BMP388 understand Data

Other Hardware Sensors
1

Hey Guys,

i'm working on a own Arduino Flightcontroller and am trying to get a BMP388 working. I want to estimate the altitude.

I got the task done with the Adafruit Library, but it takes 2.5 microseconds to read the Sensor and get the altitude. Thats way to slow for a quadrocopter.

That's why i try to write my own Code and for a performance. I think i'm able to read the 24bit pressure data from the register 0x04:

Wire.beginTransmission(BMP_ADDR);                                     //Start communication with the BMP.
 Wire.write(0x04);                                                    //Start reading @ register 0x04 and auto increment with every read.
 Wire.endTransmission();                                              //End the transmission.
 Wire.requestFrom(BMP_ADDR,6);                                        //Request 6 bytes from the gyro.    
 while(Wire.available() < 6);                                         //Wait until 6 Byte are recieved
 pressure = (Wire.read()<<16) | (Wire.read()<< 8 ) | (Wire.read());
 druck = (float)pressure/0.33;
 temperature = (Wire.read()<<16) | (Wire.read()<< 8 ) | (Wire.read())

But i'm not sure if i understand how to get the pressure in Pa the right way.
What i do:

druck = (float)pressure/0.33;

I set the Oversamplingrate of the Pressure to x8. In the Datasheet is wirtten on page 13:
x8 Oversampling 19bit/0.33 Pa resolution.

Did i calculated the pressure the right way ?

https://www.mouser.com/pdfdocs/BST-BMP388-DS001-01.pdf

2

Please remove this line:

while(Wire.available() < 6);        //Wait until 6 Byte are recieved

Explanation: Common mistakes #1.

Please show the full sketch. There is a website for it: https://snippets-r-us.com/.

The Wire.read() returns an integer. That is 16 bits on a Arduino Uno or Mega. When shifting that 16 times to the left you end up with nothing.

Adafruit library: GitHub - adafruit/Adafruit_BMP3XX.

The datasheet and the Adafruit library do many calculations to get the temperature and the pressure.
I see now that a faster sampling is possible when the temperature is not needed. The 0.33 is the resolution. As far as I can tell the number from the registers is not the pressure in Pa, I think you still have to do all those calculations. But I'm not sure, I always use a library with all those calculations for a Bosch sensor, without thinking about it.

3

My Code. I really need a faster library. It's kinda difficult, to get this done by myself.

#define BMP_ADDR 0x76
#include <Wire.h>
uint32_t pressure,temperature;
float druck;
void setup() {
  // put your setup code here, to run once:
Serial.begin(115200);
Wire.begin();
TWBR = 12;                                                                //Set the I2C clock speed to 400kHz.
SetupBMP();
}

void loop() 
{
  // put your main code here, to run repeatedly:
GetBMPData();
Serial.println(druck);
delay(20);
}
void I2C_WriteOneByte(uint8_t DevAddr, uint8_t RegAddr, uint8_t value)
{
  Wire.beginTransmission(DevAddr);
  Wire.write(RegAddr);
  Wire.write(value);
  Wire.endTransmission();
}
uint8_t I2C_ReadOneByte(uint8_t DevAddr, uint8_t RegAddr)
{
  uint8_t value;

  Wire.beginTransmission(DevAddr);
  Wire.write((byte)RegAddr);
  Wire.endTransmission();

  Wire.requestFrom(DevAddr, (byte)1);
  value = Wire.read();

  return value;
}
void SetupBMP()
{
I2C_WriteOneByte(BMP_ADDR,0x7E,0xB6); //Trigger a Softreset
delay(20);
I2C_WriteOneByte(BMP_ADDR,0x1C,0x03); //Set Oversampling for pressure (x8) and Temperature(x1)
delay(20);
I2C_WriteOneByte(BMP_ADDR,0x1D,0x02); //Set the Output Data Rate 50Hz = 20ms
delay(20);
I2C_WriteOneByte(BMP_ADDR,0x1F,0x02); //Set the IIR Filter Coefficients = 3
delay(20);
I2C_WriteOneByte(BMP_ADDR,0x1B,0x01|0x10|0x30); //Setup Serial Interface Settings
}
void GetBMPData(){
 Wire.beginTransmission(BMP_ADDR);                                     //Start communication with the gyro.
 Wire.write(0x04);                                                    //Start reading @ register 43h and auto increment with every read.
 Wire.endTransmission();                                              //End the transmission.
 Wire.requestFrom(BMP_ADDR,3);                                        //Request 14 bytes from the gyro.    
 while(Wire.available() < 3);                                         //Wait until 3 Byte are recieved
 pressure = (Wire.read()<<16) | (Wire.read()<< 8 ) | (Wire.read());
 druck = (float)pressure/0.33;
 temperature = (Wire.read()<<16) | (Wire.read()<< 8 ) | (Wire.read());
}
4

Please remove this line: while(Wire.available() < 3);

In the function GetBMPData(), you request 3 bytes, but you read 6 bytes (pressure and temperature).

To set the speed, the function Wire.setClock() can be used.

You can measure it with micros():

void loop()
{
  unsigned long t1 = micros();
  GetBMPData();
  unsigned long t2 = micros();

  Serial.print( "GetBMPData: ");
  Serial.print( druck);
  Serial.print( " (");
  Serial.print( t2 - t1);
  Serial.print( " microseconds)");
  delay(20);
}

400kHz is about 25µs per byte.
The GetBMPData transfers 6 bytes (or 9 with the bug). That is 150µs (or 225µs with the bug). Plus 50% overhead. Plus the rest of the function.

To get higher speed, you need to use the SPI bus.

5

it takes 2.5 microseconds to read the Sensor and get the altitude. Thats way to slow for a quadrocopter

Have I misunderstood something about quads?

6

TheMemberFormerlyKnownAsAWOL:
Have I misunderstood something about quads?

More cryptical pls....

My Loop refreshrate is 4ms (250 Hz). The PWM takes between 1ms - 2ms. So have 2ms for Calculations.
So 2.5 ms for reading altitude is too much.

I dont get, how modern flightcontrollers get 8kHz, but this is another story...

7

You wrote 2.5 microseconds, not milliseconds.

I don't get why you would want to calculate air pressure at 250Hz anyway. Calculate it as often as possible.