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parallax_ms5607_barometric_altimeter

Parallax_MS5607_Barometric_Altimeter

Altimeter Module reads Pressure and Temperature, Altitude must be calculated. =Introduction= The Parallax MS5607 Altimeter is a pressure and temperature sensing chip loaded with some firmware and thrown on a breakout board. They are readily available at Radio Shack. It uses i2c communication to provide values which can be used to calculate an altitude. Unfortunately, the documentation is not straight forward enough for me to easily follow. Thus this wiki page to try to get things together in a concise format.

=i2c Protocol=

Since Java doesn't support unsigned fields, any command that has a MSB (starts with) a 1 will have to be converted. Thus, the command passed to the i2c interface will be a negative byte with a different value.

In order to do this conversion, take the command byte, subtract 1, do a bitwise complement and then you will have a magnitude.

For C3:

*10100110 *-1 *10100101 *Comp *01011010 *Hex *0x5A Signed > -0x5A

Thank you Java for forcing us to use signed fields.

Commands

*Device Address= 0x76 or 0x77 (118 or 119) depending on CSB *Reset= 0x1E *Read Calibration Coefficients C1= 0xA2 > -0x5E C2= 0xA4 > -0x5C C3= 0xA6 > -0x5A C4= 0xA8 > -0x58 C5= 0xAA > -0x56 C6= 0xAC > -0x54

Parsing

The unit will return 3 byte and 2 byte values when queried. These need to be parsed into signed java fields. The following functions return longs that represent the bytes that came in over i2c. Here's that code:

<syntaxhighlight lang=“java”> public final int TwoByteToInt(byteparallax_ms5607_barometric_altimeter b)

{
int i = 0;
i |= b[[0]] & 0xFF;
i <<= 8;
i |= b[[1]] & 0xFF;
return i;
}
		

public final long ThreeByteToLong(byteparallax_ms5607_barometric_altimeter b)

{
long i = 0;
i |= b[[0]] & 0xFF;
i <<= 8;
i |= b[[1]] & 0xFF;
i <<= 8;
i |= b[[2]] & 0xFF;
return i;
}

</syntaxhighlight>

=Calculations= The following code shows a way to use java to get Temperature, Pressure, and Altitude from the module: <syntaxhighlight lang=“java”> double Temp; double dT; double Offset; double Sens; double Pressure; //Temp Compensated Pressure in mbar double baroAltPre; double pRatio; double seaPressure; int baroAlt;

<nowiki>//</nowiki>Temperature Conversion
dT= d2 - c5*Math.pow(2, 8);
Temp= 2000 + dT*c6/Math.pow(2,23);
Temp= Temp/100; <nowiki>//</nowiki>Scales for readability
			
			
<nowiki>//</nowiki>Offsets and Sensitivity
Offset= c2*Math.pow(2, 17) +(c4 *dT ) /Math.pow(2, 6);
Sens= c1*Math.pow(2, 16) + c3*dT/Math.pow(2,7);
			
			
<nowiki>//</nowiki>Pressure Calculation
Pressure= (d1*Sens/Math.pow(2,21) - Offset) / Math.pow(2,15);
Pressure= Pressure/100; <nowiki>//</nowiki>Scales for readability
<nowiki>//</nowiki>Altitude Calculation
seaPressure=1014;
pRatio=seaPressure/Pressure;
baroAltPre= (Math.pow(pRatio, .19022256)-1)*(Temp+273.15) /.0065;
baroAlt= (int) (baroAltPre * 3.28084); <nowiki>//</nowiki>Converts to feet and places in int for easy use

</syntaxhighlight>

parallax_ms5607_barometric_altimeter.txt · Last modified: 2014/09/04 14:31 (external edit)