Question about MAP shown in the runtime window of the Tuning

[jeffery_1965]

Hello,

I just finished assembly of my MJLJ, and was looking at the tuning software when I noticed that the real time display of the MAP seemed to be wrong. The sensor should output something close to 100KPa, depending on the current barrometric pressure, while it is sitting on the bench. I looked through the MAP sensor application guide and it says that the sensor is good to ~ +/- 4KPa depending on temperature and other items. That made me wonder even more at the real time reading of 92 that was showing.

Well after looking at the java and the asm source code I believe that the number shown on the real time display is actually the raw A/D output, and not the actual pressure. Is this correct Brent, or am I experiencing something strange here? I created a set of graphs which seems to prove my undersatanding. Here are the numbers where Vout is the output of the MAP, Kpa is the pressure based on the conversion formula of Vout = Vss(Pressure x 0.004 - .04):

Vout _____ Kpa _____ A/D
0.0 _____ 10.00 _____ 0.00
0.1 _____ 15.02 _____ 5.14
0.2 _____ 20.04 _____ 10.28
0.3 _____ 25.06 _____ 15.42
0.4 _____ 30.08 _____ 20.56
0.5 _____ 35.10 _____ 25.70
0.6 _____ 40.12 _____ 30.84
0.7 _____ 45.14 _____ 35.98
0.8 _____ 50.16 _____ 41.12
0.9 _____ 55.18 _____ 46.27
1.0 _____ 60.20 _____ 51.41
1.1 _____ 65.22 _____ 56.55
1.2 _____ 70.24 _____ 61.69
1.3 _____ 75.26 _____ 66.83
1.4 _____ 80.28 _____ 71.97
1.5 _____ 85.30 _____ 77.11
1.6 _____ 90.32 _____ 82.25
1.7 _____ 95.34 _____ 87.39
1.8 _____ 100.36 _____ 92.53
1.9 _____ 105.38 _____ 97.67
2.0 _____ 110.40 _____ 102.81
2.1 _____ 115.42 _____ 107.95
2.2 _____ 120.44 _____ 113.09
2.3 _____ 125.46 _____ 118.23
2.4 _____ 130.48 _____ 123.37
2.5 _____ 135.50 _____ 128.51
2.6 _____ 140.52 _____ 133.65
2.7 _____ 145.54 _____ 138.80
2.8 _____ 150.56 _____ 143.94
2.9 _____ 155.58 _____ 149.08
3.0 _____ 160.60 _____ 154.22
3.1 _____ 165.62 _____ 159.36
3.2 _____ 170.64 _____ 164.50
3.3 _____ 175.66 _____ 169.64
3.4 _____ 180.68 _____ 174.78
3.5 _____ 185.70 _____ 179.92
3.6 _____ 190.72 _____ 185.06
3.7 _____ 195.74 _____ 190.20
3.8 _____ 200.76 _____ 195.34
3.9 _____ 205.78 _____ 200.48
4.0 _____ 210.80 _____ 205.62
4.1 _____ 215.82 _____ 210.76
4.2 _____ 220.84 _____ 215.90
4.3 _____ 225.86 _____ 221.04
4.4 _____ 230.88 _____ 226.18
4.5 _____ 235.90 _____ 231.33
4.6 _____ 240.92 _____ 236.47
4.7 _____ 245.94 _____ 241.61
4.8 _____ 250.96 _____ 246.75
4.9 _____ 255.98 _____ 251.89
5.0 _____ 261.00 _____ 257.03


Admittedly there is a maximum difference of 10KPa (~ 3inHg) across the measurement range. Maybe this is done to reduce the overhead on the microprocessor, ie no math functions.

So am I goofy here Brent, or is this the way that the software functions?

Thanks,

Jeff

PS The board went together without a hitch, and looks awesome in the case. Only 4hrs start to finish, including cutting the case out. Can't wait to hook it up!

[Josh_b]

mine does the same, i was wondering if we could have some software calibration available?

[brentp]

Yep, you're right, I missed that in the MPX4250 application notes. I looked over the megasquirt code and I need to incorporate a table look up to correct the KPA value.

Look for an updated very soon that will fix this.

Thanks for pointing this out!
Brent