i2ctool dev has a -z option to use a zero-byte write request for

scanning the bus, which allows detection of more sensors such as the
MS5611 and SHT41 to be detected.

system/i2c/i2c_common.c

@@ -173,6 +173,10 @@ int i2ctool_common_args(FAR struct i2ctool_s *i2ctool, FAR char **arg)
         i2ctool->width = (uint8_t)value;
         return ret;
 
+      case 'z':
+        i2ctool->zerowrite = true;
+        return 1;
+
       default:
         goto invalid_argument;
     }

system/i2c/i2c_dev.c

@@ -64,6 +64,14 @@ int i2ccmd_dev(FAR struct i2ctool_s *i2ctool, int argc, FAR char **argv)
   saveaddr         = i2ctool->regaddr;
   i2ctool->regaddr = 0;
 
+  /* For backwards compatibility, the default behaviour while scanning will
+   * be to send a read request on the I2C bus. It is also possible to specify
+   * the use of a zero-byte write request instead, but this option will not
+   * be sticky.
+   */
+
+  i2ctool->zerowrite = false;
+
   /* Parse any command line arguments */
 
   for (argndx = 1; argndx < argc; )
@@ -126,6 +134,19 @@ int i2ccmd_dev(FAR struct i2ctool_s *i2ctool, int argc, FAR char **argv)
       goto errout;
     }
 
+  /* Display message warning user about some devices which don't appear
+   * unless using zero-byte write header.
+   */
+
+  if (!i2ctool->zerowrite)
+    {
+      i2ctool_printf(
+          i2ctool,
+          "NOTE: Some devices may not appear with this "
+          "scan.\nYou may also try a scan with the -z flag to "
+          "discover more devices using a zero-byte write request.\n");
+    }
+
   /* Probe each address */
 
   i2ctool_printf(i2ctool,
@@ -146,13 +167,22 @@ int i2ccmd_dev(FAR struct i2ctool_s *i2ctool, int argc, FAR char **argv)
 
           /* Set up data structures */
 
-          regaddr       = i2ctool->regaddr;
-
           msg.frequency = i2ctool->freq;
-          msg.addr      = addr;
-          msg.flags     = I2C_M_READ;
-          msg.buffer    = &regaddr;
-          msg.length    = 1;
+          msg.addr = addr;
+
+          if (i2ctool->zerowrite)
+            {
+              msg.flags = 0;
+              msg.buffer = NULL;
+              msg.length = 0;
+            }
+          else
+            {
+              regaddr = i2ctool->regaddr;
+              msg.flags = I2C_M_READ;
+              msg.buffer = &regaddr;
+              msg.length = 1;
+            }
 
           ret = i2cdev_transfer(fd, &msg, 1);
 

system/i2c/i2c_main.c

@@ -149,6 +149,10 @@ static int i2ccmd_help(FAR struct i2ctool_s *i2ctool, int argc,
   i2ctool_printf(i2ctool,
                  "  [-r regaddr] is the I2C device register index (hex)."
                  "  Default: not used/sent\n");
+  i2ctool_printf(
+      i2ctool,
+      "  [-z] instructs the 'dev' command to scan the I2C bus by sending "
+      "zero-byte write headers (if the architecture supports it)\n");
 
   i2ctool_printf(i2ctool, "\nNOTES:\n");
 #ifndef CONFIG_DISABLE_ENVIRON
@@ -398,6 +402,7 @@ int main(int argc, FAR char *argv[])
     }
 
   g_i2ctool.hasregindx = false;
+  g_i2ctool.zerowrite = false;
 
   /* Parse and process the command line */
 

system/i2c/i2ctool.h

@@ -127,6 +127,7 @@ struct i2ctool_s
   uint8_t  regaddr;    /* [-r regaddr] is the I2C device register address */
   uint8_t  width;      /* [-w width] is the data width (8 or 16) */
   bool     start;      /* [-s|n], send|don't send start between command and data */
+  bool     zerowrite;  /* [-z] uses a zero byte write request to scan the I2C bus */
   bool     autoincr;   /* [-i|j], Auto increment|don't increment regaddr on repetitions */
   bool     hasregindx; /* true with the use of -r */
   uint32_t freq;       /* [-f freq] I2C frequency */