LoPy and custom PCB with L76-L GPS module

[seb]

@bobie

Is the device a 5v or 3.3v device? If it is 5v you will need to use a level shifter. Do you have pull up resistors for the SDA and SCL lines?

[bobie]

This post is deleted!

[bobie]

@seb We use LoPy4 so "device" is 3.3V I believe. Quectel says:
...

• I2C_SDA/I2C_SCL is pulled up to 2.8V inside L76-L module. There is no need to add a pull-up resistor externally.
• The voltage threshold of I2C is 2.8V. If the system voltage is not consistent with it, a level shifter circuit must be used.

Quectel I2C voltage characteristics:
VILmin=-0.3V
VILmax=0.7V
VIHmin=2.1V
VIHmax= 3.1V
VOLmax=0.42V
VOHmin=2.4V
VOHnom=2.8V

[seb]

@bobie

Out modules output 3.3v which is above the maximum 3.1V specified there, to be safe I would include level shifters. Simple bi-directional level shifters can be made like so:

http://husstechlabs.com/wp-content/uploads/2010/09/Level-shifter.jpg

[bobie]

@seb OK, will try to use level shifter. Do Pycom use level shifter on its PyTrack module ?

[seb]

I just checked the Pytrack schematic and we use the i2c interface to the module and actually don't have level shifters, that must mean it works ok.

[bobie]

@seb We connected our Quectel L76-L pins:

L76-L .................. LoPy
GND------------------------ GND
VccInput ----------------- 3.3V VccOut
SDA ------------------------ P9
SCL ------------------------ P10

Then we initiated:
i2c = I2C(0, I2C.MASTER)
i2c.init(I2C.MASTER)
i2c.scan()

On our L76-L i2c.scan() produces empty array and multiple execution produces sometimes random adresses.

When we connected PyTrack PCB using only those 4 pins the L76-L from PyTrack gives proper response to i2c.scan()

What I did wrong ;-( ?

[robert-hh]

@bobie It still looks like wrong signals. If you can do so, hook up an oscilloscope and check the signal quality.

[seb]

@bobie

Do you have pullup resistors on the SDA and SCL lines? You will need to connect a 10k resistor between those lines and 3.3V. It's also worth noting that on the Pytrack the I2C is sda='P22', scl='P21'.

[bobie]

@seb No luck with pull up resistors. There is also garbage when connected on P22/P21.
PyTrack connected to the same pins with the same settings goes smooth.
While connecting our L76-L there is no response on I2C ;-( only garbage response for i2c.scan().

Maybe you reconfigure L76-L chip through UART after mounting ?

Will try to connect oscilloscope and/or logic analyzer