MPXA4115A picbasic code

[ScaleRobotics]

I need to work at this a little to explain it well. But here is a shot at explaining it generally. I am still looking for the excel spreadsheet I did to make my lookup table ... but in the mean time...

Using an excel spread sheet, put a formula for calculating altitude from pressure. Add the formula for your pressure sensor to convert its voltage to pressure. Add in the part that converts your A/D reading to a voltage. Once you get to a point where you can say a reading of xxx of your a/d conversion gives you x feet or meters, you can find some evenly spaced a/d readings, and input the corresponding feet or meters into a lookup table. I know this explanation kind of stinks, because you need some examples here. I will try to elaborate on this later.

Anyway, once you have your lookup table, you need to divide your a/d reading by a number. If you look at my lookup table, you see that I am dividing by 1000 and subtracting 15. This is for a 16 bit A/D result. (In my code, even if you decide to use 12 bit resolution, it is shifted up to 16 bits so it can use the same lookup table). The subtract 15 lets you lower the start of your max altitude. Normal folks might just want to go up to 12,000 feet, or maybe even lower, so they would have a bigger subtraction number, and a different lookup table, that started at a lower altitude.

You may notice that I am looking up two values. This is because the value from the adc will be between two of the lookup table values. The lookup values in my table are about 400 feet or so apart. To make this somewhat accurate, we need to lookup both readings in the lookup table, find the difference in feet, then take our remainder from divide by 1000 and multiply the fraction to the difference in feet. Then I subtract this from my TempAlt to get a final altitude. For this to work, my lookup table had to be made at Analog to digital steps of 1000. That makes the math a little easier.

I am pretty sure I used Alain's spreadsheet as a base to calculate my lookup table. You can find his spreadsheet in Post 33 and another one of his previous. Also BitManiac has a spreadsheet in there as well. Here are my values, calculating for even 1000 steps for the A/D value. Table step number is shown, as is feet of altitude, and pressure in Pa.

<table frame="VOID" rules="NONE" border="0" cellspacing="0" cols="4"> <colgroup><col width="83"><col width="93"><col width="107"><col width="107"></colgroup> <tbody> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20" width="83">A/D Value</td> <td align="CENTER" bgcolor="#E6E6E6" width="93">Table step </td> <td align="CENTER" bgcolor="#E6E6E6" width="107">Feet</td> <td align="CENTER" bgcolor="#E6E6E6" width="107">Pa</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">54000</td> <td align="CENTER" bgcolor="#E6E6E6">39</td> <td align="CENTER" bgcolor="#E6E6E6">-237</td> <td align="CENTER" bgcolor="#E6E6E6">102.2</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">53000</td> <td align="CENTER" bgcolor="#E6E6E6">38</td> <td align="CENTER" bgcolor="#E6E6E6">226</td> <td align="CENTER" bgcolor="#E6E6E6">100.5</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">52000</td> <td align="CENTER" bgcolor="#E6E6E6">37</td> <td align="CENTER" bgcolor="#E6E6E6">696</td> <td align="CENTER" bgcolor="#E6E6E6">98.8</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">51000</td> <td align="CENTER" bgcolor="#E6E6E6">36</td> <td align="CENTER" bgcolor="#E6E6E6">1172</td> <td align="CENTER" bgcolor="#E6E6E6">97.11</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">50000</td> <td align="CENTER" bgcolor="#E6E6E6">35</td> <td align="CENTER" bgcolor="#E6E6E6">1655</td> <td align="CENTER" bgcolor="#E6E6E6">95.41</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">49000</td> <td align="CENTER" bgcolor="#E6E6E6">34</td> <td align="CENTER" bgcolor="#E6E6E6">2145</td> <td align="CENTER" bgcolor="#E6E6E6">93.71</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">48000</td> <td align="CENTER" bgcolor="#E6E6E6">33</td> <td align="CENTER" bgcolor="#E6E6E6">2643</td> <td align="CENTER" bgcolor="#E6E6E6">92.02</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">47000</td> <td align="CENTER" bgcolor="#E6E6E6">32</td> <td align="CENTER" bgcolor="#E6E6E6">3148</td> <td align="CENTER" bgcolor="#E6E6E6">90.32</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">46000</td> <td align="CENTER" bgcolor="#E6E6E6">31</td> <td align="CENTER" bgcolor="#E6E6E6">3660</td> <td align="CENTER" bgcolor="#E6E6E6">88.62</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">45000</td> <td align="CENTER" bgcolor="#E6E6E6">30</td> <td align="CENTER" bgcolor="#E6E6E6">4181</td> <td align="CENTER" bgcolor="#E6E6E6">86.92</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">44000</td> <td align="CENTER" bgcolor="#E6E6E6">29</td> <td align="CENTER" bgcolor="#E6E6E6">4710</td> <td align="CENTER" bgcolor="#E6E6E6">85.23</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">43000</td> <td align="CENTER" bgcolor="#E6E6E6">28</td> <td align="CENTER" bgcolor="#E6E6E6">5247</td> <td align="CENTER" bgcolor="#E6E6E6">83.53</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">42000</td> <td align="CENTER" bgcolor="#E6E6E6">27</td> <td align="CENTER" bgcolor="#E6E6E6">5794</td> <td align="CENTER" bgcolor="#E6E6E6">81.83</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">41000</td> <td align="CENTER" bgcolor="#E6E6E6">26</td> <td align="CENTER" bgcolor="#E6E6E6">6349</td> <td align="CENTER" bgcolor="#E6E6E6">80.14</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">40000</td> <td align="CENTER" bgcolor="#E6E6E6">25</td> <td align="CENTER" bgcolor="#E6E6E6">6915</td> <td align="CENTER" bgcolor="#E6E6E6">78.44</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">39000</td> <td align="CENTER" bgcolor="#E6E6E6">24</td> <td align="CENTER" bgcolor="#E6E6E6">7490</td> <td align="CENTER" bgcolor="#E6E6E6">76.74</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">38000</td> <td align="CENTER" bgcolor="#E6E6E6">23</td> <td align="CENTER" bgcolor="#E6E6E6">8076</td> <td align="CENTER" bgcolor="#E6E6E6">75.04</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">37000</td> <td align="CENTER" bgcolor="#E6E6E6">22</td> <td align="CENTER" bgcolor="#E6E6E6">8672</td> <td align="CENTER" bgcolor="#E6E6E6">73.35</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">36000</td> <td align="CENTER" bgcolor="#E6E6E6">21</td> <td align="CENTER" bgcolor="#E6E6E6">9280</td> <td align="CENTER" bgcolor="#E6E6E6">71.65</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">35000</td> <td align="CENTER" bgcolor="#E6E6E6">20</td> <td align="CENTER" bgcolor="#E6E6E6">9900</td> <td align="CENTER" bgcolor="#E6E6E6">69.95</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">34000</td> <td align="CENTER" bgcolor="#E6E6E6">19</td> <td align="CENTER" bgcolor="#E6E6E6">10532</td> <td align="CENTER" bgcolor="#E6E6E6">68.26</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">33000</td> <td align="CENTER" bgcolor="#E6E6E6">18</td> <td align="CENTER" bgcolor="#E6E6E6">11176</td> <td align="CENTER" bgcolor="#E6E6E6">66.56</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">32000</td> <td align="CENTER" bgcolor="#E6E6E6">17</td> <td align="CENTER" bgcolor="#E6E6E6">11834</td> <td align="CENTER" bgcolor="#E6E6E6">64.86</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">31000</td> <td align="CENTER" bgcolor="#E6E6E6">16</td> <td align="CENTER" bgcolor="#E6E6E6">12507</td> <td align="CENTER" bgcolor="#E6E6E6">63.16</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">30000</td> <td align="CENTER" bgcolor="#E6E6E6">15</td> <td align="CENTER" bgcolor="#E6E6E6">13194</td> <td align="CENTER" bgcolor="#E6E6E6">61.47</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">29000</td> <td align="CENTER" bgcolor="#E6E6E6">14</td> <td align="CENTER" bgcolor="#E6E6E6">13896</td> <td align="CENTER" bgcolor="#E6E6E6">59.77</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">28000</td> <td align="CENTER" bgcolor="#E6E6E6">13</td> <td align="CENTER" bgcolor="#E6E6E6">14615</td> <td align="CENTER" bgcolor="#E6E6E6">58.07</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">27000</td> <td align="CENTER" bgcolor="#E6E6E6">12</td> <td align="CENTER" bgcolor="#E6E6E6">15351</td> <td align="CENTER" bgcolor="#E6E6E6">56.38</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">26000</td> <td align="CENTER" bgcolor="#E6E6E6">11</td> <td align="CENTER" bgcolor="#E6E6E6">16106</td> <td align="CENTER" bgcolor="#E6E6E6">54.68</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">25000</td> <td align="CENTER" bgcolor="#E6E6E6">10</td> <td align="CENTER" bgcolor="#E6E6E6">16879</td> <td align="CENTER" bgcolor="#E6E6E6">52.98</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">24000</td> <td align="CENTER" bgcolor="#E6E6E6">9</td> <td align="CENTER" bgcolor="#E6E6E6">17673</td> <td align="CENTER" bgcolor="#E6E6E6">51.29</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">23000</td> <td align="CENTER" bgcolor="#E6E6E6">8</td> <td align="CENTER" bgcolor="#E6E6E6">18489</td> <td align="CENTER" bgcolor="#E6E6E6">49.59</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">22000</td> <td align="CENTER" bgcolor="#E6E6E6">7</td> <td align="CENTER" bgcolor="#E6E6E6">19327</td> <td align="CENTER" bgcolor="#E6E6E6">47.89</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">21000</td> <td align="CENTER" bgcolor="#E6E6E6">6</td> <td align="CENTER" bgcolor="#E6E6E6">20190</td> <td align="CENTER" bgcolor="#E6E6E6">46.19</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">20000</td> <td align="CENTER" bgcolor="#E6E6E6">5</td> <td align="CENTER" bgcolor="#E6E6E6">21078</td> <td align="CENTER" bgcolor="#E6E6E6">44.5</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">19000</td> <td align="CENTER" bgcolor="#E6E6E6">4</td> <td align="CENTER" bgcolor="#E6E6E6">21995</td> <td align="CENTER" bgcolor="#E6E6E6">42.8</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">18000</td> <td align="CENTER" bgcolor="#E6E6E6">3</td> <td align="CENTER" bgcolor="#E6E6E6">22942</td> <td align="CENTER" bgcolor="#E6E6E6">41.1</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">17000</td> <td align="CENTER" bgcolor="#E6E6E6">2</td> <td align="CENTER" bgcolor="#E6E6E6">23921</td> <td align="CENTER" bgcolor="#E6E6E6">39.41</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">16000</td> <td align="CENTER" bgcolor="#E6E6E6">1</td> <td align="CENTER" bgcolor="#E6E6E6">24934</td> <td align="CENTER" bgcolor="#E6E6E6">37.71</td> </tr> <tr> <td align="CENTER" bgcolor="#E6E6E6" height="20">15000</td> <td align="CENTER" bgcolor="#E6E6E6">0</td> <td align="CENTER" bgcolor="#E6E6E6">25985</td> <td align="CENTER" bgcolor="#E6E6E6">36.01</td> </tr> </tbody> </table>

I have attached one interesting pdf. It is not the formula I used for my lookup table, but it is interesting.

Walter