AM2320 Sensor: CRCs doesn't match, CRC from sensor(0) - go

Question

Note: Code is cross-compiled in windows 10.
Code:
package main
import (
"fmt"
"io"
"log"
"net/http"
aosong "github.com/d2r2/go-aosong"
i2c "github.com/d2r2/go-i2c"
)
const i2CAddress = 0x5c
const i2CBus = 1
// Server struct
type Server struct {
Sensor *aosong.Sensor
I2C *i2c.I2C
}
func main() {
var err error
s := Server{Sensor: aosong.NewSensor(aosong.AM2320)}
s.I2C, err = i2c.NewI2C(i2CAddress, i2CBus)
if err != nil {
log.Printf(err.Error())
}
fmt.Println(s.Sensor.ReadRelativeHumidityAndTemperature(s.I2C))
defer s.I2C.Close()
}
Debug info:
2019-02-12T10:29:19.692 [ i2c] DEBUG Write 3 hex bytes: [030004]
2019-02-12T10:29:19.697 [ i2c] DEBUG Read 8 hex bytes: [0304012500d92045]
2019-02-12T10:29:19.698 [ i2c] DEBUG Read 8 hex bytes: [0000000000000000]
CRCs doesn't match: CRC from sensor(0) != calculated CRC(6912).
Any ideea why the CRC from sensor is 0?
I am able to read the sensor on the same bus with the same address with a python script.
#!/usr/bin/python
import posix
from fcntl import ioctl
import time
class AM2320:
I2C_ADDR = 0x5c
I2C_SLAVE = 0x0703
def __init__(self, i2cbus = 1):
self._i2cbus = i2cbus
#staticmethod
def _calc_crc16(data):
crc = 0xFFFF
for x in data:
crc = crc ^ x
for bit in range(0, 8):
if (crc & 0x0001) == 0x0001:
crc >>= 1
crc ^= 0xA001
else:
crc >>= 1
return crc
#staticmethod
def _combine_bytes(msb, lsb):
return msb << 8 | lsb
def readSensor(self):
fd = posix.open("/dev/i2c-%d" % self._i2cbus, posix.O_RDWR)
ioctl(fd, self.I2C_SLAVE, self.I2C_ADDR)
# wake AM2320 up, goes to sleep to not warm up and affect the humidity sensor
# This write will fail as AM2320 won't ACK this write
try:
posix.write(fd, b'\0x00')
except:
pass
time.sleep(0.001) #Wait at least 0.8ms, at most 3ms
# write at addr 0x03, start reg = 0x00, num regs = 0x04 */
posix.write(fd, b'\x03\x00\x04')
time.sleep(0.0016) #Wait at least 1.5ms for result
# Read out 8 bytes of result data
# Byte 0: Should be Modbus function code 0x03
# Byte 1: Should be number of registers to read (0x04)
# Byte 2: Humidity msb
# Byte 3: Humidity lsb
# Byte 4: Temperature msb
# Byte 5: Temperature lsb
# Byte 6: CRC lsb byte
# Byte 7: CRC msb byte
data = bytearray(posix.read(fd, 8))
# Check data[0] and data[1]
if data[0] != 0x03 or data[1] != 0x04:
raise Exception("First two read bytes are a mismatch")
# CRC check
if self._calc_crc16(data[0:6]) != self._combine_bytes(data[7], data[6]):
raise Exception("CRC failed")
# Temperature resolution is 16Bit,
# temperature highest bit (Bit15) is equal to 1 indicates a
# negative temperature, the temperature highest bit (Bit15)
# is equal to 0 indicates a positive temperature;
# temperature in addition to the most significant bit (Bit14 ~ Bit0)
# indicates the temperature sensor string value.
# Temperature sensor value is a string of 10 times the
# actual temperature value.
temp = self._combine_bytes(data[4], data[5])
if temp & 0x8000:
temp = -(temp & 0x7FFF)
temp /= 10.0
humi = self._combine_bytes(data[2], data[3]) / 10.0
return (temp, humi)
am2320 = AM2320(1)
(t,h) = am2320.readSensor()
print t, h

Answer 1

Seems there was an issue with the library itself which was making two reads, but because the read code was not sent the values came as 0, as it can be seen in the logs:
2019-02-12T10:29:19.692 [ i2c] DEBUG Write 3 hex bytes: [030004]
2019-02-12T10:29:19.697 [ i2c] DEBUG Read 8 hex bytes: [0304012500d92045] (first read that was ignored)
2019-02-12T10:29:19.698 [ i2c] DEBUG Read 8 hex bytes: [0000000000000000] (second one that came a 0)
CRCs doesn't match: CRC from sensor(0) != calculated CRC(6912)
Made a PR to fix the issue: https://github.com/d2r2/go-aosong/pull/3