Re: Buffert trigger and Power management support for BME680 ?

Himanshu Jha
2018-08-04T14:36:20Z

On Fri, Aug 03, 2018 at 09:14:27AM -0700, David Frey wrote:
> On 7/30/2018 6:07 AM, Jonathan Cameron wrote:
> > On Mon, 30 Jul 2018 17:42:41 +0530
> > Himanshu Jha <himanshujha199640@gmail.com> wrote:
> >
> > > Hi,
> > >
> > > The initial minimal driver has been recently merged[1] and irrespective
> > > of the ending week of Google Summer of Code'18, I would like to extend this
> > > driver and work on it.
> > >
> > > I have a few queries for the community:
> > >
> > > 1. Buffered Trigger Support:
> > > ----------------------------
> > >
> > > Is it worth to add buffer support for this device ?
> > > The device has 1Hz Forced mode which we need to enable everytime
> > > we need a reading.
> > >
> > > Bosch provides[1] BSEC: Bosch Software Environmental Cluster binaries for
> > > different platforms, and the software has various modes:
> > >
> > > * Ultra-low power mode.
> > > * Ultra-low power mode.
> > > * Continuous mode.
> > >
> > > According to datasheet "1.2 Gas sensor specification"
> > >
> > > Response time Continuous mode 0.75s
> > >
> > > Now, these details are worth noting and confusing at the same to me!
> > > Since, we only have 1Hz mode, so would it be a problem when adding this
> > > interface to get continuous measurements ?
> > > What other factors/caveats should I look for before proceeding further ?
> >
> > Technically there is no problem with adding buffered mode, but there may
> > be no real usecases at these very low rates.
> >
> > The main convenience is that you could read out the various channels in one
> > pass (I think - haven't checked the datasheet).
>
> I think that buffered mode is essential because it will allow TPHG to be
> done all at once. If you measure TPHG with the current driver, you are
> doing 4 measurements. This is bad because it takes longer, uses more power,
> the values all have slightly different timestamps.

Agreed! The buffer mode can be thought of as while(1) {} of what Bosch does
in their userspace driver. But we need remember that every time we need to
set FORCED mode to trigger measurement. And due to this discussion I was
temped to download BSEC and look upon the code. Seems like we have a lot
interesting things buried there unlike datasheet.

For instance:

At example/bsec_integration.c

<snip>
/* When the measurement is completed and data is ready for reading, the sensor must be in BME680_SLEEP_MODE.
* Read operation mode to check whether measurement is completely done and wait until the sensor is no more
* in BME680_FORCED_MODE. */
while (bme680_g.power_mode == BME680_FORCED_MODE)
{
/* sleep for 5 ms */
sleep(5);
bme680_status = bme680_get_sensor_mode(&bme680_g);
}
</snip>

And many more interesting stuff...

I mean they also have:

<snip>

typedef enum {
...
/**
* @brief Sensor heat compensated humidity [%]
*
* Relative measured by BME680 which is compensated for the influence of sensor (heater) in %.
*
* It converts the ::BSEC_INPUT_HUMIDITY from temperature ::BSEC_INPUT_TEMPERATURE to temperature
* ::BSEC_OUTPUT_SENSOR_HEAT_COMPENSATED_TEMPERATURE.
*
* @note IAQ solution: If ::BSEC_INPUT_HEATSOURCE is used for device specific temperature compensation, it will be
* effective for ::BSEC_OUTPUT_SENSOR_HEAT_COMPENSATED_HUMIDITY too.
*/
BSEC_OUTPUT_SENSOR_HEAT_COMPENSATED_HUMIDITY = 15,
...
} bsec_virtual_sensor_t;

</snip>

Their design for high precision readings is just flawless!

I need some time to get ideas so as to start with buffer support.

But what kind of buffer support should be used ?
sysfs trigger using scan_elements ?

There are no interrupt supported by the sensor! And I don't know
how hrtimer would work.

Another question arises, say for eg., we use sysfs trigger and buffer
capture comes in action:

* Would I enable FORCED mode everytime in the trigger interrupt handler ?
* Also check for config setting(such as oversampling) and set
them accordingly in the interrupt handler ?

If no, then other suggestion ?
If yes, isn't that bad to do all that in "bottom half" ?

I thought of preenable, but it works only 'once' prior to enabling but
we need to set mode every time!

> > > 2. Power Management:
> > > -------------------
> > >
> > > 3.1 Sensor modes
> > >
> > > Forced mode:
> > > * Single TPHG cycle is performed
> > > * Sensor *automatically* returns to sleep mode afterwards
> > > * Gas sensor heater only operates during gas measurement
> > > Sleep mode:
> > > * No measurements are performed
> > > * Minimal power consumption
> > >
> > > So, in my opinion this device is already power managed.
> > > Comments ?
> >
> > It seems like there is little purpose in sleep mode, are there power
> > numbers to indicate what 'minimal' means here?
>
> From the datasheet on page 3:
> Current consumption
> 2.1 μA at 1 Hz humidity and temperature
> 3.1 μA at 1 Hz pressure and temperature
> 3.7 μA at 1 Hz humidity, pressure and temperature
> 0.09‒12 mA for p/h/T/gas depending on operation mode
> 0.15 μA in sleep mode
>
> My interpretation of this is that you will consume 2.1 μA on average if you
> are reading the humidity and temperature once per second. The datasheet
> goes into some more detail in Table 2 on page 8 where it describes the
> average supply current of the gas sensor as follows:
> Ultra-low power mode 0.09 ma with response time of 92 s
> Low power mode 0.9 mA with a response time of 1.4 s
> Continuous mode 12 mA with a response time of 0.75 s
>
> I don't think there are actually different modes at all. I believe what
> they call modes are just different settings corresponding to enum values in
> the BSEC library. Maybe I'm wrong though. Please correct me if you see any
> evidence of explicit mode settings. I'm also skeptical of the current
> consumption numbers. It seems weird to me that continuous mode consumes >
> 10x the current of low power mode and only produces roughly twice as many
> readings.

Yes, corrcect! Please don't get confused between only two modes of sensor
and "modes of BSEC". BSEC is different story ...

BSEC = Userspace Bosch driver API + IAQ alogithms + very smart subroutines ;)

> It's nice to see this driver progressing. Keep up the good work!

Thanks! Would need your help with its development.