PAC1941T-1ESLASH4MX. datasheet(33/96 Pages) MICROCHIP

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DS20006543A-page 33

PAC194X

6.0

SMBUS AND I2C

COMMUNICATIONS

PROTOCOL

The PAC194X devices communicate over a two-wire

bus with a controller, using a SMBus or I2C serial com-

munication protocol. A detailed timing diagram is

shown in Figure 6-1.

Stretching of the SMCLK signal is supported.

However, the PAC194X will not stretch the clock signal

itself.

FIGURE 6-1:

SMBus Timing Diagram.

6.1

I2C/SMBus Addressing and

Control Bits

6.1.1

SMBUS ADDRESS AND RD/WR BIT

The SMBus Address byte consists of the 7-bit client

address followed by a 1-bit RD/WR indicator. If this

RD/WR bit is a logic ‘0’, the SMBus host writes data to

the client device. If this RD/WR bit is a logic ‘1’, the

SMBus host reads data from the client device.

The PAC194X I2C/SMBus address is determined by a

single pull-down resistor connected between ground

and the ADDRSEL pin, as shown in Table 6-1. The

chip translates the resistor value into an address on

power-up and the value is latched until another

power-up event takes place. The address cannot be

changed on the fly.

6.1.2

SMBUS START BIT

The SMBus Start bit is defined as a transition of the

SMBus data line from a logic ‘1’ state to a logic ‘0’

state, while the SMBus clock line is in a logic ‘1’ state.

6.1.3

SMBUS ACK AND NACK BITS

The SMBus client will ACK (acknowledge) all data

bytes that it receives. This is done by the client device

pulling the SMBus data line low after the eighth bit of

each byte that is transmitted.

6.1.4

SMBUS STOP BIT

The SMBus Stop bit is defined as a transition of the

SMBus data line from a logic ‘0’ state to a logic ‘1’

state, while the SMBus clock line is in a logic ‘1’ state.

When the PAC194X detects an SMBus Stop bit and it

has been communicating with the SMBus protocol, it

resets its client interface and prepares to receive

further communications.

SMDATA

SMCLK

BUF

S - Start Condition

P - Stop Condition

T HIGH

T LOW

HD:STA

SU:STO

HD:STA

HD:DAT

SU:DAT

SU:STA

FALL

RISE

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