LTC4240IGN datasheet(13/28 Pages) LINER

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LTC4240

4240f

APPLICATIO S I FOR ATIO

START and STOP Commands

The START command is defined as a high to low transition

of the SDA line while the SCL line is high. It is an asynchro-

nous event issued by the host, waking up all slave devices

and alerting them that a slave address is being written onto

the bus. Only the slave device that matches the address will

communicate with the host. The STOP command is de-

fined as a low to high transition on the SDA line while SCL

is high. It is also an asynchronous event issued by the host

to signal the termination of the data transfer. Other than

START and STOP commands, the SDA line is allowed to

change states only when SCL is low.

Address Byte

Once the LTC4240 has detected a START command, it

clocks in the SDA line on the succeeding 9 SCL rising

edges. The first 7 bits clocked in contain the address of the

slave device targeted by the host. The first (MSB) address

bit must be set to low and the second bit must be set to

high. The next 5 bits are fed into a digital comparator and

compared against the output of an internal 5-bit A/D. If the

comparison is true, then there is an address match and the

LTC4240 continues to communicate with the host device.

The LTC4240 proceeds to acknowledge the address match

by pulling the SDA line low while SCL is low, just before the

9th SCL rising edge. Figures 1 and 3 show a timing

diagram of the START condition and address byte for both

the Send Byte and Receive Byte protocols. Note that the

SDA bit clocked in with the 8th SCL edge determines

whether the host is sending or receiving information to/

from the LTC4240.

Send Byte Protocol

The Send Byte protocol allows a host to write information

into the LTC4240 and command the LTC4240 to perform

certain predetermined functions. The host initiates com-

munication with a START bit followed by 7 address bits.

The address bits are followed by the R/W bit, which is low

for Send Byte. The 9th bit is asserted low by the LTC4240

to acknowledge when there has been an address match.

The only time the LTC4240 writes data onto the SDA bus

during a send byte is to acknowledge the address and

command bytes. The first 8 bits are referred to collectively

as the address byte.

The command byte follows the address byte. The

command byte contains the information sent from the

host to the LTC4240. After the LTC4240 acknowledges the

address byte, each of the next 8 SCL rising edges shifts

SDA from the host into a shift register inside the LTC4240.

The first 2 bits clocked into the shift register (2 MSBs of the

command latch) are not used by the LTC4240. Only the 6

LSBs are stored in the command latch on the falling edge

of the 8th clock during the command byte. The output of

the command latch remains fixed until the next Send Byte

command overwrites it. Note that if power is turned off

(5VIN < 2V), the command and data latches will be cleared.

Figure 1 shows the timing diagram of the entire send byte

protocol. Transmission ends when the host issues a STOP

command. Table 2 defines the functions of the 6 command

bits. Note that some of these functions can override, or can

be overridden by, other circuitry and pins of the LTC4240.

Figure 2 shows the relationship between bits C1 to C3 and

other LTC4240 signals.

Receive Byte Protocol

The Receive Byte protocol is used by the host to read data

from the LTC4240 data latch. This protocol begins with a

START command, issued by the host, followed by 7

address bits. The address bits are followed by the R/W bit,

which is high for Receive Byte. The 9th bit is used by the

LTC4240 to acknowledge when there is an address match.

The data byte then follows the address byte. This byte

contains LTC4240 status information. After the LTC4240

acknowledges the address byte, it shifts 8 bits of data onto

the SDA line. Figure 3 shows the entire Receive Byte timing

diagram. Note that neither the host or the slave acknowl-

edges the data byte (SDA line stays high during 9th clock

edge of the data byte).

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