[Gta04-owner] Using BMP085

Dr. H. Nikolaus Schaller hns at goldelico.com
Tue May 29 12:32:34 CEST 2012

Am 29.05.2012 um 11:55 schrieb Denis Shulyaka:

> Hi,
> 2012/5/29 Dr. H. Nikolaus Schaller <hns at goldelico.com>:
>> Hi Neil,
>> Am 29.05.2012 um 11:18 schrieb Neil Jerram:
>>> Neil Jerram <neil at ossau.homelinux.net> writes:
>>>> Hooray, it works, thank you:
>>>> gta04:~# echo bmp085 0x77 > /sys/bus/i2c/devices/i2c-2/new_device
>>>> gta04:~# find /sys -name "*pressure*"
>>>> /sys/devices/platform/omap/omap_i2c.2/i2c-2/2-0077/pressure0_input
>>>> gta04:~# cat /sys/devices/platform/omap/omap_i2c.2/i2c-2/2-0077/pressure0_input
>>>> 92192
>>> I've been looking now at temperature too
>>> (/sys/devices/platform/omap/omap_i2c.2/i2c-2/2-0077/temp0_input).
>>> I'm getting 33 degrees C [1], although the actual temperature in my
>>> house is only 16 degrees C.  Is that because the temperature reported is
>>> intentionally that *inside* the GTA04, and hence reasonably higher than
>>> ambient?  If so, is 33 degrees a reasonable temperature to be measuring
>> Yes.
>>> inside the GTA04?
>> Yes. It can go up to 45-50 degrees if the CPU is running at full power.
>> The BMP085 is mounted directly near the CPU and the PCB also
>> distributes the heat of other high-power components (WLAN).
> What about temperature sensors inside gyro and accelerometer? Are the
> chips mounted farther from the CPU?

Yes, but the PCB distributes the energy to all corners. So they show a little
lower temperature but also not any outside temp.

BTW: I think the CPU itself has another temp sensor inside. And the UMTS module.
As well as the TSC2007. So there are approx. 4-6 temp measurement positions. But
with different offsets and precision.

So, someone could try to work out a differential equation for energy diffusion through
the board and the case and use this to estimate the outside temp from the sensor
data :)

A graphical representation of the temperature profile would be similar to a
tent being raised by the temperature sources (CPU, WLAN, UMTS). The tent pegs
would be connected to the outside air temp. And the sensors are measuring the
value at different positions in between.

A simplified model could extrapolate the temp difference between two sensors
at some distance to the outside.

>> The temp measurements of the BMP085 are done for compensating the
>> pressure readings and are not intended to indicate the correct outside temp.
>> Maybe, we can try to use a thermopile sensor in some future device.
>>> [1] To be precise, I mean that ((VALUE + 5) / 10) = 33, where VALUE is
>>> `cat /sys/devices/platform/omap/omap_i2c.2/i2c-2/2-0077/temp0_input`.
>>> Regards,
>>>        Neil
>> BR,
>> Nikolaus
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