MPXA4115A picbasic code

[Acetronics2]

Hi, Scalerobotics

Could you tell us what it shows for some close to another calibrated pressures ( water column i.e.) I'm really curious about this "oversampling method" application ...

And moreover after having carefully read the relevant AN 1152 from Microchip ...

Alain

" Surely overpaid Engineer ... ( lol ) , but Graduate Engineer ... "

[bitmaniac]

@Acetronics
VERY good infos in AN thanks for sharing!

@ Scalerobotics

You have done excellent job That was exactly I was thinking to do trying oversampling on 12f pics! so you did the job for me THANKS!

So the conclusion is that we can use 12f675 ir 12f683 as I want to just measure an alt difference of 200m

I did some calculation and I have attaches 2 XL one is the previous 10,12,14bit calculation and bolded out the 200m measurements just to see what is going on and the second one is use pic 10bit ADC but with Dual AMP LMC662 (Acetronics suggestion) to give a narrow-window measurements and the resolution improved to about 4m .

Also attached a fast schematic of my thoughts (not checked , improvements are welcome!)

Final conclusion I take the amp route or MPX connected directly to pic (with oversampling). I think as I will only want to measure 200m difference. the pcb must/and can be kept simple (low weight) and low part count.

(dual amp trick: first subtracts 2.5V from MPX vout and the result is multiplied by 3 , so we have a range just from
0-2300meters but better resolution for 10bit pic adc -see AZAltitude10bit_DUALAMP.xls)

Scalerobotics about oversampling on 12f do you think you can 'go with' just the internal 8Mhz rc oscilator or it will not do the trick?!

[Dave]

bitmaniac, Check your J1 and J2 power wiring....

Dave Purola,
N8NTA

[bitmaniac]

bitmaniac, Check your J1 and J2 power wiring....

Dave Purola,
N8NTA

Oh! you have a sharp eye!
I told you it was a fast schematic he!he!

corrected.

[ScaleRobotics]

Scalerobotics about oversampling on 12f do you think you can 'go with' just the internal 8Mhz rc oscilator or it will not do the trick?!

You can go with the internal, but at least on the 12F675 you have 4mhz internal, so your conversions are a little slower. See the TAD vs device operating frequencies part of your schematic. 1.6 us if fastest. Looks like 4mhz would be 2.0 us. So as long as you are not using 16 bit, you probably won't notice the difference in speed, and you will save one more part in your part count.

[bitmaniac]

@scalerobotics

I have import your code to Proteus Sim and playing around with your code.

It seems to work ok (I have to use meters instead of feets here ).

My MPX4115A arrived yesterday (4pieces) so it is time to make the real thing and figure out how it works. I am going to use my schematic ommiting the opamp and hope oversampling method works (12f683,internal 8Mhz,MPX,3caps -> total part count 5!)

For the moment I put everythimg in SIM (i used 12f683 with internal 8Mhz) and here what I found:
According to simulation at 50.400kPa I read 17646 feet ALT (lower limit) and
at 99.900kPa I read 5 feet ALT (upper limit) that is with this code we measure aprox. 49,500Kpa difference. (two screenshots attached)

Note: We must also use calibration data and/or Level0 default presure

(Please note that in the sim I can only vary sensor kPa in .100 increaments)

[ScaleRobotics]

Sounds good. If you beat me to it, try it with and without C3. I am guessing it may work better without it. If not without, it may work best with a much smaller capacitor. That defies most filtering logic, but I think oversampling likes a little bit of noise. I can't find my 470pf, but I know they are around here somewhere.....

[bitmaniac]

.....I can see your 470pf.... over there... yes there.....next to your Pickit2 he!he!

[bitmaniac]

3d visualization of a test pcb attached...

As you notice I didn't care much about the placement of sensor caps.
Just wanted to see the size of the board and best placement of connectors

(pcb measured... 45mm X 30 mm)

PLEASE NOTE all connectors will be male (not as seen) except for the sensor that it will be soldered on board
The sensor will be layed above C3 C2 and C1 (low profile cap)

[ScaleRobotics]

Hi, Scalerobotics

Could you tell us what it shows for some close to another calibrated pressures ( water column i.e.) I'm really curious about this "oversampling method" application ...

And moreover after having carefully read the relevant AN 1152 from Microchip ...

At my location and 14 bits, I get an output of 13255. This calculates to a pressure of 1004.86 mBar, and an altitude of 229.85 feet. I don't have a barometer here, so I can't really give you a side by side comparison on the figures. On the internet, it said a nearby weather barometer read 1014.42. But I think this is more a function of the accuracy of the MPX4115a, and to a bigger extent (since I don't have a calibrated barometer side by side) my altitude. The altitude from my calculations seem very close. I think I am between 250 and 285 feet altitude.

Oversampling is really just a form of averaging. So it can't skew your results. At worst, you could have no noise in your A/D conversion, which would give you a 10 bit result with lots of useless bits to the right. Once I find a 470pf capacitor around here, I plan to test results again. I think this will filter the result a bit too much, and take away some of the needed noise. Here, white noise is our friend.

On a side note, at 16 bits, when you watch the results at first power up, you can really see the effects of the MPX4115a and maybe the PIC12F675 warming up. For about two minutes the values really move!

[ScaleRobotics]

Here's a little code to calculate altitude from 0 to about 17,000 feet. I could not quite fit the full 85 value lookup2 table without getting some errors, so it is more of a 74 value lookup table. I may try a 74 value lookup table that covers the range to 36,000 feet later. Also will not work for Death Valley where the altitude goes to negative.

It is a pretty short program. I am using about 900 words of the PIC12F675 though, so not much space left over. Have not tested it too much, so if anyone finds issues, let me know.

DEFINE OSC          20
INCLUDE "DT_Analog.pbp"     ; DT's 16-bit Analog Module

DEFINE ADC_BITS     10  '10 BIT A/D CONVERSION RESULT
DEFINE ADC_CLOCK    2   '
DEFINE ADC_SAMPLEUS 5  'SET SAMPLE TIME IN MICROSECONDS


CMCON = 7               'TURN COMPARATORS OFF
TRISIO = %000100        'Set GSIO 0 INPUTS, others to OUTPUT
ANSEL = %00100100 	    '
ADCON0.7 = 1	        'Right Justify for 10-bit
ADCON0 = %10001001 

ADbits = 14                ; set to 14-bit resolution


DEFINE DEBUG_REG	GPIO
DEFINE DEBUG_BIT	0
DEFINE DEBUGIN_BIT	1
DEFINE DEBUG_BAUD	2400
DEFINE DEBUG_MODE	0
ALTresult var word       'lookup table output
tempALT var word         'base altitude of lookup from table
ADtemp var byte          'lookup table input
ALTdifference var word   'difference of two nearby altitudes
finalALT var word        'final calculated altitude in feet

;---------------------------------------------------------------------------
Main:
    ADchan = 2       ; Do only AN2 channel
    GOSUB GetADC            ; Get A/D value
    ADtemp = (ADvalue/100) - 60 ' get to table acceptable value for lookup
    gosub lookmeup
    tempALT = ALTresult   'save first result for use in math
    ADtemp = ADtemp - 1  'run another lookup for nearby altitude for diff calc
    gosub lookmeup
    ALTdifference = ALTresult - tempALT   'get difference in nearby alt
    finalALT = tempalt - (((ADvalue//100)* ALTdifference)/100) 
    'use difference to calculate slope & adjust altitude accordingly
    debug "Alt= ",DEC finalALT," A/D= ",DEC ADvalue,"   "
    pause 1000
GOTO Main:

End

lookmeup:
lookup2 ADtemp,[_
17646,17327,17011,16699,16389,16083,15780,15480,15183,14888,14597,14308,14022,_
13739,13458,13179,12903,12630,12359,12090,11823,11559,11297,11037,10779,10524,_
10270,10018,9769,9521,9275,9031,8789,8549,8310,8074,7838,7605,7373,7143,6915,_
6688,6463,6239,6017,5796,5577,5359,5143,4928,4715,4502,4292,4082,3874,3667,_
3462,3257,3054,2852,2652,2452,2254,2057,1861,1666,1472,1280,1088,898,708,520,_
332,146],ALTresult
return