Re: [Paparazzi-devel] BMP085 sensor temperature wiht large error

[Helge Walle]

Hi Martin,

Thank you very much, that really made a difference! I am not sure if I would have found that one.

While trying to analyze the code I noticed something in:

void baro_bmp_event( void ) {
.....
/* compensate pressure */
.....
if (bmp_b7 < 0x80000000)
   bmp_p = (bmp_b7 * 2) / bmp_b4;
else
   bmp_p = (bmp_b7 * bmp_b4) * 2;
.....

According to the BMP085 datasheet, should the last statement rather be:
bmp_p = (bmp_b7 / bmp_b4) * 2;     ?

Thanks again,
Helge.

2011/8/5 Martin Mueller <address@hidden>

Hi Helge,

the extra noise is from reading the lower three bits of the pressure result wrongly. They are in the higher bits of the XLSB, not the lower, fixed in master:

@@ -130,7 +130,7 @@ void baro_bmp_event( void ) {
      /* get uncompensated pressure, oss=3 */
      bmp_up = (bmp_trans.buf[0] << 11) |
               (bmp_trans.buf[1] << 3)  |
-                bmp_trans.buf[2];
+               (bmp_trans.buf[2] >> 5);
      /* start temp measurement */
      bmp_trans.buf[0] = BMP085_CTRL_REG;
      bmp_trans.buf[1] = BMP085_START_TEMP;

Thanks for looking into that.

As the max conversion time is 25.5ms it could be run at 30Hz without changes, maybe having the temp reading only once per second as suggested in the spec. Some averaging could be done depending on the frequency you need.

Martin

http://paparazzi.enac.fr/wiki_images/Bmp085_scp1000.png
http://paparazzi.enac.fr/wiki_images/Bmp085_scp1000_fixed.png


On 04.08.2011 11:36, Helge Walle wrote:

Hi,
To make sure that the sample rate is not set so high that it interferes
with the conversion time of the BMP sensor, I would need to check the
data flow with my logic analyzer.
I did not do that yesterday evening. However, Increasing freq from "8"
to "20" in .../paparazzi/conf/modules/baro_bmp.xml increased the data
flow and did not create any problems.
The code I used with my Arduino board that made better results was from
Sparkfun. I am quite determined to work through the code since I feel
sure that this is the right way to go.

Regards,
Helge.

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