PLEASE NOTE: This article is obsolete or related to a discontinued product.

Using the I2C bus

This article explains how to manage the I2C bus on the FOX Board LX
There are two ways to connect I2C chips on the FOX Board LX:
  • Using the kernel driver improved by Geert Vancompernolle and implemented by default in the Phrozen SDK
  • Using bit-banging routines implemented directly in the user space application

Test circuitry

To test both methods we've used an I2C 8-bit A/D converter Philips PCF8591. This is a single-chip, single-supply low power 8-bit CMOS data acquisition device with four analog inputs, one analog output and a serial I2C-bus interface. Four trimmers are used to change the input voltage on the four A/D chanels. The D/A channel is not used in this example.

As shown on the following table, the I2C driver uses the standard I2C data and clock lines located by Axis on PB0 and PB1 I/O lines. On the other side with the user space bit banging example the line used are IOG24 (data) and OG25 (clock).

The following table shows how to connect the I2C lines on both examples:

SignalsLines used by the
kernel driver
Lines used by the
user space routines
SDAJ6.32 (PB0)J7.21 (IOG24)
SCLJ6.31 (PB1) J7.13 (OG25)
3.3VJ6.1 or J6.2 or J7.39 or J7.40
GNDJ7.1 or J7.2 or J6.39 or J6.40


Wiring between the PCF8591 and the FOX board LX

Kernel driver method

This example reads the values from the four A/D channels using the I2C kernel driver and shows the voltage value to the screen:
//***************************************************
// pcf8591d.c
// 
// Example to read A/D values from a 
// 4 channel / 8 bit AD converter PCF8591
// through I2C using the I2C driver improved by
// Geert Vancompernolle
// http://www.acmesystems.it/?id=10
//***************************************************

#include "stdio.h"
#include "stdlib.h"
#include "unistd.h"
#include "sys/ioctl.h"
#include "fcntl.h"
#include "time.h"
#include "string.h"

#include "i2c_errno.h"
#include "etraxi2c.h"


int main( int argc, char **argv ) {
  int rtc;
  int fd_i2c;
  I2C_DATA i2c_d;
  int ch;
  
  printf("Reading from a PCF8591 (4 chanel A/D at 8 bits with I2C bus)\n");

  fd_i2c = open( "/dev/i2c", O_RDWR );
  if (fd_i2c<=0)     {
    printf( "Open error on /dev/i2c\n" );
    exit( 1 );
  }

  // PCF8591 address scheme
  // |  1 |  0 |  0 |  1 | A2 | A1 | A0 | R/W |
  i2c_d.slave =(0x09<<4)|(0x01<<1);

  for (ch=0;ch<=3;ch++) {  
    // Select the A/D channel
    i2c_d.wbuf[0] = ch;
    i2c_d.wlen  = 1;
    if ((rtc=ioctl(fd_i2c,_IO( ETRAXI2C_IOCTYPE, I2C_WRITE), &i2c_d))!=EI2CNOERRORS)  {
      close(fd_i2c);
      printf( "Error %d on line %d\n",rtc,__LINE__);
      return ( -1 );   
    }

    i2c_d.rlen  = 3;
    if ((rtc=ioctl(fd_i2c,_IO( ETRAXI2C_IOCTYPE, I2C_READ), &i2c_d))!=EI2CNOERRORS)  {
      close(fd_i2c);
      printf( "Error %d on line %d\n",rtc,__LINE__);
      return ( -1 );   
    }

    // Show the voltage level
    printf("Chanel %d = %.2fv (%02X hex)\n",ch,i2c_d.rbuf[2]*0.012941,i2c_d.rbuf[2]);
  }

  close(fd_i2c);
  return(0);
}

To compile this code with the SDK use this makefile (see Compile a C application):

AXIS_USABLE_LIBS = UCLIBC GLIBC
include $(AXIS_TOP_DIR)/tools/build/Rules.axis

PROGS     = pcf8591d

OBJS      = $(PROGS).o

INSTDIR = $(prefix)/usr/bin

all: $(PROGS)
$(PROGS): $(OBJS)
  $(CC) $(LDFLAGS) $^ $(LDLIBS) -o $@
  $(STRIP) $@

install: $(PROGS)
  $(INSTALL) -p -o root -g root -m 0777 $(PROGS) $(INSTDIR)

clean:
  rm -f $(PROGS) *.o core

test:
  scp $(PROGS) root@192.168.0.90:/mnt/flash

Running the example

Copy the executable file to the /mnt/flash directory and type:
[root@axis /mnt/flash]103# ./pcf8591d
Reading from 4 ch 8 bit A/D converter PCF8591
CH0 = 0.40v (1F hex)
CH1 = 1.19v (5C hex)
CH2 = 1.15v (59 hex)
CH3 = 3.30v (FF hex)

User space method

This example reads the values from the four A/D channels managing from user space directly the I/O lines used for the I2C bus and shows the voltage value to the screen:
//***************************************************
// pcf8591nd.c
// 
// Example to read A/D values from a 
// 4 channel / 8 bit AD converter PCF8591
// through I2C using bit banging method in user space
// http://www.acmesystems.it/?id=10
//***************************************************

#include "stdio.h"     
#include "unistd.h"    
#include "sys/ioctl.h"
#include "fcntl.h"     
#include "time.h"     
#include "linux/gpio_syscalls.h"

#define I2C_DATA_LINE   1<<24   // IOG24 J7.21
#define I2C_CLOCK_LINE  1<<25   //  OG25 J7.13
#define I2C_DATA_PORT   PORTG
#define I2C_CLOCK_PORT  PORTG

// Get the SDA line state

int i2c_getbit(void) {
  return (gpiogetbits(I2C_DATA_PORT,I2C_DATA_LINE)?(1):(0));
} 

// Set the SDA line as output

void i2c_dir_out(void) {
  gpiosetdir(I2C_DATA_PORT,DIROUT,I2C_DATA_LINE);
}

// Set the SDA line as input

void i2c_dir_in(void) {
  gpiosetdir(I2C_DATA_PORT,DIRIN,I2C_DATA_LINE);
}

// Set the SDA line state

void i2c_data(int state) {
  if (state==1) {
    i2c_dir_in(); 
  } else {  
    i2c_dir_out();  
    gpioclearbits(I2C_DATA_PORT, I2C_DATA_LINE);  
  } 
}

// Set the SCL line state

void i2c_clk(int state) {
  if (state==1) gpiosetbits(I2C_CLOCK_PORT, I2C_CLOCK_LINE);
  else gpioclearbits(I2C_CLOCK_PORT, I2C_CLOCK_LINE); 
}

// Read a byte from I2C bus and send the ack sequence
// Put islast = 1 is this is the last byte to receive from the slave

unsigned char i2c_inbyte(int islast) {
  unsigned char value = 0;
  int bitvalue;
  int i;

  // Read data byte

  i2c_clk(0);
  i2c_dir_in();

  for (i=0;i<8;i++) {
    i2c_clk(1);

    bitvalue = i2c_getbit();
    value |= bitvalue;
    if (i<7) value <<= 1;
    
    i2c_clk(0);
  }
  
  if (islast==0) {
    // Send Ack if is not the last byte to read
  
    i2c_dir_out();
    i2c_data(0);
    i2c_clk(1);
    i2c_clk(0);
    i2c_dir_in();
  } else {
    // Doesn't send Ack if is the last byte to read
    i2c_dir_in();
    i2c_clk(1);
    i2c_clk(0);
  }

  return value;
}


// Initializate the I2C bus

void i2c_init(void) {
  i2c_dir_in();
  i2c_clk(1);
}

// Send a start sequence to I2C bus

void i2c_start(void){
  i2c_clk(0);
  i2c_data(1);
  i2c_clk(1);
  i2c_data(0);
}

// Send a stop sequence to I2C bus

void i2c_stop(void) {
  i2c_clk(0);
  i2c_data(0);
  i2c_clk(1);
  i2c_data(1);
}

// Send a byte to the I2C bus and return the ack sequence from slave
// rtc
//  0 = Nack, 1=Ack

int i2c_outbyte(unsigned char x) {
  int i;
  int ack;

  i2c_clk(0);

  for (i=0;i<8;i++) {
    if (x & 0x80) i2c_data(1);
    else  i2c_data(0);
    i2c_clk(1);
    i2c_clk(0);
    x <<= 1;
  }

  i2c_dir_in();
  i2c_clk(1);
  ack=i2c_getbit();
  i2c_clk(0);
  
  if (ack==0) return 1;
  else return 0;
}

int main(void) {
  int ch;
  int i2c_slave;
  int ad_value;

  i2c_init();

  // PCF8591 address scheme
  // |  1 |  0 |  0 |  1 | A2 | A1 | A0 | R/W |
  i2c_slave =(0x09<<4)|(0x01<<1);

  for (ch=0;ch<=3;ch++) {  
    // Select the A/D channel
    i2c_start();
    if (i2c_outbyte(i2c_slave|0)==0) {
      printf("NACK received %d\n",__LINE__);
      i2c_stop();
      return -1;
    }
    if (i2c_outbyte(ch)==0) {
      printf("NACK received %d\n",__LINE__);
      i2c_stop();
      return -1;
    }
    i2c_stop();

    i2c_start();
    if (i2c_outbyte(i2c_slave|1)==0) {
      printf("NACK received %d\n",__LINE__);
      i2c_stop();
      return -1;
    }
    i2c_inbyte(0);
    i2c_inbyte(0);
    ad_value=i2c_inbyte(1);
    i2c_stop();

    // Show the voltage level
    printf("Chanel %d = %.2fv (%02X hex)\n",ch,ad_value*0.012941,ad_value);
  }
  
  return 0;
} 

To compile this code with the SDK (see Compile a C application) use the same makefile of the previous example changing this line:

PROGS     = pcf8591d
in
PROGS     = pcf8591nd

Running the example

Copy the executable file to the /mnt/flash directory and type:
[root@axis /mnt/flash]103# ./pcf8591nd
Reading from 4 ch 8 bit A/D converter PCF8591
CH0 = 0.40v (1F hex)
CH1 = 1.19v (5C hex)
CH2 = 1.15v (59 hex)
CH3 = 3.30v (FF hex)

Download

  • pcf8591d.c I2C sample code using I2C driver
  • pcf8591nd.c I2C sample code using the bit banging methon in user space
  • Makefile Makefile to compile the C code inside the SDK
  • etraxi2c.h Hearder filer requested by vmeter.c
  • i2c_errnoc.h Hearder filer requested by vmeter.c

Related links

FOXLX832
EUR 159.00


FOX Board LX832

Historical FOX Board version based on Axis Communications ETRAX 100LX microprocessor. The available quantity depends on how many chips are still available in the end-of-life products market.

Please note:

  • this is a brand new board built in Acme Systems during this year (2013) and not an used part so it is covered by a one year warranty (Terms & Conditions)
  • don't use this product for new design because we can't ensure product continuity
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