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AN1045

ST7 S/W IMPLEMENTATION OF I2C BUS MASTER

STMicroelectronics 

ST7 S/W IMPLEMENTATION OF I2C BUS MASTER

1.1 COMMUNICATION SPEED

The communication speed is modifiable by using the function delay(time) which waits for a

given time period and then modifies the frequency of SCL.

Here Fscl is equal to 62.5 kHz. It can be easily reduced by increasing the period between two

clock cycles, but this speed is not far from the highest speed you can have (~70 kHz).

1.2 START, STOP CONDITION AND ACKNOWLEDGE GENERATION

The Start and Stop conditions are always generated by the master. In this software, there are

no bits to set to generate these conditions like in the real peripheral: you just have to call the

corresponding function (I2Cm_Start() and I2Cm_Stop()).

An Acknowledge is sent after an address or a data byte is received. When the master has to

receive an acknowledge from the slave, you have to call the function Wait_Ack() which reads

the SDA and SCL lines to recognize the acknowledge condition (the SDA line put at the low

state by the one which sends the acknowledge during one clock pulse). And when the master

has to send an acknowledge after receiving data from the slave, you have to call the function

I2Cm_Ack().

2 ST7 I2C COMMUNICATION APPLICATION

2.1 HARDWARE CONFIGURATION

The ST7 communication application hardware is composed of a ST72324 microcontroller

(which has no I2C peripheral) and any slave (an M24C08 EEPROM for example).

Figure 1. ST7 / E2PROM I2C Communication Application

ST72324 Vdd

SCL

I2C SDA

Vss

5V

2x100W

2x12KW

M24C08

SCL

SDA

Vss

E

2/16

2

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