AN1045
APPLICATION NOTE
ST7 S/W IMPLEMENTATION OF I2C BUS MASTER
by Microcontroller Division Applications Team
INTRODUCTION
The goal of this application note is to implement an I2C communications software interface for
devices which have no I2C peripheral. The software of this application performs I2C master
transmitter and master receiver functions. The master chosen here is a ST72324 and the
slave is an EEPROM (M24C08).
The program described in this application note is in C language, a program in assembly language is also available in the software library (see ST7 CD ROM on Internet).
1 CHARACTERISTICS
The main characteristics of this I2C software are:
■
bit addressing
■
Master Transmitter/Receiver
■
Several data bytes sent and received (3 in this application)
■
Fscl = 62.5 kHz
■
Acknowledge management
■
Error management (AF)
The I2C synchronous communication needs only two signals: SCL (Serial clock line) and SDA
(Serial data line). The corresponding port pins used are PA7 for SCL and PA6 for SDA.
These two pins are configured as floating input (to have a high level applied on the pin or to receive data) or as output open drain (to have a low level applied on the pin or to output data).
Please refer to the ST7 datasheet for more details about port configuration.
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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
5V
ST72324
Vdd
2x12KW
SCL
I2C
M24C08
SCL
SDA
SDA
2x100W
Vss
Vss
E
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ST7 S/W IMPLEMENTATION OF I2C BUS MASTER
2.2 INITIATING A COMMUNICATION
To initiate an I2C communication, first a start condition has to be generated and then the selected slave address has to be sent, both by the master.
Here, this action is done by calling the function I2Cm_Start() followed by the sending of the
slave address with the least significant bit correctly set (0:transmission, 1:reception).
As the slave here is an EEPROM, two addresses have to be sent by the master to the slave:
the address of the slave and the address where you want to write or read into the EEPROM
(refer to Section 3: Communication frames).
2.3 SENDING A DATA BYTE ON THE I2C BUS
To transmit a new data byte from the ST72324, the addresses or data bytes previously transmitted have to be completed correctly. This previous byte transmission check is done with the
reception of an acknowledge condition by the master. If an error is detected (AF: Acknowledge
Failure), the AF bit of the created I2C_SR2 register is cleared and the transmission is restarted from the START condition.
When the previous data transmission is over, the application writes the new data byte to be
transmitted. The data to transmit is put on the created I2C_DR register and is sent bit by bit
through PADR (PA6=SDA), MSB first.
All the data to send to the slave (and the addresses too) are stored in a table.
2.4 RECEIVING A DATA BYTE ON THE I2C BUS
To receive a new data byte, the previous data byte to receive has to be completed correctly.
This byte reception check is done with the sending of an acknowledge condition by the
master. An AF can’t occur on the master side because it’s the master that sends the acknowledge condition. If there is a problem with the reception of this acknowledge, it’s up to the slave
to manage this problem.
The frame in this case (master receiver) is: the master after sending the first Start condition
and the two addresses, has to resend a Start condition followed by the address of the
EEPROM, but this time with the least significant bit at 1 to make the slave understand it’s
waiting for the data (refer to Section 3: Communication frames).
When the master is receiver, after receiving the last data, it has to generate a non acknowledge condition to be able to generate the STOP condition afterwards.
Note: There is no need to clear the ACK bit to disable acknowledgement before receiving the
second last byte or set the STOP bit before receiving the last byte (as is necessary in ST7
MCUs with a dedicated I2C peripheral), because here Acknowledgement and Stop condition
generation is under software control, while in the I2C Peripheral it is under hardware control.
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ST7 S/W IMPLEMENTATION OF I2C BUS MASTER
3 COMMUNICATION FRAMES
The communication protocol between the master and the slave is given in Figure 2. For more
details, please refer to the ST7 datasheet.
Figure 2. I2C Communication Protocol
Write data from ST7 to E2PROM
START E2PROM @
ACK
SUB @
ACK
DATA 1 ACK
DATA 2
DATA N-1 ACK
DATA N ACK STOP
Read data from E2PROM to ST7
START E2PROM @
ACK
SUB @
ACK START E2PROM @
ACK
DATA 1 ACK
DATA N
NACK STOP
4 FLOWCHARTS
Figure 3. Communication Application Flowchart
Master transmitter
INITIATE TRANSMISSION
(START + addresses)
no
Master receiver
INITIATE TRANSMISSION
(START+@+START+@lsb=1)
no
ACK?
ACK?
yes
yes
WAIT FOR NEXT DATA TO RECEIVE
SEND NEXT TABLE DATA
ACK
ACK?
no
yes
END OF
BUFFOUT?
yes
no
no
LAST VALUE
TO RECEIVE?
yes
NON ACK
STOP CONDITION
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Figure 4. Buffer of transmission structure
0
data nb-2
|
|
|
|
nb-3
data2
nb-2
data1
nb-1
sub @
nb
EEPROM @
The buffer of transmission contains the EEPROM address, the sub address (the address
where you want to write into the EEPROM) and then the data to transmit.
In this application, a parameter called “n” allows you to modify the number of data to transmit
and then to receive. The number of data is “n-1”, that means that in this application, as 3 data
have to be sent, “n=4”.
The transmission function is based on a double shift: a shift of the “count” variable to call 8
times the function I2Cm_TxData (to send the 8 bits of one data byte) and a shift into the
I2Cm_TxData function to always send the MSB of the data (refer to Figure 5).
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Figure 5. Flowchart of the transmission function
SDA configured as output
j=nb
count=1
I2C_DR=buffout[j]
shift of count
I2Cm_TxData
no
count overflows?
yes
wait Acknowledge
no
ACK ok?
AF=1
yes
j-no
j=FF?
yes
END
The reception function is also based on a double shift: a shift of the “count” variable to call 8
times the function I2Cm_RxData (to receive the 8 bits of one data byte) and a shift of a buffer
into the I2Cm_RxData function to receive the data bit by bit on the LSB (refer to Figure 6).
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Figure 6. Flowchart of the reception function
RCPT=1
I2Cm_Tx for addresses
transmission
yes
AF?
no
count=1
SDA configured as input
shift of count
I2Cm_RxData
j--
no
count overflows?
yes
j=FF?
END
no
SDA configured as output
Acknowledge
buffin[j]=I2C_DR
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5 SOFTWARE
The assembly code given below is for guidance only.
/**************** (c) 2003 STMicroelectronics ********************************
PROJECT : EVALUATION BOARD - ST7 I2C DEMO SYSTEM
COMPILERS : COSMIC AND METROWERKS
MODULE : i2cm_drv.c
REVISION DATE : 12/06/03
AUTHOR : Micro Controller Division Application Team
-*-*-*-*-*-*-*-*-*-*-*-*--*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*THE SOFTWARE INCLUDED IN THIS FILE IS FOR GUIDANCE ONLY. STMicroelectronics
SHALL NOT BE HELD LIABLE FOR ANY DIRECT, INDIRECT OR CONSEQUENTIAL
DAMAGES WITH RESPECT TO ANY CLAIMS ARISING FROM USE OF THIS SOFTWARE.
-*-*-*-*-*-*-*-*-*-*-*-*--*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*DESCRIPTION : ST7 I2C single master T/R peripheral software driver.
-*--*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*MODIFICATIONS :
27/08/98 - V1.0 - First version (error management:AF).
12/06/03 - V1.1 - 1) Compatibility With Cosmic And Metrowerks.
2) Generation Of Non - Ack.
3) Update for st72324
**************************************************************************/
#pragma NO_STRING_CONSTR
#define SDA
#define SCL
6
7
/* EXTERNAL DECLARATIONS ***************************************************/
/* List of all the variables defined in another module and used in this one. */
/* MODEL => #include {file_name}.h
*/
#include "map72324.h"
/* Declaration of the I2C HW registers.*/
#include "lib_bits.h"
/* Bit handling macro definitions.*/
#include "variable.h"
/* FUNCTIONDESCRIPTIONS ************************************************* /
/* Description of all the functions defined in this module.
*/
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ST7 S/W IMPLEMENTATION OF I2C BUS MASTER
/* MODEL => [static] type_name var_name; or #define
*/
void delay (unsigned char time)
{
#ifdef __HIWARE__
asm
{
nop
/* Time is stored on the Accumulator automatically */
again: DEC
A
/* When the function is called.
*/
JRNE again
/* (15+6*time) clock cycles
*/
}
#else
#ifdef __CSMC__
{
_asm ("nop");
_asm("again: DEC A");
_asm(" JRNE again");
}
#endif
#endif
}
/*-------------------------------------------------------------------------ROUTINE NAME : I2Cm_Start
INPUT/OUTPUT : None.
DESCRIPTION : Generates I2C-Bus Start Condition.
COMMENTS
:
--------------------------------------------------------------------------*/
void I2Cm_Start (void)
{
ClrBit(PADDR,SDA);
/* Configure SDA and SCL as floating input to have a high state */
ClrBit(PADDR,SCL);
delay(10);
SetBit(PADDR,SDA);
/* Configure SDA as output open drain to have a low state */
delay(4);
/* Waits 39 cycles=4.875µs at a Fcpu=8MHz to keep the high state on SCL */
SetBit(PADDR,SCL);
/* Configure SCL as output open drain to have a low state */
delay(6);
/* Delay to wait after a START
*/
}
/*-------------------------------------------------------------------------ROUTINE NAME : I2Cm_Stop
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INPUT/OUTPUT : None.
DESCRIPTION : Generates I2C-Bus Stop Condition.
COMMENTS
:
--------------------------------------------------------------------------*/
void I2Cm_Stop (void)
{
SetBit(PADDR,SDA);
/* Configure SDA and SCL as output open drain to have a low state */
SetBit(PADDR,SCL);
ClrBit(PADDR,SCL);
/* Configure SCL as floating input to have a high state
*/
delay(4);
/* Macro delay with time=4 (4.875 µs)
*/
ClrBit(PADDR,SDA);
/* Configure SDA as floating input to have a high state
*/
/* Delay after the Stop did in main.c with Wait_1ms()
*/
}
/*-------------------------------------------------------------------------ROUTINE NAME : wait_Ack
INPUT/OUTPUT : None.
DESCRIPTION : Acknowledge received?
COMMENTS
: Transfer sequence = DATA, ACK.
--------------------------------------------------------------------------*/
void wait_Ack (void)
{
SetBit(PADDR,SCL);
/* Output open drain to have a low level */
ClrBit(PADDR,SDA);
/* Floating input, the slave has to pull SDA low */
delay(1);
if (ValBit(PADR,SDA))
/* Test of SDA level, if high -> pb
*/
{
SetBit(I2C_SR2,AF);
ClrBit(I2C_SR1,ACK);
return;
}
delay(2);
if (ValBit(PADR,SDA))
/* Test of SDA level, if high -> pb
*/
{
SetBit(I2C_SR2,AF);
ClrBit(I2C_SR1,ACK);
return;
}
delay(5);
ClrBit(PADDR,SCL);
/* Start of the generation of 1 clock pulse */
delay(1);
if (ValBit(PADR,SDA))
/* Test of SDA level, if high -> pb
*/
{
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ST7 S/W IMPLEMENTATION OF I2C BUS MASTER
SetBit(I2C_SR2,AF);
ClrBit(I2C_SR1,ACK);
return;
}
delay(1);
if (ValBit(PADR,SDA))
/* Test of SDA level, if high -> pb
*/
{
SetBit(I2C_SR2,AF);
ClrBit(I2C_SR1,ACK);
return;
}
delay(1);
SetBit(PADDR,SCL);
/* End of the clock pulse
*/
SetBit(I2C_SR1,ACK);
delay(1);
ClrBit (PADR,SDA);
SetBit(PADDR,SDA);
/* Reconfigure SDA as output to proceed at the next transmission */
}
/*-------------------------------------------------------------------------ROUTINE NAME : I2C_nAck
INPUT/OUTPUT : None.
DESCRIPTION : Non acknoledge generation from now.
COMMENTS
: Transfer sequence = DATA, NACK.
--------------------------------------------------------------------------*/
void I2C_nAck (void)
{
ClrBit(I2C_SR2,ACK); /* Non acknoledge when the master is receiver */
SetBit(PADR,SDA);
/* The master pulls the SDA line high
*/
SetBit(PADDR,SCL);
/* Output open drain to have a low level
*/
delay(10);
ClrBit(PADDR,SCL);
/* The master generates a clock pulse
*/
delay(10);
SetBit(PADDR,SCL);
/* Clock pulse complete
*/
ClrBit(PADR,SDA);
/* The master pulls the SDA line high
*/
delay(3);
}
/*-------------------------------------------------------------------------ROUTINE NAME : I2Cm_Init
INPUT/OUTPUT : None.
DESCRIPTION : I2C peripheral initialisation routine.
COMMENTS
: Contains inline assembler instructions in C like mode !
--------------------------------------------------------------------------*/
void I2Cm_Init (void)
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{
count=0;
I2C_SR1=0;
I2C_SR2=0;
I2C_DR=0;
err_status=0;
t_count_err=0;
r_count_err=0;
SetBit(I2C_SR1,M_SL); /* Master mode: M_SL=1
*/
}
/*-------------------------------------------------------------------------ROUTINE NAME : I2Cm_TxData
INPUT/OUTPUT : data byte to be transfered(MSB first) / None.
DESCRIPTION : Transmits a data bit.
COMMENTS
: Transfer sequence = DATA, ACK, ...
--------------------------------------------------------------------------*/
void I2Cm_TxData (void)
{
SetBit(PADDR,SCL);
/* Low level on SCL
*/
if (I2C_SR2)
/* Check the communication error status */
{
err_status++;
t_count_err++;
if (t_count_err==0) t_count_err++;
}
else
/* If no error
*/
{
if (ValBit(I2C_DR,7))
/* Send data bit per bit, MSB first */
SetBit(PADR,SDA);
/* Send a one
*/
else
ClrBit(PADR,SDA);
/* Send a zero
*/
I2C_DR*=2;
ClrBit(PADDR,SCL);
delay(10);
/* High state on SCL
*/
}
}
/*-------------------------------------------------------------------------ROUTINE NAME : I2Cm_RxData
INPUT/OUTPUT : Last byte to receive flag (active high) / Received data bit.
DESCRIPTION : Receive a data byte.
COMMENTS
: Transfer sequence = DATA, ACK, EV7...
--------------------------------------------------------------------------*/
void I2Cm_RxData (void)
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ST7 S/W IMPLEMENTATION OF I2C BUS MASTER
{
if (!I2C_SR2)
{
buff*=2;
/* No communication error detected
/* Shift I2C_DR to receive next bit
*/
*/
#ifdef __HIWARE__
asm
{
nop
nop
nop
}
#else
#ifdef __CSMC__
{
_asm("nop");
_asm("nop");
_asm("nop");
}
#endif
#endif
ClrBit(PADDR,SCL);
/* Rise the SCL line
*/
do{
}while(ValBit(PADR,SCL)!=0);
/* Wait SCL at a high state
if(ValBit(PADR,SDA))
buff|=1;
/* The received bit is 1
*/
else
buff|=0;
/* The received bit is 0
*/
delay(10);
SetBit(PADDR,SCL);
/* SCL at a low level
*/
*/
}
else
r_count_err++;
}
/*-------------------------------------------------------------------------ROUTINE NAME : I2C_Ack
INPUT/OUTPUT : None.
DESCRIPTION : Send Ack to the slave.
COMMENTS
:
--------------------------------------------------------------------------*/
void I2C_Ack(void)
{
ClrBit(PADR,SDA);
/* The master pulls the SDA line low
*/
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SetBit(PADDR,SDA);
delay(10);
ClrBit(PADDR,SCL);
delay(10);
/* Waits the master takes the control of SDA */
SetBit(PADDR,SCL);
delay(5);
ClrBit(PADDR,SDA);
SetBit(I2C_SR1,ACK);
/* The master releases the SDA line
*/
/* ACK=1: Acknowledge sent by the master
*/
}
/*-------------------------------------------------------------------------ROUTINE NAME : I2Cm_Tx
INPUT/OUTPUT : send_tab and n, the number of data to transmit (with 2 addresses)
/ None.
DESCRIPTION : Transmit data buffer.
COMMENTS
: Most significant bytes first.
--------------------------------------------------------------------------*/
void I2Cm_Tx (char * buffout,char nb)
{
SetBit(PADDR,SDA);
/* Configure SDA as an output to send data */
for (j=nb;j!=0xFF;j--)
{
/* 2 addresses and 3 data to send: from X=n downto X=0
*/
flag=0;
if ((j==(nb-2))&&(ValBit(I2C_SR1,RCPT)))
{
I2Cm_Start();
/* Start condition
*/
j=nb;
/* EEPROM @ with the LSB at 1 to send
*/
flag=1;
}
count=1;
I2C_DR=buffout[j];
if (flag==1) I2C_DR=I2C_DR|1;
/* If master receiver, the address to send is A1
*/
do
{
I2Cm_TxData(); /* Sending of data bit per bit, MSB first */
count*=2;
}while(count!=0);
wait_Ack();
/* Wait ACK from the slave
*/
if(!ValBit(I2C_SR1,ACK))
SetBit(I2C_SR2,AF);
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if (flag==1) return;
/* If master receiver, go back to I2Cm_Rx() to receive data */
}
}
/*-------------------------------------------------------------------------ROUTINE NAME : I2Cm_Rx
INPUT/OUTPUT : @ reception buffer + n-2 data/ None.
DESCRIPTION : Receive in data buffer via I2C.
COMMENTS
: Most significant bytes first.
--------------------------------------------------------------------------*/
void I2Cm_Rx (char *buffin,char nb)
{
SetBit(I2C_SR1,RCPT);
/* Master in receiver mode
*/
I2Cm_Tx(send_tab,nb);
/* Send the addresses and wait ACK */
if (ValBit(I2C_SR2,AF)) return;
/* If AF -> go back to main and restart the reception
*/
for (j=nb-2;j!=0xFF;j--)
{
count=1;
buff=0;
ClrBit(PADDR,SDA);
/* SDA as floating input to read data from the EEPROM */
do
{
I2Cm_RxData();
/* Read data bit per bit, MSB first */
count*=2;
}while(count!=0);
I2C_DR=buff;
SetBit(PADDR,SDA);
/* Configure SDA as output
*/
if (j==0)
I2C_nAck();
/* Non acknowledge to make the master generate the STOP */
else
I2C_Ack();
/* To acknowledge read data
*/
buffin[j]=I2C_DR;
/* Store read data into buffin
*/
}
}
/******************* (c) 2003 STMicroelectronics ************ END OF FILE ***/
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THE PRESENT NOTE WHICH IS FOR GUIDANCE ONLY AIMS AT PROVIDING CUSTOMERS WITH INFORMATION
REGARDING THEIR PRODUCTS IN ORDER FOR THEM TO SAVE TIME. AS A RESULT, STMICROELECTRONICS
SHALL NOT BE HELD LIABLE FOR ANY DIRECT, INDIRECT OR CONSEQUENTIAL DAMAGES WITH RESPECT TO
ANY CLAIMS ARISING FROM THE CONTENT OF SUCH A NOTE AND/OR THE USE MADE BY CUSTOMERS OF
THE INFORMATION CONTAINED HEREIN IN CONNECTION WITH THEIR PRODUCTS.
Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences
of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted
by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publication are subject
to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not
authorized for use as critical components in life support devices or systems without the express written approval of STMicroelectronics.
The ST logo is a registered trademark of STMicroelectronics
2003 STMicroelectronics - All Rights Reserved.
Purchase of I2C Components by STMicroelectronics conveys a license under the Philips I2C Patent. Rights to use these components in an
I2C system is granted provided that the system conforms to the I2C Standard Specification as defined by Philips.
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