Hi all,

I'm trying to port a library from ESP8266 to ATmega328P (Arduino Pro Mini clone, 5V), and am running into problems working with large int - specifically bit-wise operations on 32 and 64 bit int.

My library (the offending function is below) works perfectly on the ESP8266 processor. Now trying to run it on the Arduino, I'm running into trouble. This is code to read the MS5837 pressure sensor, which requires manipulation of 32 and 64 bit integers. In many cases there are multiplications and divisions by large numbers, all powers of two, which I replaced by the equivalent direct bit operations (left and right shifts).

The first that didn't work:

```
uint32_t D2 = (Wire.read() << 16) | (Wire.read() << 8) | Wire.read();
```

I had to replace this by:

```
uint32_t D1 = 0;
D1 = Wire.read();
D1 = (D1 << 8) | Wire.read();
D1 = (D1 << 8) | Wire.read();
```

The question I have is not how to make this work again (I know how), the question is more academic: why doesn't it work on the ATmega328, while it works on the ESP8266? I know the ESP8266 is 32-bit while the ATmega328 is 16-bit but that shouldn't be the issue as in the end both can deal with 64-bit values just fine. The problem is in the bitwise shifts.

The complete function:

```
void MS5837::read() {
// Check if sensor was read less than MIN_INTERVAL milliseconds ago. If so, don't read it again
// at this time.
if ((millis() - lastReadTime) < MIN_INTERVAL) return;
lastReadTime = millis();
// Request D1 (pressure) conversion using OSR = 8192
Wire.beginTransmission(MS5837_ADDR);
Wire.write(MS5837_CONVERT_D1);
Wire.endTransmission();
delay(20); // Wait for conversion to complete, maximum 17.2 ms according to the datasheet.
// Request the ADC to provide us with the result.
Wire.beginTransmission(MS5837_ADDR);
Wire.write(MS5837_ADC_READ);
Wire.endTransmission();
// Request the 3 bytes of data that should be ready for us.
Wire.requestFrom(MS5837_ADDR, 3);
// uint32_t D1 = (Wire.read() << 16) | (Wire.read() << 8) | Wire.read();
uint32_t D1 = 0;
D1 = Wire.read();
D1 = (D1 << 8) | Wire.read();
D1 = (D1 << 8) | Wire.read();
Serial.print("D1: ");
Serial.println(D1);
// Request D2 (temperature) conversion using OSR = 8192
Wire.beginTransmission(MS5837_ADDR);
Wire.write(MS5837_CONVERT_D2);
Wire.endTransmission();
delay(20); // Wait for conversion to complete, maximum 17.2 ms according to the datasheet.
// Request the ADC to provide us with the result.
Wire.beginTransmission(MS5837_ADDR);
Wire.write(MS5837_ADC_READ);
Wire.endTransmission();
// Request the 3 bytes of data that should be ready for us.
Wire.requestFrom(MS5837_ADDR, 3);
// uint32_t D2 = (Wire.read() << 16) | (Wire.read() << 8) | Wire.read();
uint32_t D2 = 0;
D2 = Wire.read();
D2 = (D2 << 8) | Wire.read();
D2 = (D2 << 8) | Wire.read();
Serial.print("D2: ");
Serial.println(D2);
// Calculate temperature (first order compensation).
int32_t dT = D2 - (C[5] << 8);
temperature = 2000 + ((int64_t(dT) * C[6]) >> 23);
// Calculate temperature compensated pressure (first order compensation).
int64_t off = (int64_t(C[2]) << 17) + ((int64_t(C[4]) * dT) >> 6); // Offset at actual temperature.
int64_t sens = (int64_t(C[1]) << 16) + ((int64_t(C[3]) * dT) >> 7); // Sensitivity at actual temperature.
pressure = (((D1 * sens) >> 21) - off) >> 15; // Temperature compensated pressure x100.
// If the temperature is < 20 C, calculate the second order compensation.
if (temperature < 2000) {
uint64_t ti = (11 * dT * dT) >> 35;
int64_t offi = (31 * (temperature - 2000) * (temperature - 2000)) >> 3;
int64_t sensi = (63 * (temperature - 2000) * (temperature - 2000)) >> 5;
off -= offi;
sens -= sensi;
temperature -= ti;
pressure = (((D1 * sens) >> 21) - off) >> 15;
}
}
```

Another library for the same sensor; this one does work on both the ESP8266 and ATmega328. It uses the large numbers for the divisions and multiplications.

```
void MS5837::read() {
// Request D1 conversion
Wire.beginTransmission(MS5837_ADDR);
Wire.write(MS5837_CONVERT_D1_8192);
Wire.endTransmission();
delay(20); // Max conversion time per datasheet
Wire.beginTransmission(MS5837_ADDR);
Wire.write(MS5837_ADC_READ);
Wire.endTransmission();
Wire.requestFrom(MS5837_ADDR,3);
D1 = 0;
D1 = Wire.read();
D1 = (D1 << 8) | Wire.read();
D1 = (D1 << 8) | Wire.read();
// Request D2 conversion
Wire.beginTransmission(MS5837_ADDR);
Wire.write(MS5837_CONVERT_D2_8192);
Wire.endTransmission();
delay(20); // Max conversion time per datasheet
Wire.beginTransmission(MS5837_ADDR);
Wire.write(MS5837_ADC_READ);
Wire.endTransmission();
Wire.requestFrom(MS5837_ADDR,3);
D2 = 0;
D2 = Wire.read();
D2 = (D2 << 8) | Wire.read();
D2 = (D2 << 8) | Wire.read();
calculate();
}
void MS5837::calculate() {
// Given C1-C6 and D1, D2, calculated TEMP and P
// Do conversion first and then second order temp compensation
int32_t dT = 0;
int64_t SENS = 0;
int64_t OFF = 0;
int32_t SENSi = 0;
int32_t OFFi = 0;
int32_t Ti = 0;
int64_t OFF2 = 0;
int64_t SENS2 = 0;
// Terms called
dT = D2-uint32_t(C[5])*256l;
if ( _model == MS5837_02BA ) {
SENS = int64_t(C[1])*65536l+(int64_t(C[3])*dT)/128l;
OFF = int64_t(C[2])*131072l+(int64_t(C[4])*dT)/64l;
P = (D1*SENS/(2097152l)-OFF)/(32768l);
} else {
SENS = int64_t(C[1])*32768l+(int64_t(C[3])*dT)/256l;
OFF = int64_t(C[2])*65536l+(int64_t(C[4])*dT)/128l;
P = (D1*SENS/(2097152l)-OFF)/(8192l);
}
// Temp conversion
TEMP = 2000l+int64_t(dT)*C[6]/8388608LL;
//Second order compensation
if ( _model == MS5837_02BA ) {
if((TEMP/100)<20){ //Low temp
Serial.println("here");
Ti = (11*int64_t(dT)*int64_t(dT))/(34359738368LL);
OFFi = (31*(TEMP-2000)*(TEMP-2000))/8;
SENSi = (63*(TEMP-2000)*(TEMP-2000))/32;
}
} else {
if((TEMP/100)<20){ //Low temp
Ti = (3*int64_t(dT)*int64_t(dT))/(8589934592LL);
OFFi = (3*(TEMP-2000)*(TEMP-2000))/2;
SENSi = (5*(TEMP-2000)*(TEMP-2000))/8;
if((TEMP/100)<-15){ //Very low temp
OFFi = OFFi+7*(TEMP+1500l)*(TEMP+1500l);
SENSi = SENSi+4*(TEMP+1500l)*(TEMP+1500l);
}
}
else if((TEMP/100)>=20){ //High temp
Ti = 2*(dT*dT)/(137438953472LL);
OFFi = (1*(TEMP-2000)*(TEMP-2000))/16;
SENSi = 0;
}
}
OFF2 = OFF-OFFi; //Calculate pressure and temp second order
SENS2 = SENS-SENSi;
if ( _model == MS5837_02BA ) {
TEMP = (TEMP-Ti);
P = (((D1*SENS2)/2097152l-OFF2)/32768l)/100;
} else {
TEMP = (TEMP-Ti);
P = (((D1*SENS2)/2097152l-OFF2)/8192l)/10;
}
}
```