MPXA4115A picbasic code

[bitmaniac]

@scalerobotics

Of course you have not notice that in the last attached XL I upload there are three XL sheet in the file 10,12,14bit

Test it

My only concern is what method to choose (to start building) pic A/D with oversampling tehnicue , 12bit MAX and pic or DIF AMP + MAX + pic?

I cannot decide yet.

[Acetronics2]

Hi, Alexandros

I'd recommend you to read the Freescale AN 1646 ...

Interesting matter !

take care to the LM 358 output voltage range ... I think it's not the good choice ...
LMC662 or AD 623 would be better ...

Have a good Evening

Alain

[ScaleRobotics]

@scalerobotics

Of course you have not notice that in the last attached XL I upload there are three XL sheet in the file 10,12,14bit

Actually, I did notice, and it looks to be 100% correct. I am only pointing out that the increments with the 14 bit is 0.553 meters (near sea level), and not the ~1 meter that you mentioned in your previous post.

Thanks,

Walter

[ScaleRobotics]

I just tested Darrel's oversampling, using the MPX4115a and a PIC12F675 that I quickly soldered up, and it works very well. I just used the two capacitors on the power side of the MPX4115a, like the data sheet shows, but with no filter capacitor on the output. Even at 16 bit resolution, the A/D output values only varied by +/- 1 digit. It varied more when I breathed though! 16 bit is really sensitive!

My vote is for oversampling, parts count is at 6, running the board from the power of the PicKit2.

Pic12f675
3 capacitors
mpx4115a
20 mhz resonator

[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]

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

[Bill Legge]

I don't see the problem with dividing by 1024?
The process is quantising a variable. Say our variable voltage lies in the range 0 to 10 Volts and we have one bit to use for the digital result of the quantising (2 to the power of 1 = 2, so we have two levels to play with):

A. The result 0 means that the voltage was in the range 0 to 5 Volts (not that it was zero).
B. The result 1 means the voltage was in the range 5 to 10 volts (not that it was 10 volts)

So each level is 10/2 volts wide. Not 10/1 volt wide.

The bit of cake after carving it up into portions has a slice 0 - it is not any smaller than any other slice.

This is not jibberish created by highly paid engineers it is simple maths.

I wonder how many devices you have launched on the world that are happily 'over' measuring analog quantities by assuming that each slice is bigger than it actualy is?

Kind regards to you all

Bill Legge

[Acetronics2]

Hi,

The problem is very simple : the servo signal frame length is too long ( or too short ??? ) and the ESC do not recognise it as a valid frame ...

The solution is not so simple : the total time of the frame must be 30 or 35 ms as a MAXIMUM and , say 17-18 ms as a minimum.

That's why servo works and not the ESC ...

Alain

[bitmaniac]

Sure there is a timing (delay) problem.
You gave a more sophisticating explanation.
You gave me an idea.May be we I will hook the oscilloscope to see the exact pulse..

[Acetronics2]

Hi Alexandros

I had committed that from what you wrote ...

'****************************************************************************
' (c)2009 Alexandros Zahariadis (E.A.TH.) Greece *
' Electric Glider electronic switch to be used in F3J-GR competitions.
' The device will cut-off motor if 200m ALT reached or 30seconds passed
' whichever comes first!
' There is also a status led option which idicates if you have reached 200m
' or not (may be because of luck of power!)
' steady led -> 200m ALT reached , blinking led -> 30 secs passed.
' NOTE: after a lot of experiments and because of code execution speed
' I use direct ADC raw sample values without converting them to meters first!
'    :: thanks to Acetronics (Alain) and picbasic forum for codding ideas ::
'****************************************************************************

'@ DEVICE FCMEN_OFF
'@ DEVICE IESO_OFF
'@ DEVICE BOD_ON
'@ DEVICE CPD_OFF
'@ DEVICE PROTECT_OFF
'@ DEVICE MCLR_OFF
'@ DEVICE PWRT_OFF
'@ DEVICE WDT_OFF
'@ DEVICE INTRC_OSC

@    __config _INTRC_OSC_NOCLKOUT & _WDT_ON & _PWRTE_ON & _MCLRE_OFF & _CP_OFF & _BOD_ON

INCLUDE "modedefs.bas"

clear
DEFINE OSC 4
define PULSIN_MAX 250
;DEFINE DEBUG_REG GPIO
;DEFINE DEBUG_BIT 2
;DEFINE DEBUG_BAUD 9600
;DEFINE DEBUG_MODE 1

OSCCON = $60           ' $60:4 Mhz $70:8 Mhz int.
CMCON0 = 7              'TURN COMPARATORS OFF
TRISIO = %00001010      'Set GPIO1 and GPIO3 0 INPUTS, others to OUTPUT
ANSEL =  %00000100      'Set AN2 , Fosc/32
ADCON0 = %00000000      'AN2 select & Right Justify for 10-bit
;---------------------------------------------------------------------------   
THROTTLE    VAR GPIO.0  'Outputs to throttle servo 
RADIO       VAR GPIO.1  'Input from receiver
LED         var GPIO.2  'status indicator
ADC_DATA	var	GPIO.3	' adc serial data
ADC_SCLK    var	GPIO.5	' adc serial clock
ADC_CS		var	GPIO.4	' adc chip select
;--------------------------------------------------------------------------- 
                       
advalue 	var	word    ' value read from adc
groundALT   var word
difalt      var word
P           VAR WORD    ' This stores the PULSIN value
x           var byte    ' general var
y           var byte    ' general var
SEC_COUNT   VAR BYTE    ' seconds counter
O_FLOWS     VAR BYTE    ' holds the number of timer1 overflows
BRAKE_PULSE var word    ' servo pulse to STOP (brake on) THR servo
BRAKE_PULSE = 100

'-= vars for averaging routine =-
AvgCount	CON  6		' = Number of samples to average
FAspread	CON  1000	' = Fast Average threshold +/-
Value		VAR  WORD
'-= end of vars for averaging routine =-

'******************************************************************************
' TIMER1 Cycle MUST be longer than a R/C frame duration to be sure not to miss any ticks.
' 50 ms is a good value for timing precision.
' PL = 15543 @ 4Mhz ( reload = 7 cycle )
'******************************************************************************

T1LO CON $49            'timer1 preload values
T1HI CON $C3            '$C349 -> -15543
low throttle

   INTCON  = 0          ' disable interrupts
   T1CON = %00010000    ' 1:2 prescale (1uS per tick), timer1 off (max 1:8!)
                        ' 11 = 1:8 ,10 = 1:4, 01 = 1:2, 00 = 1:1 Prescale Value
   TMR1H = T1HI         ' 65,536-15536 = 50.000
   TMR1L = T1LO         ' 50.000 ticks * 1uS =50.000usec.=100msec
                        ' 1 second= 100*10
                        ' every 10 timer1 overflows = 1 second   1uS=
'---------------------------------------------------------------------------
pause 3000

for x=1 to 4
	high led
	pause 200
	low led
	pause 100
next x



'******************************************************************************
CAL_BP: ' CALIBRATING Brake_Pulse ( P )
'******************************************************************************

	Pause 50
	LOW led
 	P = 0
 
	For x = 1 TO 16

		PulsIn radio,1,P
		
		IF ( P < 80 ) OR ( P > 220 ) Then
			Led = 1
			GoTo CAL_BP
		EndIF
		
	
		IF P = $00 Then prems
	
		IF ( P < (Brake_Pulse -1)) OR ( P > (Brake_Pulse +1)) THEN
			Led = 1
			GoTo CAL_BP
		ENDIF

prems:	Brake_Pulse = P
	
	Next x

LOW Throttle

''******************************************************************************
'calibrate:             'check for valid signal
'
'For x = 1 TO 2
'
'	PulsIn radio,1,P
'	pauseus 10
'	IF ( P < 80 ) OR ( P > 200 ) Then calibrate
'	next x
'	
'for x=1 to 3
'	PulsIn radio,1,P
'	brake_pulse = P
'next x
'
'low led
'
'low throttle

'*****************************************************************************
precheck:            'leave some seconds to do a motor test
						'then motor brake is ON for some seconds
for y=1 to 3
         
	for x=1 to 255
		PULSIN RADIO,1,P
				
		IF ( P < 80 ) OR ( P > 220 ) Then
			Led = 1
			GoTo Precheck
		EndIF
		
		Led = 0
		LOW Throttle            
		pULSOUT THROTTLE, p
		
	next x
	
next y

for x=1 to 160

	LOW Throttle
	PULSOUT THROTTLE, brake_pulse   
	pause 18
	
next x

low THROTTLE

GOSUB GetAdc         						'Get ground ALT
groundalt=value

'******************************************************************************
armcheck:									' Now, we can launch the beast ...
            
PULSIN RADIO,1,P            
	IF ( P < 80 ) OR ( P > 220 ) Then
		Led = 1								' Show Problem !!!		
		LOW Throttle
		GoTo Armcheck
	ENDIF
	
Led = 0		
low throttle
PULSOUT THROTTLE, p

if difalt < 2 OR difalt > 10000 then Armcheck 'Arm if>=5mALT (in adc samples) reached!!


'******************************************************************************
'******************************************************************************
start:                  				' We are flying , now ... preset timer1
 
   SEC_COUNT=28         				' load for 30 to 0 seconds countdown
   O_FLOWS=0            				' clear overflow count
   PIR1.0=0             				' clear timer1 overflow flag
   T1CON.0=1            				' start timer1
   
low throttle							' to be sure ...

'******************************************************************************
main:					
    
gosub GetADC

PULSIN RADIO,1,P            
pauseUS 10

pULSOUT THROTTLE, p

' Note: Timer1 overflows every 0.050 seconds

   IF PIR1.0 THEN           			' if timer1 overflow then
      O_FLOWS = O_FLOWS + 1     		' inc oveflow counter
      
      IF O_FLOWS = 20 THEN    			' if 1 second elapsed 
	      SEC_COUNT = SEC_COUNT - 1 	' decrement seconds count
	      O_FLOWS=0             		' reset overflow counter
      ENDIF
      
      PIR1.0=0              			' clear timer1 overflow flag
      T1CON.0 = 0           			' stop timer1
      
      TMR1H = T1HI 						' Reload values 
      TMR1L = T1LO

      T1CON.0=1              			' re-start timer1
     
   ENDIF
   
if difalt>=89 and difalt<10000 then KILL_MOTOR_ALT 'Check if 100mALT
                                                   '(in adc samples) reached!!
if sec_count<=0 then KILL_MOTOR_TMR

goto main


'                                SUBROUTINES
'******************************************************************************
'******************************************************************************
GetADC:									'Read MAX187 12bit ADC value

low ADC_CS
PAUSEUS 10 
shiftin ADC_DATA,ADC_SCLK,msbpost,[advalue\12]
high ADC_CS


Average:' -=-=-=-=-=-=  Average 6 X Analog values -=-=-=-=-=-=-=-=-=-=
    IF Value = advalue Then NoChange
    
    IF ABS (Value - advalue) > FAspread OR Value < AvgCount Then FastAvg
    IF ABS (Value - advalue) < AvgCount Then RealClose
    
    advalue = advalue - (advalue/AvgCount)
    advalue = advalue + (Value/AvgCount)
    GoTo AVGok
    
FastAvg:
    advalue = Value
    GoTo AVGok
    
RealClose:'If averaging more than 8 samples,change to (AvgCount/4)

    advalue = advalue - (advalue/(AvgCount/2)) 
    advalue = advalue + (Value/(AvgCount/2))
    
AVGok:
    Value = advalue					' Put Average back into Value
    
NoChange:' -=-=-=-=-=-=  End of Averageing -=-=-=-=-=-=-=-=-=-=

difalt=value-groundalt

return
END

'******************************************************************************   
KILL_MOTOR_ALT:
low throttle
high led

LOOP1:
PULSOUT throttle, brake_pulse   ; Kill motor - adjust this value. 4Mhz ->> 10uSec
pause 18                        
goto LOOP1

END
'******************************************************************************
KILL_MOTOR_TMR:
x = 0

LOOP2:

low throttle
PULSOUT throttle, brake_pulse   ; Kill motor - adjust this value. 4Mhz ->> 10uSec

IF X = 0  THEN led = led ^ 1
x = ( x+1 ) // 18							' Counter for led Blinking @ 1.5 Hz

pause 18                      
goto LOOP2

END
'******************************************************************************
'******************************************************************************

...

But did not try it ...


have fun

Alain

[ScaleRobotics]

Hey Bitmaniac,

You could try using a 20ms interrupt, and start your pulsout on interrupt. That would make things more consistent, and probably keep your motor controller happy.