micropython_eeprom/fram/FRAM_SPI.md

1. A MicroPython SPI FRAM driver

A driver to enable MicroPython hosts to access Ferroelectric RAM (FRAM) boards from Adafruit, namely the 256KiB board and the 512KiB board. 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. Compared to a Micro SD card fitted to the Pyboard it offers lower power consumption and longer endurance, albeit at a smaller capacity.

An arbitrary number of boards may be used to construct a nonvolatile memory array with size from 256KiB upwards. The driver allows the memory either to be mounted in the host filesystem as a disk device or to be addressed as an array of bytes.

For users interested in the technology this is worth reading. Clue: the FRAM cell contains no iron.

Main readme

2. Connections

Any SPI interface may be used. The table below assumes a Pyboard running SPI(2) as per the test program. To wire up a single FRAM BOARD, connect to a Pyboard as below (n/c indicates no connection):

FRAM Signal	PB	Signal
Vin	3V3	3V3
3V3	n/c	n/c
Gnd	Gnd	Gnd
SCK	Y6	SCK
MISO	Y7	MISO
MOSI	Y8	MOSI
CS	Y5	SS/
WP/	n/c	n/c
HOLD/	n/c	n/c

For multiple boards a separate CS pin must be assigned to each one: each pin must be wired to a single board's CS line. Multiple boards should have Vin, Gnd, SCK, MOSI and MISO lines wired in parallel.

If you use a Pyboard D and power the devices from the 3V3 output you will need to enable the voltage rail by issuing:

machine.Pin.board.EN_3V3.value(1)
time.sleep(0.1)  # Allow decouplers to charge

Other platforms may vary.

At the time of writing schematics for the Adafruit boards were unavailable but measurement indicated that CS, WP/ and HOLD/ are pulled up with 10KΩ. It is therefore safe to leave WP/ and HOLD/ unconnected, and CS will behave properly at power-up.

3. Files

1. fram_spi.py Device driver.
2. bdevice.py (In root directory) Base class for the device driver.
3. fram_spi_test.py Test programs for above. Assumes two 512KiB boards with CS connected to pins Y4 and Y5 respectively. Adapt for other configurations.
4. fram_fs_test.py A torture test for littlefs.

Installation: copy files 1 and 2 to the target filesystem. fram_spi_test.py has a function test() which provides quick verification of hardware, but cspins and get_fram at the start of the file may need adaptation to your hardware.

4. The device driver

The driver supports mounting the FRAM chips as a filesystem. Initially the device will be unformatted so it is necessary to issue code along these lines to format the device. Code assumes one or more devices and also assumes the littlefs filesystem:

import os
from machine import SPI, Pin
from fram_spi import FRAM
cspins = (Pin(Pin.board.Y5, Pin.OUT, value=1),)
fram = FRAM(SPI(2, baudrate=25_000_000), cspins)
# Format the filesystem
os.VfsLfs2.mkfs(fram)  # Omit this to mount an existing filesystem
os.mount(fram,'/fram')

The above will reformat a drive with an existing filesystem: to mount an existing filesystem simply omit the commented line.

Note that, at the outset, you need to decide whether to use the array as a mounted filesystem or as a byte array. The filesystem is relatively small but has high integrity owing to the hardware longevity. Typical use-cases involve files which are frequently updated. These include files used for storing Python objects serialised using pickle/ujson or files holding a btree database.

The SPI bus must be instantiated using the machine module. The chips are specified to a baudrate of 40MHz. I tested on a Pyboard D, specifying 25MHz - this produced an actual baudrate of 18MHz.

4.1 The FRAM class

An FRAM instance represents a logical FRAM: this may consist of multiple physical devices on a common SPI bus.

4.1.1 Constructor

This checks each CS line for an attached board of the correct type and of the specified size. A RuntimeError will occur in case of error, e.g. bad ID, no device detected or size not matching that specified to the constructor. If all is OK an FRAM instance is created.

Arguments:

1. spi Mandatory. An initialised SPIbus created by machine.
2. cspins A list or tuple of Pin instances. Each Pin must be initialised as an output (Pin.OUT) and with value=1 and be created by machine.
3. size=512 Chip size in KiB.
4. verbose=True If True, the constructor issues information on the FRAM devices it has detected.
5. block_size=9 The block size reported to the filesystem. The size in bytes is 2**block_size so is 512 bytes by default.

4.1.2 Methods providing byte level access

It is possible to read and write individual bytes or arrays of arbitrary size. Arrays will be somewhat faster owing to more efficient bus utilisation.

4.1.2.1 __getitem__ and __setitem__

These provides single byte or multi-byte access using slice notation. Example of single byte access:

from machine import SPI, Pin
from fram_spi import FRAM
cspins = (Pin(Pin.board.Y5, Pin.OUT, value=1),)
fram = FRAM(SPI(2), cspins)
fram[2000] = 42
print(fram[2000])  # Return an integer

It is also possible to use slice notation to read or write multiple bytes. If writing, the size of the slice must match the length of the buffer:

from machine import SPI, Pin
from fram_spi import FRAM
cspins = (Pin(Pin.board.Y5, Pin.OUT, value=1),)
fram = FRAM(SPI(2), cspins)
fram[2000:2002] = bytearray((42, 43))
print(fram[2000:2002])  # Returns a bytearray

Three argument slices are not supported: a third arg (other than 1) will cause an exception. One argument slices (fram[:5] or fram[32760:]) and negative args are supported.

4.1.2.2 readwrite

This is a byte-level alternative to slice notation. It has the potential advantage when reading of using a pre-allocated buffer. Arguments:

1. addr Starting byte address
2. buf A bytearray or bytes instance containing data to write. In the read case it must be a (mutable) bytearray to hold data read.
3. read If True, perform a read otherwise write. The size of the buffer determines the quantity of data read or written. A RuntimeError will be thrown if the read or write extends beyond the end of the physical space.

4.1.3 Other methods

The len() operator

The size of the FRAM array in bytes may be retrieved by issuing len(fram) where fram is the FRAM instance.

4.1.4 Methods providing the block protocol

These are provided by the base class. For the protocol definition see the pyb documentation also here.

These methods exist purely to support the block protocol. They are undocumented: their use in application code is not recommended.

readblocks()
writeblocks()
ioctl()

5. Test program fram_spi_test.py

This assumes a Pyboard 1.x or Pyboard D with FRAM(s) wired as above. It provides the following.

5.1 test()

This performs a basic test of single and multi-byte access to chip 0. The test reports how many chips can be accessed. Existing array data will be lost. This primarily tests the driver: as a hardware test it is not exhaustive.

5.2 full_test()

This is a hardware test. Tests the entire array. Fills each 128 byte page with random data, reads it back, and checks the outcome. Existing array data will be lost.

5.3 fstest(format=False)

If True is passed, formats the FRAM array as a FAT filesystem and mounts the device on /fram. If no arg is passed it mounts the array and lists the contents. It also prints the outcome of uos.statvfs on the array.

5.4 cptest()

Tests copying the source files to the filesystem. The test will fail if the filesystem was not formatted. Lists the contents of the mountpoint and prints the outcome of uos.statvfs.

5.5 File copy

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):

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

See upysh in micropython-lib for more fully developed filesystem tools for use at the REPL.

6. Low power operation

In the absence of an SPI clock signal the chip is specified to draw 50μA max. This can be reduced to 8μA max by issuing a sleep command. Code to support this is provided in fram_spi.py but is commented out; it is a somewhat specialised requirement.

7. References

256KiB Adafruit board
512KiB Adafruit board
256KiB Chip datasheet
512KiB Chip datasheet
Technology