reading from the device before the

initiation of a new conversion.

Data Transferring

After the Start condition, the I2C bus is busy and data

transfer can begin between the master and the addressed

slave. Data is transferred over the bus in groups of nine

bits, one byte followed by one acknowledge (ACK) bit.

The master releases the SDA line during the ninth SCL

clock cycle. The slave device can issue an ACK by pulling

SDA low or issue a Not Acknowledge (NAK) by leaving

the SDA line high impedance (the external pull-up resistor

will hold the line high). Change of data only occurs while

the clock line (SCL) is low.

DATA FORMAT

After a Start condition, the master sends a 7-bit address

followed by a read/write (R/W) bit. The R/W bit is 1 for a

read request and 0 for a write request. If the 7-bit address

matches the hard wired, LTC2497’s address (one of 27

pin-selectable addresses) the device is selected. When

the device is addressed during the conversion state, it will

not acknowledge R/W requests and will issue a NAK by

leaving the SDA line high. If the conversion is complete,

the LTC2497 issues an ACK by pulling the SDA line low.

SDA

SCL

SSr

tHD(SDA)

tHD(DAT)

tSU(STA)

tSU(STO)

tSU(DAT)

tLOW

tHD(SDA)

tSP

tBUF

tHIGH

2497 F02

Figure 2. Deﬁnition of Timing for Fast/Standard Mode Devices on the I2C Bus

APPLICATIONS INFORMATION

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