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1. A MicroPython Flash memory driver

1.1 Device support

This driver supports the Cypress S25FL256L and S25FL128L chips, providing 32MiB and 16MiB respectively. These have 100K cycles of write endurance (compared to 10K for Pyboard Flash memory). These were the largest capacity available with a sector size small enough for microcontroller use.

Thanks to a patch from Daniel Thompson this now supports a variety of NOR Flash chips including those with 24-bit addressing. He tested an XPX XT25F32B; I tested Winbond W25Q32JV 4MiB and Cypress S25FL064L 8MiB devices.

It is likely that other chips with 4096 byte blocks will work but I am unlikely to be able to support hardware I don't possess. Users should check datasheets for compatibility.

1.2 The driver

Multiple chips may be used to construct a single logical nonvolatile memory module. The driver allows the memory either to be mounted in the target filesystem as a disk device or to be addressed as an array of bytes.

The driver has the following attributes:

It supports multiple Flash chips to configure a single array.
It is cross-platform.
The SPI bus can be shared with other chips.
It supports filesystem mounting.
Alternatively it can support byte-level access using Python slice syntax.

Flash technology requires a sector buffer. Consequently this driver uses 4KiB of RAM (compared to minuscule amounts for the FRAM and EEPROM drivers). This is an inevitable price for the large capacity of flash chips.

FAT and littlefs filesystems are supported but the latter is preferred owing to its resilience and wear levelling characteristics.

Arguably byte level access on such large devices has few use cases other than for facilitating effective hardware tests and for diagnostics.

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 flash chip, connect to a Pyboard as below. Pin numbers relate to an 8 pin SOIC or WSON package. Inputs marked nc may be connected to 3V3 or left unconnected.

Flash	Signal	PB	Signal
1	CS/	Y5	SS/
2	SO	Y7	MISO
3	WP/	nc	-
4	Vss	Gnd	Gnd
5	SI	Y8	MOSI
6	SCK	Y6	SCK
7	RESET/	nc	-
8	Vcc	3V3	3V3

For multiple chips a separate CS pin must be assigned to each chip, each one being wired to a single chip's CS line. The test program assumes a second chip with CS connected to Y4. Multiple chips should have 3V3, Gnd, SCL, MOSI and MISO lines wired in parallel.

If you use a Pyboard D and power the chips 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 devices may vary but the Cypress chips require a 3.3V supply.

It is wise to add a pullup resistor (say 10KΩ) from each CS/ line to 3.3V. This ensures that chips are deselected at initial power up when the microcontroller I/O pins are high impedance.

2.1 SPI Bus

The devices support baudrates up to 50MHz. In practice MicroPython targets do not support such high rates. The test programs specify 20MHz, but in practice the Pyboard D delivers 15MHz. Testing was done at this rate. In testing a "lashup" breadboard was unsatisfactory: a problem entirely fixed with a PCB. Bus lines should be short and direct.

3. Files

flash_spi.py Device driver.
bdevice.py (In root directory) Base class for the device driver.
flash_test.py Test programs for above.
littlefs_test.py Torture test for the littlefs filesystem on the flash array. Requires flash_test.py which it uses for hardware configuration.
wemos_flash.py Test program running on a Wemos D1 Mini ESP8266 board.

Installation: copy files 1 and 2 (3 - 5 are optional) to the target filesystem. The flash_test script assumes two chips connected to SPI(2) with CS/ pins wired to Pyboard pins Y4 and Y5. Device size is detected at runtime. The get_device function may be adapted for other setups and is shared with littlefs_test.

For a quick check of hardware issue:

import flash_test
flash_test.test()

4. The device driver

The driver supports mounting the Flash 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 two devices and the (recommended) littlefs filesystem:

import os
from machine import SPI, Pin
from flash_spi import FLASH
cspins = (Pin(Pin.board.Y5, Pin.OUT, value=1), Pin(Pin.board.Y4, Pin.OUT, value=1))
flash = FLASH(SPI(2, baudrate=20_000_000), cspins)
# Format the filesystem
os.VfsLfs2.mkfs(flash)  # Omit this to mount an existing filesystem
os.mount(flash,'/fl_ext')

The above will reformat a drive with an existing filesystem erasing all files: to mount an existing filesystem 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. Most use cases for flash will require a filesystem, although byte level reads may be used to debug filesystem issues.

The SPI bus must be instantiated using the machine module.

4.1 The FLASH class

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

4.1.1 Constructor

This tests each chip in the list of chip select pins - if a chip is detected on each chip select line a flash array is instantiated. A RuntimeError will be raised if a device is not detected on a CS line. The test has no effect on the array contents.

Arguments. In most cases only the first two mandatory args are required:

spi An initialised SPI bus created by machine.
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.
size=None Chip size in KiB. The size is read from the chip. If a value is passed, the actual size is compared with the passed value: a mismatch will raise a ValueError. A ValueError will also occur if chips in the array have differing sizes. See table below for values of chips tested to date.
verbose=True If True, the constructor issues information on the flash devices it has detected.
sec_size=4096 Chip sector size.
block_size=9 The block size reported to the filesystem. The size in bytes is 2**block_size so is 512 bytes by default.
cmd5=None Flash chips can support two low level command sets, an old 4 byte set and a newer 5 byte set capable of supporting a larger address space. By default if the size read from the chip's ID is <= 4096KiB the 4 byte set is used oterwise the 5 byte set is adopted. This works for supported chips. Setting cmd5 True forces 5 byte commands, False forces 4 byte. This override is necessary for certain chip types (e.g. WinBond W25Q64FV).

Size values (KiB):

Chip	Size
Cypress S25FL256L	32768
Cypress S25FL128L	16384
Cypress S25FL064L	8192
Winbond W25Q32JV	4096

4.1.2 Methods providing byte level access

It is possible to read and write individual bytes or arrays of arbitrary size. Because of the very large size of the supported devices this mode is most likely to be of use for debugging. When writing in this mode it is necessary to be aware of the characteristics of flash devices. The memory is structured in blocks of 4096 bytes. To write a byte a block has to be read into RAM and the byte changed. The block on chip is erased then the new data written out. This process is slow (~300ms). In practice writing is deferred until it is necessary to access a different block: it is therefore faster to write data to consecutive addresses. Writing individual bytes to random addresses would be slow and cause undue wear because of the repeated need to erase and write sectors.

The examples below assume two devices, one with CS connected to Pyboard pin Y4 and the other with CS connected to Y5.

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 flash_spi import FLASH
cspins = (Pin(Pin.board.Y5, Pin.OUT, value=1), Pin(Pin.board.Y4, Pin.OUT, value=1))
flash = FLASH(SPI(2, baudrate=20_000_000), cspins)
flash[2000] = 42
print(flash[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 flash_spi import FLASH
cspins = (Pin(Pin.board.Y5, Pin.OUT, value=1), Pin(Pin.board.Y4, Pin.OUT, value=1))
flash = FLASH(SPI(2, baudrate=20_000_000), cspins)
flash[2000:2002] = bytearray((42, 43))
print(flash[2000:2002])  # Returns a bytearray

Three argument slices are not supported: a third arg (other than 1) will cause an exception. One argument slices (flash[:5] or flash[13100:]) 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:

addr Starting byte address
buf A bytearray or bytes instance containing data to write. In the read case it must be a (mutable) bytearray to hold data read.
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

sync

This causes the cached sector to be written to the device. In normal filesystem use this need not be called. If byte-level writes have been performed it should be called prior to power down.

The len operator

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

scan

Args:

verbose bool. If True print information on chips detected.
size int or None. If an int is passed a ValueError is thrown if the detected chip size does not match the passed value.

Activate each chip select in turn checking for a valid device and returns the size in KiB of one instance of the flash devices detected. A RuntimeError will be raised if any CS pin does not correspond to a valid chip. A ValueError is thrown if the detected chips are not of the same size.

Other than for debugging there is no need to call scan(): it is called by the constructor.

erase

Erases the entire array. Beware: this takes many minutes.

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.

readblocks()
writeblocks()
ioctl()

5. Test program flash_test.py

This assumes a Pyboard 1.x or Pyboard D with two chips wired to SPI(2) as above with chip selects connected to pins Y4 and Y5. 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. It does provide a quick verification that all chips can be accessed.

5.2 full_test(count=10)

This is a hardware test. Tests the entire array. Creates an array of 256 bytes of random data and writes it to a random address. After synchronising the cache with the hardware, reads it back, and checks the outcome. Existing array data will be lost. The arg determines the number of passes.

5.3 fstest(format=False)

If True is passed, formats the flash array as a littlefs filesystem deleting existing contents. In both cases of the arg it mounts the device on /fl_ext lists the contents of the mountpoint. It also prints the outcome of uos.statvfs on the mountpoint.

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 flash becoming full) it is up to the caller to handle it. For example (assuming the flash is mounted on /fl_ext):

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

See upysh in micropython-lib for other filesystem tools for use at the REPL.

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