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howtos:hardware:arm:interfacing_i2c_devices [2014/03/22 22:09 (UTC)]
louigi600 [Communicating With An I2C Device]
howtos:hardware:arm:interfacing_i2c_devices [2019/07/31 01:39 (UTC)] (current)
rramp [Voltage Level Shifting]
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 A bash script is really not the most suitable way to read data from I2C devices, a faster means of managing the data from the devices is really mandatory in order to do calibration,​ data averaging and what more to make the information consistent and useful for further calculations. I found the Linux kernel i2c documentation a usefull reference (<kernel source tree>/​Documentation/​i2c/​dev-interface);​ it's not the only way that data can be read but it's a good starting point. A bash script is really not the most suitable way to read data from I2C devices, a faster means of managing the data from the devices is really mandatory in order to do calibration,​ data averaging and what more to make the information consistent and useful for further calculations. I found the Linux kernel i2c documentation a usefull reference (<kernel source tree>/​Documentation/​i2c/​dev-interface);​ it's not the only way that data can be read but it's a good starting point.
  
-I hate showing my poor C programming capabilities but here's some code that uses i2c-dev to read stuff from the ITG3200:+I hate showing my poor C programming capabilities but here's some code that uses i2c-dev to read stuff from the ITG3200 ​and takes an average over 10 readings:
  
   #include <​sys/​ioctl.h>​   #include <​sys/​ioctl.h>​
Line 151: Line 151:
   ​   ​
   #define I2C_DEVICE "/​dev/​i2c-1"​   #define I2C_DEVICE "/​dev/​i2c-1"​
-  #define I2C_DEV_ADDR 0x69 
-  #define I2C_DEV_SELF 0x0 
-  #define I2C_DEV_INT 0x1a 
-  #define I2C_DEV_REG_START_ADDR 0x1b 
-  #define I2C_DEV_REG_END_ADDR 0x22 
   ​   ​
-  #​define ​TEMP_RAW_OFFSET ​13200 +  ​/​*ITG3200*/​ 
-  #​define ​TEMP_RAW_SENSITIVITY ​280 +  ​#​define ​ITG3200_ADDR 0x69 
-  #​define ​TEMP_OFFSET ​35+  #define ITG3200_SELF 0x0 
 +  #define ITG3200_INT 0x1a 
 +  #define ITG3200_TH 0x1b /*2 bytes Hight byte and Low byte*/ 
 +  #define ITG3200_TL 0x1c 
 +  #define ITG3200_XRH 0x1d /*2 byte Hight byte and Low byte*/ 
 +  #define ITG3200_XRL 0x1e 
 +  #define ITG3200_YRH 0x1f /*2 byte Hight byte and Low byte*/ 
 +  #define ITG3200_YRL 0x20 
 +  #define ITG3200_ZRH 0x21 /*2 byte Hight byte and Low byte*/ 
 +  #define ITG3200_ZRL 0x22 /*2 byte Hight byte and Low byte*/ 
 +  #define ITG3200_TEMP_RAW_OFFSET ​13200 
 +  #​define ​ITG3200_TEMP_RAW_SENSITIVITY ​280 
 +  #​define ​ITG3200_TEMP_OFFSET ​35 
 +  #define ITG3200_ROT_RAW_SENSITIVITY 14.375
   ​   ​
-  ​#define ROT_RAW_SENSITIVITY 14.375+  ​int twosc2int(int twoscomplimentdata) 
 +  { int retval; 
 +    if( twoscomplimentdata > 32768 ) retval = twoscomplimentdata - 65536; 
 +    else retval = twoscomplimentdata;​ 
 +    return retval; 
 +  } 
 +   
 +  float ITG3200_rot_conv(int rawdata) 
 +  { float retval; 
 +    int raw; 
 +   
 +    raw=twosc2int(rawdata);​ 
 +    retval = (float)raw / (float)ITG3200_ROT_RAW_SENSITIVITY;​ 
 +    return retval; 
 +  } 
 +   
 +  float ITG3200_temp_conv(int rawdata) 
 +  { float retval; 
 +    int raw; 
 +   
 +    raw=twosc2int(rawdata);​ 
 +    retval = (float)ITG3200_TEMP_OFFSET + (((float)raw + ITG3200_TEMP_RAW_OFFSET) / ITG3200_TEMP_RAW_SENSITIVITY);​ 
 +    return retval; 
 +  }
   ​   ​
-  void read_registers ​(int file, int *raw) +  void ITG3200_read ​(int file, int *raw, int *reg_array,​int size
-  { char buf[256] = {0};+  { __s32 res;
     int i,j,k;     int i,j,k;
   ​   ​
-  /*For some unexpected reason I'm getting responses from the other I2C devices +    for(i=0;​i<​size;​i++
-    on the same bus so I'm ignoring data that does not match the WHO AM I reg +    ​{ k=0
-    and also data that has not set the interrupt register ​(meaning that data is +      for (j=0;j<2;j++
-    not really avalible) +      { 
-  */ +        ​if( (res = i2c_smbus_read_byte_data(file,*(reg_array + i + j)) )< 0 
-    ​while (buf[0] != 0x69 || buf[26] == 0) +        ​printf("​Failed to read from the i2c bus.\n"​);​ 
-    { if (read(file,buf,sizeof(buf)) != sizeof(buf))   +          exit(1); 
-      { /* ERROR HANDLING: i2c transaction failed */ +        } 
-        printf("​Failed to read from the i2c bus.\n"​);​ +        if (j == 0) k=(int)res << 8; 
-        exit(1);+        else 
 +        { k += (int)res; 
 +          *(raw + (i/2))=k; 
 +        }
       }       }
-    } 
-  ​ 
-    j=0; 
-    for(i=I2C_DEV_REG_START_ADDR;​i<​=I2C_DEV_REG_END_ADDR;​i++) 
-    { k= (buf[i] << 8) + buf[i+1]; 
-      if ( k > 32768 ) *(raw+j)= k - 65536; 
-      else *(raw+j)=k; 
       i++;       i++;
-      j++; +    ​}
-    ​+
   }   }
   ​   ​
   main ()   main ()
-  { int file,buffer;+  { int file;
     int i,j,k;     int i,j,k;
-    ​int raw_data[4];​ +    float data[4]={0};​
-    ​float data[4],​tdata[10],​rxdata[10],​rydata[10],​rzdata[10];​ +
-    char buf[256] = {0};+
   ​   ​
-    if ((file = open(I2C_DEVICE,​ O_RDWR)) < 0)  +    ​int ITG3200_REGS[8]={ITG3200_TH,​ITG3200_TL,​ITG3200_XRH,​ITG3200_XRL,​ 
-    { /* ERROR HANDLING: you can check errno to see what went wrong */ +      ITG3200_YRH,​ ITG3200_YRL,​ITG3200_ZRH,​ITG3200_ZRL};​ 
-      ​perror("​Failed to open the i2c bus");+    int ITG3200_RAW_DATA[4];​ 
 +    float ITG3200_DATA[4];​ 
 +   
 +    ​if ((file = open(I2C_DEVICE,​ O_RDWR)) < 0) 
 +    { perror("​Failed to open the i2c bus");
       exit(1);       exit(1);
-    } +    }
   ​   ​
-    if (ioctl(file,​ I2C_SLAVE, ​I2C_DEV_ADDR) < 0) +    if (ioctl(file,​ I2C_SLAVE, ​ITG3200_ADDR) < 0)
     { printf("​Failed to acquire bus access and/or talk to slave.\n"​);​     { printf("​Failed to acquire bus access and/or talk to slave.\n"​);​
-      /* ERROR HANDLING; you can check errno to see what went wrong */ 
       exit(1);       exit(1);
     }     }
   ​   ​
-    ​for(i=0;​i<​10;​i++) +  /*Take an avarage over 10 consecuitve readings on the ITG3200*/ 
-    { read_registers(file,&raw_data[0]); +    ​for (i=0;​i<​10;​i++) 
-      tdata[i]=TEMP_OFFSET + (((float)raw_data[0] + TEMP_RAW_OFFSET) / TEMP_RAW_SENSITIVITY);​ +    { ITG3200_read(file,&ITG3200_RAW_DATA[0],&​ITG3200_REGS[0],sizeof(ITG3200_REGS)/sizeof(ITG3200_REGS[0]));
-      rxdata[i]=(float)raw_data[1]ROT_RAW_SENSITIVITY;​ +
-      rydata[i]=(float)raw_data[2]/ ROT_RAW_SENSITIVITY;​ +
-      rzdata[i]=(float)raw_data[3]/​ ROT_RAW_SENSITIVITY;​ +
-    } +
-    close(file);+
   ​   ​
-    for(i=0;​i<​10;​i++) +      ​data[0] ​+ITG3200_temp_conv(ITG3200_RAW_DATA[0])
-    { data[0]=data[0] + tdata[i]+      data[1] ​+ITG3200_rot_conv(ITG3200_RAW_DATA[1])
-      data[1]=data[1] + rxdata[i]+      data[2] ​+ITG3200_rot_conv(ITG3200_RAW_DATA[2])
-      data[2]=data[2] + rydata[i]+      data[3] ​+ITG3200_rot_conv(ITG3200_RAW_DATA[3]);
-      data[3]=data[3] + rzdata[i];+
     }     }
-    for(i=0;​i<​4;​i++) ​data[i]=data[i]/10;+    for(i=0;​i<​4;​i++) data[i] /10; 
   ​   ​
     printf("​Temp. : %2.2f \n",​data[0]);​     printf("​Temp. : %2.2f \n",​data[0]);​
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     printf("​Rot. Y : %2.2f \n",​data[2]);​     printf("​Rot. Y : %2.2f \n",​data[2]);​
     printf("​Rot. Z : %2.2f \n",​data[3]);​     printf("​Rot. Z : %2.2f \n",​data[3]);​
 +  ​
 +    close(file);​
   }   }
 +
 +
 ====== Voltage Level Shifting ====== ====== Voltage Level Shifting ======
 You may end up with heterogeneous voltage level devices and if you have many devices the correct way to work around this problem is by using bidirectional I2C voltage level shifters like the [[ http://​www.ti.com/​lit/​ds/​symlink/​pca9306.pdf |PCA9306]], but if you only have a few devices all grouped up in a neat PCB like the 10DOF IMU unit you might want to give a simpler system a try. You may end up with heterogeneous voltage level devices and if you have many devices the correct way to work around this problem is by using bidirectional I2C voltage level shifters like the [[ http://​www.ti.com/​lit/​ds/​symlink/​pca9306.pdf |PCA9306]], but if you only have a few devices all grouped up in a neat PCB like the 10DOF IMU unit you might want to give a simpler system a try.
-This is how I connected my 5v IMU pcb to a 3.3v I2C bus on my RaspberryPI:​ I took as educated guess that 4.4v (5v with a 4148 diode in series) would still be a tolerable power supply voltage for the whole IMU pcb, this would most likely allow all the I2C devices on the IMU pcb to recognize a minimum of 3.08v (4.4 * 0.7) as the lowest reliable logic level 1 tension allowing it to inter-operate with the PI's 3.3v logig levels. I was not able to find if the PI has internal pullups on the I2C bus or if they have to be externally placed so in doubt I put in 10k pullups between the 4.4v power line and the 2 data lines. I was the able to correctly detect the sensors on the IMU pcb.   +This is how I connected my 5v IMU pcb to a 3.3v I2C bus on my RaspberryPI:​ I took as educated guess that 4.4v (5v with a 4148 diode in series) would still be a tolerable power supply voltage for the whole IMU pcb, this would most likely allow all the I2C devices on the IMU pcb to recognize a minimum of 3.08v (4.4 * 0.7) as the lowest reliable logic level 1 tension allowing it to inter-operate with the PI's 3.3v logic levels. I was not able to find if the PI has internal pullups on the I2C bus or if they have to be externally placed so in doubt I put in 10k pullups between the 4.4v power line and the 2 data lines. I was the able to correctly detect the sensors on the IMU pcb.   
  
 ====== Sources ====== ====== Sources ======

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