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https://github.com/rolandvs/micropython-fram

Pyboard driver for Ferroelectric RAM module
https://github.com/rolandvs/micropython-fram

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Pyboard driver for Ferroelectric RAM module

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# micropython-fram

A driver to enable the Pyboard to access the Ferroelectric RAM (FRAM) board from

[Adafruit](http://www.adafruit.com/product/1895). FRAM is a technology offering

nonvolatile memory with extremely long endurance and fast access, avoiding the

limitations of Flash memory. Its endurance is specified as 10^13 writes,

contrasted with 10,000 which is the quoted endurance of the Pyboard's onboard

Flash memory. In data logging applications the latter can be exceeded relatively

rapidly. Flash writes can be slow because of the need for a sector erase: this

is not a fast process. FRAM is byte addressable and is not subject to this

limitation. The downside is limited capacity. Compared to a Micro SD card fitted

to the Pyboard it offers lower power consumption and longer endurance.



From one to eight boards may be used to construct a nonvolatile memory module

with size ranging from 32KB to 256KB. The driver allows the memory either to be

mounted in the Pyboard filesystem as a disk device or to be addressed as an

array of bytes.



Now accepts an I2C bus as argument to facilitate the use of multiple devices on

the bus. Also adapted to work on the ESP8266.



# Connections



To wire up a single FRAM module, connect to the Pyboard as below (nc indicates

no connection).



| FRAM    |  L  |  R  |

|:-------:|:---:|:---:|

| Vcc     | 3V3 | 3V3 |

| Gnd     | GND | GND |

| WP      | nc  | nc  |

| SCL     | X9  | Y9  |

| SDA     | X10 | Y10 |

| A2      | nc  | nc  |

| A1      | nc  | nc  |

| A0      | nc  | nc  |



For multiple modules the address lines A0, A1 and A2 of each module need to be

wired to 3V3 in such a way as to give each device a unique address. These must

start at zero and be contiguous. Thus with three modules, the first would have

address lines unconnected (address 0), the second would have A0 connected to 3V3

(address 1) and the third would have A1 connected to 3V3 (address 2).



Multiple modules should have 3V3, Gnd, SCL and SDA lines wired in parallel.



# Driver



The driver supports mounting the FRAM modules as a filesystem. Initially the

device will be unformatted so it is necessary to issue code along these lines to

format the device.



```python

import pyb

from fram import FRAM

i2c = pyb.I2C(2, pyb.I2C.MASTER)

f = FRAM(i2c)

pyb.mount(f, '/fram', mkfs = True)

pyb.mount(None, '/fram')

```



At the time of writing the ``pyb`` module provides no way to reformat a drive

which already has a filesystem. As a workround the following will perform this:



```python

import pyb 

from fram import FRAM

i2c = pyb.I2C(2, pyb.I2C.MASTER)

f = FRAM(i2c)

f.low_level_format()

pyb.mount(f, '/fram', mkfs = True)

pyb.mount(None, '/fram')

```



Note that, at the outset, you need to decide whether to use the array as a

mounted filesystem or as a byte array. As a filesystem the limited size is an

issue, but a potential use case is for pickling Python objects for example to

achieve persistence when issuing ``pyb.standby()``. Also for holding a small

frequently updated persistent btree database.



### Constructor



``FRAM()`` Takes two arguments:  

 1. ``i2c`` Mandatory. An initialised master mode I2C bus.  

 2. ``verbose`` (default False). If True, the constructor issues information on

 the FRAM devices it has detected.



A ``FRAMException`` will be raised if a device is not detected or if device

address lines are not wired as  described in Connections above.



### Methods providing the block protocol



For the protocol definition see

[the pyb documentation](http://docs.micropython.org/en/latest/library/pyb.html)



``readblocks()``  

``writeblocks()``  

``count()``  

``ioctl()``



### Methods providing byte level access



The following methods are available for general use.



``readwrite()`` Provides byte level access to the memory array. Arguments:  

 1. ``addr`` Starting byte address  

 2. ``buf`` A buffer containing the data to write or to hold read data  

 3. ``read`` If True, perform a read otherwise write. The size of the buffer

 determines the quantity of data read or written. A ``FRAMException`` will be

 thrown if the read or write extends beyond the end of the array.



``low_level_format()`` Erases the filesystem! Currently (this may change) the

pyb module doesn't provide a means of forcing a drive format. Issuing a

``low_level_format()`` followed by ``pyb.mount()`` with ``mkfs-True`` will

format the drive deleting all files.



``available()`` Returns True if the device is detected and is supported.



Other than for debugging there is no need to call ``available()``: the

constructor will throw a ``FRAMException`` if it fails to communicate with and

correctly identify the chip.



## File copy



This is really a piece of history now superseded. upysh in

[micropython-lib](https://github.com/micropython/micropython-lib.git) is a much

more capable solution.



A rudimentary ``cp(source, dest)`` function is provided as a generic file copy

routine for setup and debugging purposes at the REPL. The first argument is the

full pathname to the source file. The second may be a full path to the

destination file or a directory specifier which must have a trailing '/'. If an

OSError is thrown (e.g. by the source file not existing or the FRAM becoming

full) it is up to the caller to handle it. For example (assuming the FRAM is

mounted on /fram):



```python

cp('/flash/main.py','/fram/')

```



# ESP8266



The driver now supports this module. The file ``framtest_esp8266.py`` demonstrates

its use. Note that currently the ESP8266 does not support concurrent mounting

of multiple filesystems. Consequently the onboard flash must be unmounted (with

``uos.umount()``) before the FRAM can be mounted.