1. MicroPython drivers for nonvolatile memory

These drivers support nonvolatile memory chips and the littlefs filesystem.

Currently supported devices include technologies having superior performance compared to flash. Resultant storage has much higher write endurance. In some cases read and write access times may be shorter. EEPROM and FRAM chips have much lower standby current than SD cards, benefiting micropower applications.

The drivers present a common API having the features listed below.

1.1 Features common to all drivers

The drivers have the following common features:

1. Support for single or multiple chips on the same bus. Multiple chips are automatically configured as a single array.
2. This can be accessed as an array of bytes, using Python slice syntax or via a readwrite method.
3. Alternatively the array can be formatted and mounted as a filesystem using methods in the uos module. Any filesystem supported by the MicroPython build may be employed: FAT and littlefs have been tested. The latter is recommended.
4. Drivers are portable: buses and pins should be instantiated using the machine module.
5. Buses may be shared with other hardware. This assumes that the application pays due accord to differing electrical constraints such as baudrate.

1.2 Technologies

Currently supported technologies are Flash, EEPROM and FRAM (ferroelectric RAM). The latter two are nonvolatile random access storage devices with much higher endurance than flash memory. Flash has a typical endurance of 10-100K writes per page. The figures for EEPROM and FRAM are 1-4M and 10^12 writes respectively. In the case of the FAT filing system 1M page writes probably corresponds to 1M filesystem writes because FAT repeatedly updates the allocation tables in the low numbered sectors. Under littlefs I would expect the endurance to be substantially better owing to its wear levelling architecture; over-provisioning should enhance this.

1.3 Organisation of this repo

The directory structure is technology/interface where supported chips for a given technology offer SPI and I2C interfaces; where only one interface exists the interface subdirectory is omitted. The file bdevice.py is common to all drivers and is in the root directory.

1.4 Supported chips

These currently include Microchip and STM EEPROM chips and this Adafruit FRAM board. Note that the largest EEPROM chip uses SPI: see below for a discussion of the merits and drawbacks of each interface.

Supported devices. Microchip manufacture each chip in different variants with letters denoted by "xx" below. The variants cover parameters such as minimum Vcc value and do not affect the API. There are two variants of the STM chip, M95M02-DRMN6TP and M95M02-DWMN3TP/K. The latter has a wider temperature range.

In the table below the Interface column includes page size in bytes.

Manufacturer	Part	Interface	Bytes	Technology	Docs
Cypress	S25FL256L	SPI 4096	32MiB	Flash	FLASH.md
Cypress	S25FL128L	SPI 4096	16MiB	Flash	FLASH.md
STM	M95M02-DR	SPI 256	256KiB	EEPROM	SPI.md
Microchip	25xx1024	SPI 256	128KiB	EEPROM	SPI.md
Microchip	24xx512	I2C 128	64KiB	EEPROM	I2C.md
Microchip	24xx256	I2C 128	32KiB	EEPROM	I2C.md
Microchip	24xx128	I2C 128	16KiB	EEPROM	I2C.md
Microchip	24xx64	I2C 128	8KiB	EEPROM	I2C.md
Adafruit	1895	I2C n/a	32KiB	FRAM	FRAM.md

1.5 Performance

FRAM is truly byte-addressable: its speed is limited only by the speed of the I2C interface.

Reading from EEPROM chips is fast. Writing is slower, typically around 5ms. However where multiple bytes are written, that 5ms applies to a page of data so the mean time per byte is quicker by a factor of the page size (128 or 256 bytes depending on the device).

The drivers provide the benefit of page writing in a way which is transparent. If you write a block of data to an arbitrary address, page writes will be used to minimise total time.

In the case of flash, page writing is mandatory: a sector is written by first erasing it, a process which is slow. This physical limitation means that the driver must buffer an entire 4096 byte sector. This contrasts with FRAM and EEPROM drivers where the buffering comprises a few bytes.

2. Choice of interface

The principal merit of I2C is to minimise pin count. It uses two pins regardless of the number of chips connected. It requires pullup resistors on those lines, although these may be provided on the target device. The supported EEPROM devices limit expansion to a maximum of 8 chips on a bus.

SPI requires no pullups, but uses three pins plus one for each connected chip. It is much faster than I2C, but in the case of EEPROMs the benefit is only apparent on reads: write speed is limited by the EEPROM device. In principle expansion is limited only by the number of available pins. (In practice electrical limits may also apply).

The larger capacity chips generally use SPI.

3. Design details

A key aim of these drivers is support for littlefs. This requires the extended block device protocol as described here and in the uos doc. This protocol describes a block structured API capable of handling offsets into the block. It is therefore necessary for the device driver to deal with any block structuring inherent in the hardware. The device driver must enable access to varying amounts of data at arbitrary physical addresses.

These drivers achieve this by implementing a device-dependent readwrite method which provides read and write access to arbitrary addresses, with data volumes which can span page and chip boundaries. A benefit of this is that the array of chips can be presented as a large byte array. This array is accessible by Python slice notation: behaviour provided by the hardware-independent base class.

A consequence of the above is that the page size in the ioctl does not have any necessary connection with the memory hardware, so the drivers enable the value to be specified as a constructor argument. Littlefs requires a minimum size of 128 bytes - theoretically 104. The drivers only allow powers of 2: in principle 128 bytes could be used. The default in MicroPython's littlefs implementation is 512 bytes and all testing was done with this value. FAT requires 512 bytes minimum: FAT testing was done with the same block size.

3.1 Developer Documentation

This doc has information on the base classes for those wishing to write drivers for other memory devices.

4. littlefs support

The test programs use littlefs and therefore require MicroPython V1.12 or later. On platforms that don't support littlefs the options are either to adapt the test programs for FAT (code is commented out) or to build firmware with littlefs support. This can be done by passing MICROPY_VFS_LFS2=1 to the make command.