LoPy4 - I2C - FRAM Memory - MicroPython Library

[Eric Tremblay]

Hi!,
I am working on a project where I am building an industrial lubrication system controller, my application requires me to have a way to save user-selected parameters for the controller to access and config on boot. I got a custom-made shield for my project that takes a LoPy4 as a controller.
my shield is equipped with a FRAM - MB85RC256VPF-G-JNERE2 chip SDA->P9, SCL->P10, and Write Protect on P2.
It's my first project with MicroPython and Pycom. I managed to have all my other functionalities figured out from the documentation except my FRAM.
I am having problems finding a library or information to help me achieve my results. at this point, I think I may need to write my own firmware which is way above my abilities for now.

So far in my research, I found a discussion and a link to PeterHinch's Github repo that brings me to a dead end. I was looking at is GitHub projects and i did not see anything referring to FRAM I2C but he has is lots of other amazing work I am glad I found. I think it may have been deleted overtime.
I also found a Repo from Adafruit but it is for circuit python. https://github.com/adafruit/Adafruit_CircuitPython_FRAM.

I am wondering if anyone knows about or has seen something that could help me out.

Thanks
Eric

[Elizar]

There is no need for a "firmware" when it comes to using an FRAM. You just need to understand how an I²C bus operates.

Reading or writing is a straight forward process:

1. Create and i2c-Object (see example in the pycom docs)
2. Write data by

i2c.writeto_mem(ADDR_FRAM, Offset, bytearray(Data), addrsize=16)
DataByteArray = i2c.readfrom_mem(ADDR_FRAM, Offset, Size, addrsize=16)

Page 8 of the datasheet explains which bytes sequence has to be sent: two bytes for the target address, then the data byte(s).

Then write yourself two functions WriteFRAM(Offset, Byte) and ReadFRAM(Offset) and you are done. The whole thing may finally have just about 20 lines of code.

Don't spend too much time on searching for an already working program. Writing your own FRAM code is a very instructive task and will also lead to a better understanding of the possible pitfalls. Just as an example: how do you plan to react when a bus transmission fails due to a temporary electrical interference?

[Eric Tremblay]

@Elizar Thank you for the pointer I was definitely overthinking this. Altho I am still in a bind
when I call a writeto_mem() method It returns a number of bytes written like intended and then when call readfrom_mem() it returns me a byte array representing all 0, I played with baud rates and also with toggle my Write Protect Pin H to L with no success.
I'm a bit puzzled but I am learning a lot about i2c so it's a good thing :)

>>> i2c.init(I2C.MASTER, baudrate=20000)
>>> i2c.scan()
[81]
>>> i2c.writeto_mem(81,3,4,addrsize=8)
1
>>> i2c.readfrom_mem(81,2,6,addrsize=8)
b'\x00\x00\x00\x00\x00\x00'
>>> i2c.writeto_mem(0x51,0x2,bytearray(1),addrsize=8)
1
>>> i2c.readfrom_mem(81,2,6,addrsize=8)
b'\x00\x00\x00\x00\x00\x00'
>>> 

[Eric Tremblay]

I was reading the docs for the FRAM module and saw that the first byte that needs to be sent after the start condition is met is 4 bits for the device address which are always 0101 then 3 bits to address multiple FRAM which mine are not connected. so they are 000. the last bit is 0 for a write operation and 1 for a read operation. when I scan the 12C bus I get back. int81 so 01010001. That's the address that I pass in my writeto_mem. I tried passing a 01010000 thinking maybe it would require a 0 to allow a write operation and it returns a 12C bus error.

Eric

[jcaron]

@Eric-Tremblay the address pins on your FRAM module should not be left unconnected. That would result in unpredictable values.

As specified in the datasheet they should be connected to either VSS (GND) or VDD (power supply) to achieve the right address. In your case they should probably all be connected to GND to achieve 000.

[Eric Tremblay]

@jcaron thanks for the pointer.
I went back to the person who designed my board and I had an old unfinished copy of the wiring schematics, the pins are actually A0 to VDD, A1, and A2 to VSS. If I read the docs correctly should give me an address of 0101001(0or1). I took a multimeter to confirm everything was effectively like on the schematics and that all pins are ok I also check to make sure my write protect was in fact going from L to H and it seems ok. I'm pretty confident the hardware side is ok.
when I scan the bus I still get an Int 81 back as an address (01010001). I try to pass in 01010011 but I get an I2C bus exception.

i Tried different Baudrate. i wonder what i should be running.

[jcaron]

@Eric-Tremblay An I2C address is 7 bits, in this case it is indeed 81 (0x51, 101 0001): 1010 fixed bits, 001 selected address. This is the address you should provide to the I2C functions, they will add the 0 or 1 to performs reads or writes.

The memory address is 16 bits (2 x 8 bits), so you should use addrsize=16..

This should write:

i2c.writeto_mem(0x51,0x2,bytearray(0x42),addrsize=16)

This should read back the data:

i2c.readfrom_mem(0x51,0x2,1,addrsize=16)

[Eric Tremblay]

@jcaron said in LoPy4 - I2C - FRAM Memory - MicroPython Library:

i2c.readfrom_mem(0x51,0x2,1,addrsize=16)

Does
this make any sense? What should I be using for baudrate?

>>> from machine import I2C
>>> i2c = I2C(0,I2C.MASTER, baudrate=100000)
>>> 
>>> i2c.scan()
[81]
>>> i2c.writeto_mem(0x51,0x2,bytearray(0x42),addrsize=16)
66
>>> i2c.readfrom_mem(0x51,0x2,1,addrsize=16)
b'\x00'
>>> 

[Eric Tremblay]

Just check my WP pin status to make sure it was "L"

>>> wp = Pin("P2", mode = Pin.OUT)
>>> 
>>> wp()
0
>>> i2c.writeto_mem(0x51,0x2,bytearray(0x42),addrsize=16)
66
>>> i2c.readfrom_mem(0x51,0x2,1,addrsize=16)
b'\x00'
>>> 

[jcaron]

@Eric-Tremblay do you have pull ups on SCL and SDA? Is the chip powered at 3.3V (rather than 5V)?

Do you have a scope you could use to capture SCL and SDA? Have you checked actual voltage on WP?

[Eric Tremblay]

@jcaron

There is 100k pull up on both SDA and SCL I checked voltage on wp and i get 3.3vdc when H and 0 when L

[jcaron]

@Eric-Tremblay can you check if VDD is 3.3V as well? Can you capture an exchange with a scope or other tool?

Just for reference, can you run:

i2c.readfrom_mem(0x51,0x0,32768,addrsize=16)

That should read the whole memory. It would be interesting to see if there’s something other than zeroes in the output.

[Eric Tremblay]

@jcaron i dont have a scope but i think i may need one to help me troubleshoot this out. Will see if i can buy one. There is 13.3 on the VDD

[Eric Tremblay]

Sorry typo i meant 3.3

[jcaron]

@Eric-Tremblay you may want to try the “base” I2C commands instead of the memory commands, there may be something slightly different in the implementation which isn’t immediately apparent and wouldn’t be compatible with the *mem functions.

Use:

i2c.writeto(0x51, bytes(0,0,1,2,3,4))

This should write bytes 1,2,3,4 at addresses 0,1,2,3.

Then:

i2c.writeto(0x51, bytes(0,0))

This should reset the pointer to address 0

Then:

i2c.readfrom(0x51, 4)

To read back the data.

The result of the full read would be interesting as well.

[Elizar]

@Eric-Tremblay Since the I²C bus can be clocked very slow, say 1 Hz (1 clk pulse per second), you may not need a scope. You can use two simple LEDs (each in series with a resistor of about 5 kohm. The LEDs will be a bit dark but not load the bus too much). Connect the LEDs from SDA via resistor to GND and do the same for SCL.

Then you can follow the signals on the I²C bus and see what happens. Pay special attention to the final bit of each transmitted byte (the acknowledge bit). Then compare the results with the waveforms from the datasheet.

Nevertheless, a scope with at least two channels would make the observation much, much simpler. If you can afford it, buy it. It will save you many painful hours of guessing.

[jcaron]

@Elizar interesting idea! You probably want to use a camera to record that, though, it’ll make transcription easier.

[robert-hh]

@Elizar There are low cost logic analyzers available for about 10€, with 8 channels at 24MHz max sampling rate. Software is available as supporting the most common serial protocols. For I2C, UART, SPI that is extremely valuable. It does not help you though when there are analogue problems like with voltage levels.

[Eric Tremblay]

@jcaron

Base i2c gives me and TypeError when I pass in a byte array

>>> i2c.writeto(0x51, 2,)
1
>>> i2c.readfrom(0x51, 5)
b'\x00\x00\x00\x00\x00'
>>> i2c.writeto(0x51, bytes(0,0,1,2,3,4))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: wrong number of arguments
>>> 

This returns the entire memory all zero

>>> i2c.readfrom_mem(0x51,0x0,32768,addrsize=16)
b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\........   Returns all memory 

[jcaron]

@Eric-Tremblay 100K may be too weak (i.e. the numerical value is too high) for the pull-up. Can you easily change those to a different value? Usual recommandations are 4.7K or 10K, though some use much lower values.

It seems weird that the scan still works if that is indeed the problem, but it might be an edge case due to rise/fall times.

You may also want to try different speeds to see if that changes anything. Lower speeds are less sensitive to pull-up issues.