MPL3115A2 hangs intermittently

[on4aa]

I am experiencing exactly the same problem with the following short piece of code. Within a minute or two (it varies) the thing hangs without any error message.

Serial communication also breaks down and a power cycle is required to regain communication with the LoPy Pysense combo.

import pycom
import pysense
import machine
from MPL3115A2 import MPL3115A2, PRESSURE
import utime
import sys

class Device:

    def __init__(self):
        self.iteration = 0
        self.sense(self)
        self.timer = machine.Timer.Alarm(self.sense, 3, periodic=True)

    def sense(self, device):
        # barometer mode, not raw, oversampling 128, minimum time 512 ms
        self.p = MPL3115A2(board, mode=PRESSURE).pressure()    # in Pa 
        self.iteration += 1
        print('%f Pa | iteration %d' % (self.p, self.iteration))
        pycom.rgbled(0x002200)
        utime.sleep(0.1)
        pycom.rgbled(0x000000)

pycom.wifi_on_boot(False)
pycom.heartbeat(False)
board = pysense.Pysense()
device = Device()

try:
    while True:
        pass
except KeyboardInterrupt:
    device.timer.cancel()
    sys.exit()

[on4aa]

I opened a bug report here:
https://github.com/pycom/pycom-libraries/issues/46

[on4aa]

@catalin I posted example code to reproduce the MPL3115A2 hang-up within minutes.

[catalin]

Hi guys, I do reproduce the bug, I am trying to investigate.

[on4aa]

@catalin I do not know if this is related, but it might be helpful. A similar problem with an MPL3115A2 has been reported a while back on a Raspberry Pi.

i2c i2c-1: transfer setup timed out was the error given by the Raspberry Pi.

[on4aa]

@catalin Does the Pysense employ I2C for communicating with the MPL3115A2?
If so, I am pretty sure we are dealing with an I2C problem.

The following reading material hints into this direction:

• Obscure I2C problems
• SPI vs I2C

[on4aa]

@catalin Lowering the I2C clock frequency can do wonders.
A frequency as low as 50kHz would probably do fine and increase reliability.
Also, keep the trace capacitance of SCL and SDA as low as possible.

[catalin]

Hi guys,

Sorry for the delayed answer.
I was able to have the MPL3115A2 interrogation run for ~14 hours, by simply creating the object once, and periodically just calling pressure() method. Would this solve your problem?
Probably, a better solution is to have this MPL3115A2 class made static (as in Java).
With the previous version, the problem could have been the garbage collector, as for each new instance, new memory(heap) is allocated.

Regarding I2C lines, they were designed for up to 400kHz baudrate.

Code is here:

import pycom
import pysense
import machine
from MPL3115A2 import MPL3115A2, PRESSURE
import utime
import sys

class Device:

    def __init__(self):
        self.iteration = 0
        self.p = MPL3115A2(board, mode=PRESSURE)    # create object just once
        self.sense(self)
        self.timer = machine.Timer.Alarm(self.sense, 3, periodic=True)

    def sense(self, device):
        # barometer mode, not raw, oversampling 128, minimum time 512 ms
        self.iteration += 1
        print('%f Pa | iteration %d' % (self.p.pressure(), self.iteration))
        pycom.rgbled(0x002200)
        utime.sleep(0.1)
        pycom.rgbled(0x000000)

pycom.wifi_on_boot(False)
pycom.heartbeat(False)
board = pysense.Pysense()
device = Device()

try:
    while True:
        pass
except KeyboardInterrupt:
    device.timer.cancel()
    sys.exit()

[catalin]

I've verified the assumption that garbage collector is the problem, by calling it every loop, gc.collect(). In about 3 minutes, it's still blocked.
The problem seems to be the I2C module.
Meanwhile, I guess, you can use the workaround.

[on4aa]

@catalin Dear Catalin, the workaround indeed works fine. You should add it to the Pysense documentation to prevent other people falling in the same trap.

However, I am still left with one concern. Could it be that the main while True: pass loop is consuming too much processing power? I have seen this on Linux systems using htop and it was also reported on StackExchange.

Hence, my remaining questions are:

• How can I check upon MCU usage as htop is not available?
• Should I add a utime.sleep(0.2) to the main while True: loop?
• When are we going to see uasyncio being support by Pycom? Under my incentive, a couple of good souls removed the last obstacles for uasyncio adoption.

[smbunn]

Has this problem been fixed? MY code wants to call the MPL3115A2 twice, once in pressure mode and once in atitude mode so needs to be called repeatedly.

for count in range (2000):
    print("Count=", count)
    vt = py.read_battery_voltage()
    print("Battery voltage: " +str(vt))
    dew = si.dew_point()
    print("Dew point: "+ str(dew) + " deg C")
    mp = MPL3115A2(py,mode=ALTITUDE) # Returns height in meters. Mode may also be set to PRESSURE, returning a value in Pascals
    print("MPL3115A2 temperature: " + str(mp.temperature()))
    temp1 = mp.temperature()
    alt1 = mp.altitude()+100.0
    print("Altitude: " + str(alt1))
    mpp = MPL3115A2(py,mode=PRESSURE) # Returns pressure in Pa. Mode may also be set to ALTITUDE, returning a value in meters
    press1 = mpp.pressure()
    print("Pressure: " + str(press1))

so declaring it only once probably will not work?

[catalin]

hi @smbunn,
The code was not modified.
On a second thought, the altitude is just a simple function of the pressure, considering the same sea-level pressure. This is not ok, as the sea-level pressure varies in time, and from place to place.
So, I would recommend, to use just the pressure, and if altitude is required, some arithmetics code has to be added, to get the right altitude. You could use some weather forecast service to obtain the accurate and real-time sea-level pressure.