AP_ADC: ADS1115: fix driver after conversion to I2CDevice

In _start_conversion(), the check for return code of _dev->transfer() was
inverted. The structure also needs to be PACKED, otherwise there will be
a hole in the middle.  Fix these issues and use be16_t where it makes
sense.

In read() we need to check for the second byte of config register, so
either make it an array of uint8_t or convert from big endian to host
endianness.  It's simpler to leave it as it was, accessing just the
first byte. Also the conversion value is in be16 type an needs to be
converted to host endiannes, not the opposite.

Fix bus number: all boards that use it expect it to be on bus 1, not 0.

libraries/AP_ADC/AP_ADC_ADS1115.cpp

@@ -7,7 +7,9 @@
 #define ADS1115_ADDRESS_ADDR_VDD    0x49 // address pin high (VCC)
 #define ADS1115_ADDRESS_ADDR_SDA    0x4A // address pin tied to SDA pin
 #define ADS1115_ADDRESS_ADDR_SCL    0x4B // address pin tied to SCL pin
-#define ADS1115_DEFAULT_ADDRESS     ADS1115_ADDRESS_ADDR_GND
+
+#define ADS1115_I2C_ADDR            ADS1115_ADDRESS_ADDR_GND
+#define ADS1115_I2C_BUS             1
 
 #define ADS1115_RA_CONVERSION       0x00
 #define ADS1115_RA_CONFIG           0x01
@@ -106,9 +108,9 @@ static const uint16_t mux_table[ADS1115_CHANNELS_COUNT] = {
 };
 
 
-AP_ADC_ADS1115::AP_ADC_ADS1115():
+AP_ADC_ADS1115::AP_ADC_ADS1115()
-    _dev(hal.i2c_mgr->get_device(0, ADS1115_DEFAULT_ADDRESS)),
+    : _dev(hal.i2c_mgr->get_device(ADS1115_I2C_BUS, ADS1115_I2C_ADDR))
-    _channel_to_read(0)
+    , _channel_to_read(0)
 {
     _samples = new adc_report_s[_channels_number];
 }
@@ -127,22 +129,17 @@ bool AP_ADC_ADS1115::init()
 
 bool AP_ADC_ADS1115::_start_conversion(uint8_t channel)
 {
-    struct {
+    struct PACKED {
         uint8_t reg;
-        uint16_t w;
+        be16_t val;
     } config;
 
-    config.w = ADS1115_OS_ACTIVE | _gain | mux_table[channel] |
-    ADS1115_MODE_SINGLESHOT | ADS1115_COMP_QUE_DISABLE | ADS1115_RATE_250;
-
     config.reg = ADS1115_RA_CONFIG;
-    config.w = htobe16(config.w);
+    config.val = htobe16(ADS1115_OS_ACTIVE | _gain | mux_table[channel] |
+                         ADS1115_MODE_SINGLESHOT | ADS1115_COMP_QUE_DISABLE |
+                         ADS1115_RATE_250);
 
-    if (_dev->transfer((uint8_t *)&config, sizeof(config), nullptr, 0)) {
+    return _dev->transfer((uint8_t *)&config, sizeof(config), nullptr, 0);
-        return false;
-    }
-
-    return true;
 }
 
 size_t AP_ADC_ADS1115::read(adc_report_s *report, size_t length) const
@@ -202,32 +199,32 @@ void AP_ADC_ADS1115::_update()
         return;
     }
 
-    uint16_t word, config;
+    uint8_t config[2];
+    be16_t val;
 
     if (!_dev->get_semaphore()->take_nonblocking()) {
         return;
     }
 
-    if (!_dev->read_registers(ADS1115_RA_CONFIG, (uint8_t *)&config, sizeof(config))) {
+    if (!_dev->read_registers(ADS1115_RA_CONFIG, config, sizeof(config))) {
         error("i2c->read_registers failed in ADS1115");
         _dev->get_semaphore()->give();
         return;
     }
 
     /* check rdy bit */
-    if ((config & 0x80) != 0x80 ) {
+    if ((config[1] & 0x80) != 0x80 ) {
         _dev->get_semaphore()->give();
         return;
     }
 
-    if (!_dev->read_registers(ADS1115_RA_CONVERSION, (uint8_t *)&word,  sizeof(word))) {
+    if (!_dev->read_registers(ADS1115_RA_CONVERSION, (uint8_t *)&val,  sizeof(val))) {
         _dev->get_semaphore()->give();
         return;
     }
 
-    word = htobe16(word);
 
-    float sample = _convert_register_data_to_mv(word);
+    float sample = _convert_register_data_to_mv(be16toh(val));
 
     _samples[_channel_to_read].data = sample;
     _samples[_channel_to_read].id = _channel_to_read;