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Forum Index : Microcontroller and PC projects : Bosch BMP085 Barometer FP Pres. Cal.
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| DuinoMiteMegaAn Senior Member  Joined: 17/11/2011 Location: AustraliaPosts: 231 |
This sensor provides barometric pressure and temperature. It can also be used to calculate altitude. Both Sparkfun and Adafruit vendors have this precision sensor. ($20 USD) This BMP085 is an excellant choice for use in a weather station for predicting weather. I just might add it to my Outback Dot Clock project! The BMP085 I2C 3.3v barometric sensor mates well with the MaxiMite. The complex floating point (instead of integer math) can be easily done in MMBasic. Below is the precision floating point "test" code I converted to MMBasic. The I2C communicating code will be posted later after my BMP085 hardware arrives. Ref. Doc. 2012-03-07_000021_BMP085-Calcs.pdf 'File: BMP085R1.bas 'Project: Outback Dot Clock Rev. 1.0b 'Author: Hacker - DuinoMiteMegaAndy 'Date: 3/6/2012 'App. Version: Rev. 1.0b 'MMBasic Firmware: 3.1 MaxiMite CGMMStick1 'Platform: Win XP/SP3 '=========================================================== 'Bosch BMP085 Barometer Floating Point Pressure Calculations 'April 25, 2011 '=========================================================== '========================================== ' BMP085 Calibration Data - device specific '========================================== BMP085_AC1 = 7911 BMP085_AC2 = -934 BMP085_AC3 = -14306 BMP085_AC4 = 31567 BMP085_AC5 = 25671 BMP085_AC6 = 18974 BMP085_B1 = 5498 BMP085_B2 = 46 BMP085_MB = -32768 BMP085_MC = -11075 BMP085_MD = 2432 '=============================== 'BMP085 Derived Calibration Data '=============================== 'This data remains constant for any given sensor and need not be read and 'computed every time the sensor is powered on. Once all values are computed, 'it is not necessary to retain the values of BMP085_c3, BMP085_c4, BMP085_b1. 'The first three values are only used 'in computing the final constants below and can be discarded afterwards. BMP085_c3 = 160*2^-15*BMP085_AC3 ? "BMP085_c3 = ", BMP085_c3 BMP085_c4 = 10^-3*2^-15*BMP085_AC4 ? "BMP085_c4 = ", BMP085_c4 BMP085_b1 = 160^2*2^-30*BMP085_B1 ? "BMP085_b1 = ", BMP085_b1 'The next four constants are used in the computation of temperature. BMP085_c5 = 2^-15/160*BMP085_AC5 ? "BMP085_c5 = ", BMP085_c5 BMP085_c6 = BMP085_AC6 ? "BMP085_c6 = ", BMP085_c6 BMP085_mc = 2^11/160^2*BMP085_MC ? "BMP085_mc = ", BMP085_mc BMP085_md = BMP085_MD/160 ? "BMP085_md = ", BMP085_md 'Three second order polynomials are used to compute pressure, and they 'require another nine constants (three for each polynomial). BMP085_x0 = BMP085_AC1 ? "BMP085_x0 = ", BMP085_x0 BMP085_x1 = 160*2^-13*BMP085_AC2 ? "BMP085_x1 = ", BMP085_x1 BMP085_x2 = 160^2*2^-25*BMP085_B2 ? "BMP085_x2 = ", BMP085_x2 BMP085_y0 = BMP085_c4*2^15 ? "BMP085_y0 = ", BMP085_y0 BMP085_y1 = BMP085_c4*BMP085_c3 ? "BMP085_y1 = ", BMP085_y1 BMP085_y2 = BMP085_c4*BMP085_b1 ? "BMP085_y2 = ", BMP085_y2 BMP085_p0 = (3791-8)/1600 ? "BMP085_p0 = ", BMP085_p0 BMP085_p1 = 1-(7357*2^-20) ? "BMP085_p1 = ", BMP085_p1 BMP085_p2 = 3038*100*2^-36 ' This constant is wrong in the PDF !!!! ? "BMP085_p2 = ", BMP085_p2 'The coefficients above have been scaled so that the final results will be 'temperature (BMP085_T) in degrees Celsius, and pressure (BMP085_P) in millibars (a.k.a hecto- 'Pascals) 'This wraps up the calculation of floating point constants that are used for 'converting the raw, integer temperature and pressure data into degrees Celsius 'and millibars. '================ ' BMP085 Formulas '================ 'The real-time data from the BMP085 will be represented by tu for temperature 'and pu for pressure. The temperature value is just equal to the integer 'value that is read from the barometer. The pressure reading is formatted a bit 'different however; it is treated like a fixed point fraction with the radix point just 'to the right of the LSB. After these integer values are acquired, all remaining 'calculations below are floating point. These first two formulas treat the integer 'register values as unsigned quantities. '0x69EC I2C Temperature 'BMP085_tu = 256 ·MSB + LSB BMP085_tu = 256*&H69 + &hEC ? "BMP085_tu = ", BMP085_tu '0x982FC0 I2C Pressure 'BMP085_pu = 256 ·MSB + LSB + XLSB/256 BMP085_pu = (256*&H98)+&H2F+(&HC0/256) ? "BMP085_pu = ", BMP085_pu 'Defining pu in this way alleviates the need to make any further adjustments 'for the over-sampling ratio (OSS). If OSS is zero, it is not necessary to read the 'XLSB register and its value can be set to zero. 'The temperature computation is quite simple. First we define a new value, 'and then compute temperature in terms of this new variable: BMP085_a = BMP085_c5*(BMP085_tu - BMP085_c6) BMP085_T = BMP085_a + (BMP085_mc / (BMP085_a + BMP085_md)) BMP085_Tc = BMP085_T ? "BMP085_Tc degrees C = ", BMP085_Tc BMP085_Tf = 1.8 * BMP085_T + 32 ? "BMP085_Tf degrees F = ", BMP085_Tf 'Computing pressure requires the temperature (BMP085_T) computed above, and uses 'three second-order polynomials. The first two generate offset (BMP085_x) and scaling 'factors (y) which are functions of the difference between the current temperature 'and 25C (which we’ll call BMP085_s). BMP085_s = BMP085_T - 25 ? "BMP085_s = ", BMP085_s BMP085_x = BMP085_x2*BMP085_s^2 + BMP085_x1*BMP085_s + BMP085_x0 ? "BMP085_x = ", BMP085_x BMP085_y = BMP085_y2*BMP085_s^2 + BMP085_y1*BMP085_s + BMP085_y0 ? "BMP085_y = ", BMP085_y 'These two factors are now used to compute an initial pressure value, BMP085_z: BMP085_z = (BMP085_pu - BMP085_x) / BMP085_y ? "BMP085_z = ", BMP085_z 'A polynomial in BMP085_z gives the final corrected pressure value. BMP085_P = BMP085_p2*BMP085_z^2 + BMP085_p1*BMP085_z + BMP085_p0 ? "BMP085_P millibars = ", BMP085_P End Test results BMP085_c3 = -69.8535 BMP085_c4 = 0.000963348 BMP085_b1 = 0.131083 BMP085_c5 = 0.00489635 BMP085_c6 = 18974 BMP085_mc = -886 BMP085_md = 15.2 BMP085_x0 = 7911 BMP085_x1 = -18.2422 BMP085_x2 = 0.0350952 BMP085_y0 = 31.567 BMP085_y1 = -0.0672933 BMP085_y2 = 0.000126278 BMP085_p0 = 2.36438 BMP085_p1 = 0.992984 BMP085_p2 = 4.42087e-06 BMP085_tu = 27116 BMP085_pu = 38959.8 BMP085_Tc degrees C = 23.7764 BMP085_Tf degrees F = 74.7975 BMP085_s = -1.22363 BMP085_x = 7933.37 BMP085_y = 31.6495 BMP085_z = 980.311 BMP085_P millibars = 980.045 Enjoy  |
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Juri74 Senior Member Joined: 06/02/2012 Location: ItalyPosts: 162 |
that's very useful! great work! Do you have some mathematical formula to calculate altitude too? thank you |
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| DuinoMiteMegaAn Senior Member Joined: 17/11/2011 Location: AustraliaPosts: 231 |
Both Sparkfun and Adafruit provide detailed on-line tutorials for the complex Bosch BMP085. Both vendors also include the altitude calculation and weather prediction algorithms from the barometric pressure and temperature readiings.  |
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| DuinoMiteMegaAn Senior Member Joined: 17/11/2011 Location: AustraliaPosts: 231 |
MaxiMite application test code for the BMP085 Digital Pressure Sensor by Bosch Sensortec. This code is still in the beta testing <------<<<< Any calculation errors please report them on this board's topic link. This code was made for testing the BMP085 on the MaxiMite and all algorithms can be greatly reduced by the end user! This code is "as is" No warranty implied. Use at your own risk! <------<<<<< What's working: #1. I2C reading/calculating constants from the BMP085 #2. I2C reading/calculating temperature from the BMP05 #3. I2C reading/calculating barometric pressure from the BMP085 with all oversampling modes working. #4. Altitude calculation - needs work #5. Weather prediction / calculation - needs work What's not working - unknown? More testing on-going. Applications: Model rocketry --> Launch a CGMMStick1 with the BMP085 and measure altitude. Weather forcasting. Pressure measurement Temperature measurement Usage Hookup the BMP085 to the MaxiMite with proper pullups and power (+3 VDC ONLY!!!!) load / run "bmp085R7.bas" Check static FP calculation by calling subroutine "FP_BMP085_Test_R1" (Just type in this string) Check all measurement by by calling subroutine "read_BMP085" Check pressure oversampling modes by calling subroutine "check_BMP085_OSS_Modes" Enjoy
Ref. Links: Sparkfun tutorial <----<<<< helpful http://www.sparkfun.com/tutorials/253 Adafruit tutorial <----<<< helpfu http://www.ladyada.net/learn/sensors/bmp085.html Docs Spec. sheet bmp085_datasheet_rev.1.0_1July2008.pdf 2012-03-13_040342_BMP085_DataSheet_Rev.1.0_01July2008.pdf Bosch BMP085 Barometer Floating Point Pressure Calculations ...and some analysis...April 25, 2011 BMP085-Calcs.pdf 2012-03-13_040302_BMP085-Calcs.pdf Code '--------- BMP085R7.bas --------------------
'Project: 'Author: Hacker - DuinoMiteMegaAndy 'Date: 3/12/2012 'App. Version: Rev. 1.0b 'MMBasic Firmware: 3.1 MaxiMite CGMMStick1 'Platform: Win XP/SP3 ' 'Testing conditions: 'MaxiMite aka CGMMStick1 'I2C with 2.2K pullups @ 100 Khz ' 'Warning ... BMP085 is light sensitive - keep port shielded away from direct light ' 'There are two pullup resistors on the breakout module! 'I had to remove them because I had other pullups elsewhere. OPTION BASE 0 DIM BMP085_buff(22) BMP085_I2CADDR = &h77 BMP085_ULTRALOWPOWER = 0 BMP085_STANDARD = 1 BMP085_HIGHRES = 2 BMP085_ULTRAHIGHRES = 3 BMP085_CONTROL = &hF4 BMP085_TEMPDATA = &hF6 BMP085_PRESSUREDATA = &hF6 BMP085_READTEMPCMD = &h2E BMP085_READPRESSURECMD = &h34 BMP085_altitude_M = 0 ' altitude in meters BMP085_altitude_F = 0 ' altitude in feet Raw_Pres = 0 '----------------------------------------------------------- -------------------- '----------------------------------------------------------- -------------------- SUB FP_BMP085_Test_R1 ' 'Author: Hacker 'Date: 3/9/2012 'Rev: 1.0b 'Description: From PDF ---> "Bosch BMP085 Barometer Floating Point Pressure Calculations.PDF" April 25, 2011 'for the PIC32 MaxiMite - for reference only. 'This test does NOT include the I2C code for the BMP085. 'See other subroutines below for the I2C interface. ' 'This test shows the results from the complex floating point calculations 'for the Bosch BMP085 - see pdf - these calculations are "spot on" and matches 'this reference document. ' '====================================================== ' BMP085 Calibration Data - device specific - Test only '====================================================== BMP085_AC1 = 7911 BMP085_AC2 = -934 BMP085_AC3 = -14306 BMP085_AC4 = 31567 'Unsigned? BMP085_AC5 = 25671 'Unsigned? BMP085_AC6 = 18974 'Unsigned? BMP085_B1 = 5498 BMP085_B2 = 46 BMP085_MB = -32768 BMP085_MC = -11075 BMP085_MD = 2432 '=============================== 'BMP085 Derived Calibration Data '=============================== 'This data remains constant for any given sensor and need not be read and 'computed every time the sensor is powered on. Once all values are computed, 'it is not necessary to retain the values of BMP085_c3, BMP085_c4, BMP085_b1. 'The first three values are only used 'in computing the final constants below and can be discarded afterwards. BMP085_c3 = 160*2^-15*BMP085_AC3 ? "BMP085_c3 = ", BMP085_c3 BMP085_c4 = 10^-3*2^-15*BMP085_AC4 ? "BMP085_c4 = ", BMP085_c4 BMP085_b1 = 160^2*2^-30*BMP085_B1 ? "BMP085_b1 = ", BMP085_b1 'The next four constants are used in the computation of temperature. BMP085_c5 = 2^-15/160*BMP085_AC5 ? "BMP085_c5 = ", BMP085_c5 BMP085_c6 = BMP085_AC6 ? "BMP085_c6 = ", BMP085_c6 BMP085_mc = 2^11/160^2*BMP085_MC ? "BMP085_mc = ", BMP085_mc BMP085_md = BMP085_MD/160 ? "BMP085_md = ", BMP085_md 'Three second order polynomials are used to compute pressure, and they 'require another nine constants (three for each polynomial). BMP085_x0 = BMP085_AC1 ? "BMP085_x0 = ", BMP085_x0 BMP085_x1 = 160*2^-13*BMP085_AC2 ? "BMP085_x1 = ", BMP085_x1 BMP085_x2 = 160^2*2^-25*BMP085_B2 ? "BMP085_x2 = ", BMP085_x2 BMP085_y0 = BMP085_c4*2^15 ? "BMP085_y0 = ", BMP085_y0 BMP085_y1 = BMP085_c4*BMP085_c3 ? "BMP085_y1 = ", BMP085_y1 BMP085_y2 = BMP085_c4*BMP085_b1 ? "BMP085_y2 = ", BMP085_y2 BMP085_p0 = (3791-8)/1600 ? "BMP085_p0 = ", BMP085_p0 BMP085_p1 = 1-(7357*2^-20) ? "BMP085_p1 = ", BMP085_p1 BMP085_p2 = 3038*100*2^-36 ' This constant is wrong in the PDF !!!! ? "BMP085_p2 = ", BMP085_p2 'The coefficients above have been scaled so that the final results will be 'temperature (BMP085_T) in degrees Celsius, and pressure (BMP085_P) in millibars (a.k.a hecto- 'Pascals) 'This wraps up the calculation of floating point constants that are used for 'converting the raw, integer temperature and pressure data into degrees Celsius 'and millibars. '================ ' BMP085 Formulas '================ 'The real-time data from the BMP085 will be represented by BMP085_tu for temperature 'and BMP085_pu for pressure. The temperature value is just equal to the integer 'value that is read from the barometer. The pressure reading is formatted a bit 'different however; it is treated like a fixed point fraction with the radix point just 'to the right of the LSB. After these integer values are acquired, all remaining 'calculations below are floating point. These first two formulas treat the integer 'register values as unsigned quantities. '0x69EC I2C Temperature 'BMP085_tu = 256 ·MSB + LSB BMP085_tu = 256*&H69 + &hEC ? "BMP085_tu = ", BMP085_tu '0x982FC0 I2C Pressure 'BMP085_pu = 256 ·MSB + LSB + XLSB/256 BMP085_pu = (256*&H98)+&H2F+(&HC0/256) ? "BMP085_pu = ", BMP085_pu 'Defining BMP085_pu in this way alleviates the need to make any further adjustments 'for the over-sampling ratio (OSS). If OSS is zero, it is not necessary to read the 'XLSB register and its value can be set to zero. 'The temperature computation is quite simple. First we define a new value, 'and then compute temperature in terms of this new variable: BMP085_a = BMP085_c5*(BMP085_tu - BMP085_c6) BMP085_T = BMP085_a + (BMP085_mc / (BMP085_a + BMP085_md)) BMP085_Tc = BMP085_T ? "BMP085_Tc degrees C = ", BMP085_Tc BMP085_Tf = 1.8 * BMP085_T + 32 ? "BMP085_Tf degrees F = ", BMP085_Tf 'Computing pressure requires the temperature (BMP085_T) computed above, and uses 'three second-order polynomials. The first two generate offset (BMP085_x) and scaling 'factors (y) which are functions of the difference between the current temperature 'and 25C (which we’ll call BMP085_s). BMP085_s = BMP085_T - 25 ? "BMP085_s = ", BMP085_s BMP085_x = BMP085_x2*BMP085_s^2 + BMP085_x1*BMP085_s + BMP085_x0 ? "BMP085_x = ", BMP085_x BMP085_y = BMP085_y2*BMP085_s^2 + BMP085_y1*BMP085_s + BMP085_y0 ? "BMP085_y = ", BMP085_y 'These two factors are now used to compute an initial pressure value, BMP085_z: BMP085_z = (BMP085_pu - BMP085_x) / BMP085_y ? "BMP085_z = ", BMP085_z 'A polynomial in BMP085_z gives the final corrected pressure value. BMP085_P = BMP085_p2*BMP085_z^2 + BMP085_p1*BMP085_z + BMP085_p0 ? "BMP085_P millibars = ", BMP085_P END SUB '----------------------------------------------------------- -------------------- SUB read_BMP085_Raw_Temp 'BMP085_I2CADDR &h77 'BMP085_CONTROL &hF4 'BMP085_TEMPDATA &hF6 'BMP085_PRESSUREDATA &hF6 'BMP085_READTEMPCMD &h2E 'BMP085_READPRESSURECMD &h34 DIM temp_buff(2) I2CEN 100,100 'Enable I2C 100 Khz 100 ms timeout I2CSEND BMP085_I2CADDR,0,2,BMP085_CONTROL,BMP085_READTEMPCMD' write I2CDIS Pause 5 ' I2CEN 100,100 ' Enable I2C 100 Khz 100 ms timeout I2CRCV BMP085_I2CADDR, 0, 2, temp_buff(0),1,BMP085_TEMPDATA 'write then read I2CDIS BMP085_tu = (temp_buff(0)*2^8) + temp_buff(1) Erase temp_buff END SUB '----------------------------------------------------------- -------------------- SUB read_BMP085_Raw_Pres (mode) local OSS ' for ref only 'BMP085_I2CADDR &h77 'BMP085_ULTRALOWPOWER 0 'BMP085_STANDARD 1 'BMP085_HIGHRES 2 'BMP085_ULTRAHIGHRES 3 'BMP085_CONTROL &hF4 'BMP085_TEMPDATA &hF6 'BMP085_PRESSUREDATA &hF6 'BMP085_READTEMPCMD &h2E 'BMP085_READPRESSURECMD &h34 if (mode > BMP085_ULTRAHIGHRES) then mode = BMP085_ULTRAHIGHRES oversampling = mode if oversampling = 0 then OSS = &h34 if oversampling = 1 then OSS = &h74 if oversampling = 2 then OSS = &hB4 if oversampling = 3 then OSS = &hF4 DIM pres_buff(3) I2CEN 100,100 'Enable I2C 100 Khz 100 ms timeout 'I2CSEND BMP085_I2CADDR,0,2,BMP085_CONTROL, (BMP085_READPRESSURECMD + (oversampling*2^6))' write 'simplified I2C write I2CSEND BMP085_I2CADDR,0,2,BMP085_CONTROL,OSS ' write I2CDIS 'wait for oversampling to finish - timed IF (oversampling = BMP085_ULTRALOWPOWER) THEN Pause 5 ' ms ELSEIF (oversampling = BMP085_STANDARD) THEN Pause 8 ELSEIF (oversampling = BMP085_HIGHRES) THEN Pause 14 ELSE Pause 26' ms ENDIF ' I2CEN 100,100 ' Enable I2C 100 Khz 100 ms timeout I2CRCV BMP085_I2CADDR, 0, 3, pres_buff(0),1,BMP085_PRESSUREDATA 'write then read I2CDIS ' BMP085_pu =(PRES_buff(0)*(2^16) + PRES_buff(1)*(2^8) + PRES_buff(2)) \ 2^(8-oversampling) raw_pres = BMP085_pu if oversampling = 0 then BMP085_pu = BMP085_pu ' 1 sample if oversampling = 1 then BMP085_pu = BMP085_pu / 2 ' 2 samples if oversampling = 2 then BMP085_pu = BMP085_pu / 4 ' 4 samples if oversampling = 3 then BMP085_pu = BMP085_pu / 8 ' 8 samples Erase pres_buff END SUB '----------------------------------------------------------- -------------------- '----------------------------------------------------------- -------------------- '----------------------------------------------------------- -------------------- ' I2C BMP085 Algorithms for the MaxiMite PIC32 '----------------------------------------------------------- -------------------- SUB read_BMP085_Constants I2CEN 100,100 ' Enable I2C 100 Khz 100 ms timeout I2CSEND BMP085_I2CADDR,0,1,&hAA ' write I2CDIS ' I2CEN 100,100 ' Enable I2C 100 Khz 100 ms timeout I2CRCV BMP085_I2CADDR, 0, 22, BMP085_buff(0), 1, &hAA 'write then read I2CDIS End Sub SUB convert_BMP085_Constants 'Lowbyte is MSB (e.g $AA), Highbyte is LSB (e.g. $AB) 'Device stores the MSB in the Low byte, LSB in the High byte ' 'Assuming twos complement: 'read your pair of bytes, combine them into a 16 bit number as shown above, 'check if the number is equal or greater than 0x8000 (means it is negative), 'if not then treat it as positive else invert it (number XOR &HFFFF) and add '1 and then change the sign to negative. BMP085_AC1 = (BMP085_buff(0)*2^8) + BMP085_buff(1) If BMP085_AC1 >= &H8000 Then BMP085_AC1 = BMP085_AC1 Xor &HFFFF BMP085_AC1 = BMP085_AC1 + &h1 BMP085_AC1 = BMP085_AC1 * (-1) EndIf BMP085_AC2 = (BMP085_buff(2)*2^8) + BMP085_buff(3) If BMP085_AC2 >= &H8000 Then BMP085_AC2 = BMP085_AC2 Xor &HFFFF BMP085_AC2 = BMP085_AC2 + &h1 BMP085_AC2 = BMP085_AC2 * (-1) EndIf BMP085_AC3 = (BMP085_buff(4)*2^8) + BMP085_buff(5) If BMP085_AC3 >= &H8000 Then BMP085_AC3 = BMP085_AC3 Xor &HFFFF BMP085_AC3 = BMP085_AC3 + &h1 BMP085_AC3 = BMP085_AC3 * (-1) EndIf BMP085_AC4 = (BMP085_buff(6)*2^8) + BMP085_buff(7) ' UNSIGNED <----<<<< BMP085_AC5 = (BMP085_buff(8)*2^8) + BMP085_buff(9) ' UNSIGNED <----<<<< BMP085_AC6 = (BMP085_buff(10)*2^8) + BMP085_buff(11) ' UNSIGNED <----<<<< BMP085_B1 = (BMP085_buff(12)*2^8) + BMP085_buff(13) If BMP085_B1 >= &H8000 Then BMP085_B1 = BMP085_B1 Xor &HFFFF BMP085_B1 = BMP085_B1 + &h1 BMP085_B1 = BMP085_B1 * (-1) EndIf BMP085_B2 = (BMP085_buff(14)*2^8) + BMP085_buff(15) If BMP085_B2 >= &H8000 Then BMP085_B2 = BMP085_B2 Xor &HFFFF BMP085_B2 = BMP085_B2 + &h1 BMP085_B2 = BMP085_B2 * (-1) EndIf BMP085_MB = (BMP085_buff(16)*2^8) + BMP085_buff(17) If BMP085_MB >= &H8000 Then BMP085_MB = BMP085_MB Xor &HFFFF BMP085_MB = BMP085_MB + &h1 BMP085_MB = BMP085_MB * (-1) EndIf BMP085_MC = (BMP085_buff(18)*2^8) + BMP085_buff(19) If BMP085_MC >= &H8000 Then BMP085_MC = BMP085_MC Xor &HFFFF BMP085_MC = BMP085_MC + &h1 BMP085_MC = BMP085_MC * (-1) EndIf BMP085_MD = (BMP085_buff(20)*2^8) + BMP085_buff(21) If BMP085_MD >= &H8000 Then BMP085_MD = BMP085_MD Xor &HFFFF BMP085_MD = BMP085_MD + &h1 BMP085_MD = BMP085_MD * (-1) EndIf END SUB '----------------------------------------------------------- -------------------- SUB print_BMP085_Constants ? "BMP085_AC1",BMP085_AC1 ? "BMP085_AC2",BMP085_AC2 ? "BMP085_AC3",BMP085_AC3 ? "BMP085_AC4",BMP085_AC4 'Unsigned? ? "BMP085_AC5",BMP085_AC5 'Unsigned? ? "BMP085_AC6",BMP085_AC6 'Unsigned? ? "BMP085_B1", BMP085_B1 ? "BMP085_B2", BMP085_B2 ? "BMP085_MB", BMP085_MB ? "BMP085_MC", BMP085_MC ? "BMP085_MD", BMP085_MD END SUB '----------------------------------------------------------- -------------------- SUB BMP085_Derived_Calib_Data BMP085_c3 = 160*2^-15*BMP085_AC3 '? "BMP085_c3 = ", BMP085_c3 BMP085_c4 = 10^-3*2^-15*BMP085_AC4 '? "BMP085_c4 = ", BMP085_c4 BMP085_b1 = 160^2*2^-30*BMP085_B1 '? "BMP085_b1 = ", BMP085_b1 BMP085_c5 = 2^-15/160*BMP085_AC5 '? "BMP085_c5 = ", BMP085_c5 BMP085_c6 = BMP085_AC6 '? "BMP085_c6 = ", BMP085_c6 BMP085_mc = 2^11/160^2*BMP085_MC '? "BMP085_mc = ", BMP085_mc BMP085_md = BMP085_MD/160 '? "BMP085_md = ", BMP085_md BMP085_x0 = BMP085_AC1 '? "BMP085_x0 = ", BMP085_x0 BMP085_x1 = 160*2^-13*BMP085_AC2 '? "BMP085_x1 = ", BMP085_x1 BMP085_x2 = 160^2*2^-25*BMP085_B2 '? "BMP085_x2 = ", BMP085_x2 BMP085_y0 = BMP085_c4*2^15 '? "BMP085_y0 = ", BMP085_y0 BMP085_y1 = BMP085_c4*BMP085_c3 '? "BMP085_y1 = ", BMP085_y1 BMP085_y2 = BMP085_c4*BMP085_b1 '? "BMP085_y2 = ", BMP085_y2 BMP085_p0 = (3791-8)/1600 '? "BMP085_p0 = ", BMP085_p0 BMP085_p1 = 1-(7357*2^-20) '? "BMP085_p1 = ", BMP085_p1 BMP085_p2 = 3038*100*2^-36 '? "BMP085_p2 = ", BMP085_p2 END SUB '----------------------------------------------------------- -------------------- SUB init_BMP085 'call I2C constants only once! <-------------<<<<< Call this first! read_BMP085_Constants convert_BMP085_Constants 'print_BMP085_Constants' - before derived cal. BMP085_Derived_Calib_Data 'print_BMP085_Constants' - after derived cal. END SUB '----------------------------------------------------------- -------------------- SUB cal_BMP085_Temperature ' <----------<<<<<< Always call this second! read_BMP085_Raw_Temp 'compute temp BMP085_a = BMP085_c5*(BMP085_tu - BMP085_c6) BMP085_T = BMP085_a + (BMP085_mc / (BMP085_a + BMP085_md)) BMP085_Tc = BMP085_T '? "BMP085_Tc degrees C = ", BMP085_Tc BMP085_Tf = 1.8 * BMP085_T + 32 '? "BMP085_Tf degrees F = ", BMP085_Tf '? " " END SUB '----------------------------------------------------------- -------------------- SUB cal_BMP085_Pressure 'used by altitude, weather calculations 'Get BMP085 temp first - for the pressure cal <----<<<< cal_BMP085_Temperature ' 'For ref. only 'BMP085_ULTRALOWPOWER 0 'BMP085_STANDARD 1 'BMP085_HIGHRES 2 'BMP085_ULTRAHIGHRES 3 ' 'USE ONLY ONE ! Comment the other out ! ' 'read_BMP085_Raw_Pres BMP085_ULTRALOWPOWER ' User selectable SELECT ONLY ONE <------<<<<< 'read_BMP085_Raw_Pres BMP085_STANDARD ' User selectable SELECT ONLY ONE <------<<<<< 'read_BMP085_Raw_Pres BMP085_HIGHRES ' User selectable SELECT ONLY ONE <------<<<<< read_BMP085_Raw_Pres BMP085_ULTRAHIGHRES ' User selectable SELECT ONLY ONE <------<<<<< ' ' Compute pressure BMP085_s = BMP085_T - 25 '? "BMP085_s = ", BMP085_s BMP085_x = BMP085_x2*BMP085_s^2 + BMP085_x1*BMP085_s + BMP085_x0 '? "BMP085_x = ", BMP085_x BMP085_y = BMP085_y2*BMP085_s^2 + BMP085_y1*BMP085_s + BMP085_y0 '? "BMP085_y = ", BMP085_y 'These two factors are now used to compute an initial pressure value, BMP085_z: BMP085_z = (BMP085_pu - BMP085_x) / BMP085_y '? "BMP085_z = ", BMP085_z 'A polynomial in BMP085_z gives the final corrected pressure value. BMP085_P = BMP085_p2*BMP085_z^2 + BMP085_p1*BMP085_z + BMP085_p0 '? "BMP085_P millibars = ", BMP085_P ' <-----------------------------<<<<<< In millibars! END SUB '----------------------------------------------------------- -------------------- SUB Compute_BMP085_pressure 'Needs derived constants and temperature <------<<<< BMP085_s = BMP085_T - 25 '? "BMP085_s = ", BMP085_s BMP085_x = BMP085_x2*BMP085_s^2 + BMP085_x1*BMP085_s + BMP085_x0 '? "BMP085_x = ", BMP085_x BMP085_y = BMP085_y2*BMP085_s^2 + BMP085_y1*BMP085_s + BMP085_y0 '? "BMP085_y = ", BMP085_y 'These two factors are now used to compute an initial pressure value, BMP085_z: BMP085_z = (BMP085_pu - BMP085_x) / BMP085_y '? "BMP085_z = ", BMP085_z 'A polynomial in BMP085_z gives the final corrected pressure value. BMP085_P = BMP085_p2*BMP085_z^2 + BMP085_p1*BMP085_z + BMP085_p0 '? "BMP085_P millibars = ", BMP085_P ' <-------<<<<<< In millibars! END SUB '----------------------------------------------------------- -------------------- SUB check_BMP085_OSS_Modes Local avg,idx ' read_BMP085_Constants convert_BMP085_Constants BMP085_Derived_Calib_Data ' cal_BMP085_Temperature read_BMP085_Raw_Pres BMP085_ULTRALOWPOWER ? "<BMP085 Pressure ULTRALOWPOWER Mode>" ? "Raw Pressure - BMP085 1 sample = ", raw_pres Compute_BMP085_pressure ? "BMP085_P millibars hPa = ", BMP085_P ? " " cal_BMP085_Temperature read_BMP085_Raw_Pres BMP085_STANDARD ? "<BMP085 Pressure STANDARD Mode>" ? "Raw Pressure - BMP085 2 samples = ", raw_pres Compute_BMP085_pressure ? "BMP085_P millibars hPa = ", BMP085_P ? " " cal_BMP085_Temperature read_BMP085_Raw_Pres BMP085_HIGHRES ? "<BMP085 Pressure HIGHRES Mode>" ? "Raw Pressure - BMP085 4 samples = ", raw_pres Compute_BMP085_pressure ? "BMP085_P millibars hPa = ", BMP085_P ? " " cal_BMP085_Temperature read_BMP085_Raw_Pres BMP085_ULTRAHIGHRES ? "<BMP085 Pressure ULTRAHIGHRES Mode>" ? "Raw Pressure - BMP085 8 samples = ", raw_pres Compute_BMP085_pressure ? "BMP085_P millibars hPa = ", BMP085_P ? " " ' Special MaxiMite Average Sample Mode ' blocks of 8 samples are averaged 75 times (600 samples) ' 26 ms/ 8 block * 75 = ~ 2 seconds avg = 0 FOR IDX = 1 TO 75 cal_BMP085_Temperature read_BMP085_Raw_Pres BMP085_ULTRAHIGHRES Compute_BMP085_pressure ' Millibars avg = avg + BMP085_P NEXT avg = avg/75 ? "MaxiMite BMP085 (600 samples) millibars hPa = ", avg ? " " END SUB '----------------------------------------------------------- -------------------- SUB cal_Altitude local Pres_x, sealevel_pres, cal_constant sealevel_pres = 101325 ' Pascals <----- Fixed constant ? cal_constant = 0 ' 0 millibars ' convert millibars or hectorPascals to Pascals Pres_x = (BMP085_P - cal_constant) * 100 ' millibars to Pascals ' In meters 'BMP085_altitude_M = 44330 * (1.0 - ((Pres_x /sealevel_pres)^ 0.1903)) 'better precision BMP085_altitude_M = 44330 * (1.0 - ((Pres_x /sealevel_pres)^ 0.190294957184)) '? "BMP085 alt. M = ", BMP085_altitude_M ' ' In feet BMP085_altitude_F = BMP085_altitude_M * 3.2808399 '? "BMP085 alt. F = ", BMP085_altitude_F '? " " END SUB '----------------------------------------------------------- -------------------- SUB cal_Weather Local ePressure_Pa, weatherDiff, BMP085_Pa, cal_constant cal_BMP085_Pressure ' get pressure (in millibars/hectorPascals) cal_Altitude cal_constant = 0 ' 0 millibars BMP085_Pa = (BMP085_P - cal_constant) * 100 ' Convert to Pascals ePressure_Pa = BMP085_Pa * (1.0 - ((BMP085_altitude_M / 44330)^ 5.255)) ' In meters and Pascals weatherDiff = BMP085_Pa - ePressure_pa ? "BMP085 Weather BMP085_Pa = ",BMP085_Pa ? "BMP085 Weather ePressure Pa = ",ePressure_Pa ? "BMP085 Weather Diff. Pa = ",weatherDiff if(weatherDiff > 250) then ?"Sunny!" ELSEIF ((weatherDiff <= 250) OR (weatherDiff >= -250)) THEN ?"Partly Cloudy" ELSEIF if (weatherDiff > -250) THEN ?"Rain" ENDIF END SUB '----------------------------------------------------------- -------------------- '----------------------------------------------------------- -------------------- SUB read_BMP085 'Init Bmp085 'call I2C constants only once! ? "<BMP085 Constants>" read_BMP085_Constants convert_BMP085_Constants print_BMP085_Constants ? " " ? "<BMP085 Derived Constants>" BMP085_Derived_Calib_Data print_BMP085_Constants ? " " cal_BMP085_Pressure ' calls temperature first for pressure compensation ? "<BMP085 Temperature>" ? "BMP085_Tc degrees C = ", BMP085_Tc ? "BMP085_Tf degrees F = ", BMP085_Tf ? " " ? "<BMP085 Pressure>" ? "BMP085_P millibars hPa = ", BMP085_P ' Millibars ? "BMP085_P Pascals = ", BMP085_P * 100 ? " " ? "<BMP085 Altitude>" cal_altitude ' Needs sea level pressure and BMP085 Pa pressure ? "BMP085 Alt. M = ", BMP085_altitude_M ? "BMP085 Alt. F = ", BMP085_altitude_F ? " " ? "<BMP085 Weather>" cal_weather ' Needs altitude. Crude but needs more precision END SUB '----------------------------------------------------------- -------------------- '----------------------------------------------------------- -------------------- END ' end of program |
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| DuinoMiteMegaAn Senior Member Joined: 17/11/2011 Location: AustraliaPosts: 231 |
FYIO ... Altitude Testing What is your altitude? You need to know this when working with the BMP085. Since I have an airport 500 meters away on flat ground, I took their readings. Elevation 902 feet. Another way to check altitude use Google Earth or this link http://www.daftlogic.com/sandbox-google-maps-find-altitude.h tm Google Earth produced 896 feet. Google Maps produced 896.363 feet I used my handheld GPS and it showed 1000 feet?. <------ Do not use this reference! GPS are NOT known by their precision altitude readings/calculations! Sparkfun shows an error of 69 feet and I showed an error of 105 feet. <--- edited I just added/installed a "very small" calibration offset in the software.
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BobD Guru  Joined: 07/12/2011 Location: AustraliaPosts: 935 |
after I gave you info on twos complement recently I was looking at the method and thought it can be done easier. Your code, much as I suggested, BMP085_AC1 = (BMP085_buff(0)*2^8) + BMP085_buff(1)
If BMP085_AC1 >= &H8000 Then BMP085_AC1 = BMP085_AC1 Xor &HFFFF BMP085_AC1 = BMP085_AC1 + &h1 BMP085_AC1 = BMP085_AC1 * (-1) EndIf may be simplified as the following: BMP085_AC1 = (BMP085_buff(0)*2^8) + BMP085_buff(1)
If BMP085_AC1 >= &H8000 Then BMP085_AC1 = BMP085_AC1 - 65536 EndIf This includes the correct sign. The general formula is (number - 2^n) where n is the number of bits in "number". So for 16 bit negative TC numbers subtract 2^16 which is &H10000 or 65536. |
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| DuinoMiteMegaAn Senior Member Joined: 17/11/2011 Location: AustraliaPosts: 231 |
@BobD - Thanks for the reduced code algorithm for 2's complement. That got me over the hump until I "hit the wall" with the two complex formulas for altitude and weather calculations. altitude BMP085_altitude_M = 44330 * (1.0 - ((Pres_x /sealevel_pres)^ 0.190294957184))
weather ePressure_Pa = BMP085_Pa * (1.0 - ((BMP085_altitude_M / 44330)^ 5.255))
See anything obvious that is wrong?
Ref. Sparkfun Device BMP085 (quickstart tutorial) |
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| BobD Guru Joined: 07/12/2011 Location: AustraliaPosts: 935 |
Andy I wouldn't have thought that the revised TC stuff was over the hump material. Are you struggling with free memory? What problems do you have now? The exponent in the altitude calculation may run into loss of accuracy problems. It will probably get truncated. I would be inclined to use 0.1903 instead. Just looked at your code and you are already there. I'm no help there. If you need a way to look at your intermediate results you could try this bit of code SUB Wait
? +++++++++++++++++++++ ? ' print any variables that may be important ? " Press any key to continue > "; DO LOOP UNTIL INKEY$ <> "" END SUB when you want to use it just include a WAIT in your program and edit the SUB to print the variables. The ++++++s are just used as markers to indicate that it is not usual program output. They can be left out. |
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| DuinoMiteMegaAn Senior Member Joined: 17/11/2011 Location: AustraliaPosts: 231 |
What I was asking if both forumulas "matched" in format the formuluas by sparkfun and by Bosch? The weather algorithm produced 0 differential? Looking at the weather calculation, I had to insert a "fixed" value for the altitude and replaced BMP085_Pa with the pressure at sea level eg 101325 Pa. by doing so, the "epressure_Pa" seems to always stay around at 101325. The formula seems "not to change" but it gives the needed diff. "leg" for the epressure_Pa difference. SUB cal_Weather
' algorithm fix - the differential was always zero because the altitude was variable not fixed. ' epressure calculation should have sealevel Pa! Local ePressure_Pa, weatherDiff, BMP085_Pa, cal_constant, fixed_user_altitude_M cal_BMP085_Pressure ' get pressure (in millibars/hectorPascals) 'cal_Altitude <--- do not use - for ref. only fixed_user_altitude_M = 273.1 ' <-----<<<<< Local user altitude in meters cal_constant = 0 ' 0 millibars - for calbibration if needed. ePressure_Pa = 101325 * (1.0 - ((fixed_user_altitude_M / 44330)^ 5.255)) ' In meters and Pascals BMP085_Pa = (BMP085_P - cal_constant) * 100 ' Convert to Pascals with cal constant weatherDiff = BMP085_Pa - ePressure_pa ? "BMP085 Weather BMP085_Pa = ",BMP085_Pa ? "BMP085 Weather ePressure Pa = ",ePressure_Pa ? "BMP085 Weather Diff. Pa = ",weatherDiff if(weatherDiff > 250) then ?"Sunny!" ELSEIF ((weatherDiff <= 250) OR (weatherDiff >= -250)) THEN ?"Partly Cloudy" ELSEIF if (weatherDiff > -250) THEN ?"Rain" ENDIF END SUB Are you struggling with free memory?
Yes, but the workaround is to run "blocks of code" from the SD drive which has 16 GB. |
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| DuinoMiteMegaAn Senior Member Joined: 17/11/2011 Location: AustraliaPosts: 231 |
Weather calculation This code snippet is still wrong! ePressure_Pa = 101325 * (1.0 - ((fixed_user_altitude_M / 44330)^ 5.255))
Sparkfun weather calculation. ePressure_Pa = 101325 * (1 - fixed_user_altitude_M / 44330)^ 5.255 ' In meters and Pascals The formula recomputes what the barometric pressure should be at a given elevation. At sea level (101325 Pa) the elevation is 0 meters so the ref. is 101325 Pa. At higher elevations the pressure decreases so at my elevation (273.1 meters) the calculated air pressure reference (for weather) is 98087.409 Pa. Using the above calcuation they read the present barometric pressure to get the difference between the two values to compute the projected weather. <----- very cool So the correct weather formula is ePressure_Pa = 101325 * (1 - fixed_user_altitude_M / 44330)^ 5.255 ' In meters and Pascals
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| DuinoMiteMegaAn Senior Member Joined: 17/11/2011 Location: AustraliaPosts: 231 |
I read the Sparkfun and Adafruit tutorials on the BMP085 and they left out some important points. <BMP085 Q&A> 1. Why doesn't the RAW BMP085 Barometric pressure reading match the local weather station? Note: In most cases the local weather station will read higher (lower elevation) than the RAW pressure BMP085 reading. In my case, 32 millibars lower for 273.1 meters in elevation for my location. Note: The BMP085 is good for +- 2 millibars. All weather stations in the US and other countries? uses sea level as the pressure reference. This means, what you see from a weather station is without the change in pressure due to elevation changes.You can go from one weather station to the next and they keep the same pressure reference without changes elevation pressure. Note: The pressure at sea level is 101325 Pa and the pressure goes down when there is an increase in elevation.(Air pressure is lighter [less pressure] at higher elevations) To use the BMP085 as a true weather indicator you will need to add a calibration offset to the raw BMP085 pressure reading. By doing so, you are removing "your offset" due to elevation height. This way it will track other area professional weather station readings. If you don't have a weather station close by to calibrate the BMP085 then use this website link: http://www.csgnetwork.com/barcorrecthcalc.html You need to know your exact elevation (in feet or meters) to add their factor for your BMP085 pressure calibration. Do not use a GPS to determine your elevation! Please note: Sparkfun used the raw (uncalibrated) BMP085 pressure for weather prediction calculations. This is fine and will work but beware it will not match other area professional weather station readings without this calibration offset added. 2. Why doesn't the BMP085 / Sparkfun elevation calculation match the elevation at your location? If it does, consider yourself extremely lucky. This means the weather conditions, temperature, humidity at your location and at sea level is perfect! The Sparkfun altitude calculation uses a fixed sea level pressure (101325 Pa). This is fine but the weather, temperature and humidity at sea level and at your location is not always perfect. For better precision you need to find out the pressure at sea level for your location "area". Website link: http://aviationweather.gov/adds/metars/ select Metars enter your 4 letter airport station iden. example "KOSU" select translated - get the exact sea level, for the area.(Used by pilots) This pressure is due to weather changes at sea level Note: Australians need to find a similiar website that calculates the sea level pressure to put in the altitude calculation (for your country) Enjoy
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| BobD Guru Joined: 07/12/2011 Location: AustraliaPosts: 935 |
The Australian Bureau of Meteorology gives these stats. Here is my local weather station and an explanation of terms can be found on that page (and many others) if you click on the link About weather observations . The columns of interest are the second and third from the right. The altitude is shown on the page. Other weather stations in Victoria can be found here and if you want other states then look around. They are easily found. One thing that confuses me is the obs at Falls Creek which is a lot higher than Yarrawonga, show a higher pressure reading. ?????? |
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