ST72340 datasheet(129/191 Pages) STMICROELECTRONICS

129 / 191 page

ST72340, ST72344, ST72345

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I2C3S INTERFACE (Cont’d)

11.7.3 General Description

In addition to receiving and transmitting data,

I2C3S converts it from serial to parallel format and

vice versa. The interrupts are enabled or disabled

by software. The I2C3S is connected to the I2C

bus by a data pin (SDA) and by a clock pin (SCL).

It can be connected both with a standard I2C bus

and a Fast I2C bus. The interface operates only in

Slave mode as transmitter/receiver.

In order to fully emulate standard I2C EEPROM

devices with highest transfer speed, the peripheral

prevents I2C clock signal stretching and performs

data transfer between the shift register and the

RAM buffers using DMA.

11.7.3.1 Communication Flow

A serial data transfer normally begins with a start

condition and ends with a stop condition. Both

start and stop conditions are generated by an ex-

ternal master. Refer to Figure 64 for the standard

protocol. The I2C3S is not a master and is not ca-

pable of generating a start/stop condition on the

SDA line. The I2C3S is capable of recognising 3

slave addresses which are user programmable.

The three I2C slave addresses can be individually

enabled/disabled by software.

Since the I2C3S interface always acts as a slave it

does not generate a clock. Data and addresses

are transferred as 8-bit bytes, MSB first. The first

byte following the start condition contains the

slave address. A 9th clock pulse follows the 8

clock cycles of a byte transfer, during which the re-

ceiver must send an acknowledge bit to the trans-

mitter.

11.7.3.2 SDA/SCL Line Control

When the I2C3S interface is enabled, the SDA and

SCL ports must be configured as floating inputs. In

this case, the value of the external pull-up resistor

used depends on the application.

When the I2C3S interface is disabled, the SDA

and SCL ports revert to being standard I/O port

pins.

Figure 69. I2C BUS Protocol

SCL

SDA

MSB

ACK

STOP

START

CONDITION

VR02119B

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