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1. A MicroPython I2C EEPROM driver

This driver supports chips from the 64KiB 25xx512 series and related chips with smaller capacities, now including chips as small as 2KiB with single byte addressing.

From one to eight chips may be used to construct a nonvolatile memory module with sizes upto 512KiB. 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. Where multiple chips are used, all must be the same size.

The work was inspired by this driver. This was written some five years ago. The driver in this repo employs some of the subsequent improvements to MicroPython to achieve these advantages:

  1. It supports multiple EEPROM chips to configure a single array.
  2. Writes are up to 1000x faster by using ACK polling and page writes.
  3. Page access improves the speed of multi-byte reads.
  4. It is cross-platform.
  5. The I2C bus can be shared with other chips.
  6. It supports filesystem mounting.
  7. Alternatively it can support byte-level access using Python slice syntax.
  8. RAM allocations are reduced.

1.1 Release Notes

January 2024 Fixes a bug whereby incorrect page size caused data corruption. Thanks are due to Abel Deuring for help in diagnosing and fixing this, also for educating me on the behaviour of various types of EEPROM chip. This release also supports some chips of 2KiB and below which store the upper three address bits in the chip address. See 6. Small chips case study.

Main readme

2. Connections

Any I2C interface may be used. The table below assumes a Pyboard running I2C(2) as per the test program. To wire up a single EEPROM chip, connect to a Pyboard or ESP8266 as below. Any ESP8266 pins may be used, those listed below are as used in the test program.

EEPROM Pin numbers assume a PDIP package (8 pin plastic dual-in-line).

EEPROM PB ESP8266
1 A0 Gnd Gnd
2 A1 Gnd Gnd
3 A2 Gnd Gnd
4 Vss Gnd Gnd
5 Sda Y10 12 D6
6 Scl Y9 13 D7
7 WPA1 Gnd Gnd
8 Vcc 3V3 3V3

For multiple chips the address lines A0, A1 and A2 of each chip need to be wired to 3V3 in such a way as to give each device a unique address. In the case where chips are to form a single array these must start at zero and be contiguous:

Chip no. A2 A1 A0
0 Gnd Gnd Gnd
1 Gnd Gnd 3V3
2 Gnd 3V3 Gnd
3 Gnd 3V3 3V3
4 3V3 Gnd Gnd
5 3V3 Gnd 3V3
6 3V3 3V3 Gnd
7 3V3 3V3 3V3

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

The I2C interface requires pullups, typically 3.3KΩ to 3.3V although any value up to 10KΩ will suffice. The Pyboard 1.x has these on board. The Pyboard D has them only on I2C(1). Even if boards have pullups, additional externalresistors will do no harm.

If you use a Pyboard D and power the EEPROMs 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.

3. Files

  1. eeprom_i2c.py Device driver.
  2. bdevice.py (In root directory) Base class for the device driver.
  3. eep_i2c.py Pyboard test programs for above (adapt for other hosts).

3.1 Installation

This installs the above files in the lib directory.

On networked hardware this may be done with mip which is included in recent firmware. On non-networked hardware this is done using the official mpremote utility which should be installed on the PC as described in this doc.

Any hardware

On the PC issue:

$ mpremote mip install "github:peterhinch/micropython_eeprom/eeprom/i2c"

Networked hardware

At the device REPL issue:

>>> import mip
>>> mip.install("github:peterhinch/micropython_eeprom/eeprom/i2c")

4. The device driver

The driver supports mounting the EEPROM 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 64KiB devices and also assumes the littlefs filesystem:

import os
from machine import I2C
from eeprom_i2c import EEPROM, T24C512
eep = EEPROM(I2C(2), T24C512)
# Format the filesystem
os.VfsLfs2.mkfs(eep)  # Omit this to mount an existing filesystem
os.mount(eep,'/eeprom')

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 I2C bus must be instantiated using the machine module.

4.1 The EEPROM class

An EEPROM instance represents a logical EEPROM: this may consist of multiple physical devices on a common I2C bus.

4.1.1 Constructor

This scans the I2C bus - if one or more correctly addressed chips are detected an EEPROM array is instantiated. A RuntimeError will be raised if no device is detected or if device address lines are not wired as described in Connections.

Arguments:

  1. i2c Mandatory. An initialised master mode I2C bus created by machine.
  2. chip_size=T24C512 The chip size in bits. The module provides constants T24C32, T24C64, T24C128, T24C256, T24C512 for the supported chip sizes.
  3. verbose=True If True, the constructor issues information on the EEPROM devices it has detected.
  4. block_size=9 The block size reported to the filesystem. The size in bytes is 2**block_size so is 512 bytes by default.
  5. addr Override base address for first chip. See 4.1.6 Special configurations.
  6. max_chips_count Override max_chips_count - see above reference.
  7. page_size=None EEPROM chips have a page buffer. By default the driver determines the size of this automatically. It is possible to override this by passing an integer being the page size in bytes: 16, 32, 64, 128 or 256. See 4.1.5 Page size for issues surrounding this.

In most cases only the first two arguments are used, with an array being instantiated with (for example):

from machine import I2C
from eeprom_i2c import EEPROM, T24C512
eep = EEPROM(I2C(2), T24C512)

4.1.2 Methods providing byte level access

It is possible to read and write individual bytes or arrays of arbitrary size. Larger arrays are faster, especially when writing: the driver uses the chip's hardware page access where possible. Writing a page takes the same time (~5ms) as writing a single byte.

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 I2C
from eeprom_i2c import EEPROM, T24C512
eep = EEPROM(I2C(2), T24C512)
eep[2000] = 42
print(eep[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 I2C
from eeprom_i2c import EEPROM, T24C512
eep = EEPROM(I2C(2), T24C512)
eep[2000:2002] = bytearray((42, 43))
print(eep[2000:2002])  # Returns a bytearray

Three argument slices are not supported: a third arg (other than 1) will cause an exception. One argument slices (eep[:5] or eep[13100:]) and negative args are supported. See section 4.2 for a typical application.

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 EEPROM array in bytes may be retrieved by issuing len(eep) where eep is the EEPROM instance.

scan

Scans the I2C bus and returns the number of EEPROM devices detected.

Other than for debugging there is no need to call scan(): the constructor will throw a RuntimeError if it fails to communicate with and correctly identify the chip.

get_page_size

Return the page size in bytes.

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()

4.1.5 Page size

EEPROM chips have a RAM buffer enabling fast writing of data blocks. Writing a page takes the same time (~5ms) as writing a single byte. The page size may vary between chips from different manufacturers even for the same storage size. Specifying too large a value will most likely lead to data corruption in write operations and will cause the test script's basic test to fail. Too small a value will impact write performance. The correct value for a device may be found in in the chip datasheet. It is also reported if verbose is set and when running the test scripts.

Auto-detecting page size carries a risk of data loss if power fails while auto-detect is in progress. In production code the value should be specified explicitly.

4.1.6 Special configurations

It is possible to configure multiple chips as multiple arrays. This is done by means of the addr and max_chips_count args. Examples:

eeprom0 = EEPROM(i2c, max_chips_count = 2)
eeprom1 = EEPROM(i2c, addr = 0x52, max_chips_count = 2)

1st array uses address 0x50 and 0x51 and 2nd uses address 0x52 and 0x53.

Individual chip usage:

eeprom0 = EEPROM(i2c, addr = 0x50, max_chips_count = 1)
eeprom1 = EEPROM(i2c, addr = 0x51, max_chips_count = 1)

4.2 Byte addressing usage example

A sample application: saving a configuration dict (which might be large and complicated):

import ujson
from machine import I2C
from eeprom_i2c import EEPROM, T24C512
eep = EEPROM(I2C(2), T24C512)
d = {1:'one', 2:'two'}  # Some kind of large object
wdata = ujson.dumps(d).encode('utf8')
sl = '{:10d}'.format(len(wdata)).encode('utf8')
eep[0 : len(sl)] = sl  # Save data length in locations 0-9
start = 10  # Data goes in 10:
end = start + len(wdata)
eep[start : end] = wdata

After a power cycle the data may be read back. Instantiate eep as above, then issue:

slen = int(eep[:10].decode().strip())  # retrieve object size
start = 10
end = start + slen
d = ujson.loads(eep[start : end])

It is much more efficient in space and performance to store data in binary form but in many cases code simplicity matters, especially where the data structure is subject to change. An alternative to JSON is the pickle module. It is also possible to use JSON/pickle to store objects in a filesystem.

5. Test program eep_i2c.py

This assumes a Pyboard 1.x or Pyboard D with EEPROM(s) wired as above. On other hardware, adapt get_eep at the start of the script. 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. The current page size is printed and its validity is tested. 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 the array with random data in blocks of 256 byes. After each block is written, it is read back and the contents compared to the data written. Existing array data will be lost.

5.3 fstest(format=False)

If True is passed, formats the EEPROM array as a littlefs filesystem and mounts the device on /eeprom. 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. This test does not run on ESP8266 owing to a missing Python language feature. Use File Copy or upysh as described below to verify the filesystem.

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

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

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

6. Small chips case study

A generic 2KiB EEPROM was tested. Performing an I2C scan revealed that it occupied 8 I2C addresses starting at 80 (0x50). Note it would be impossible to configure such chips in a multi-chip array as all eight addresses are used: the chip can be regarded as an array of eight 256 byte virtual chips. The driver was therefore initialised as follows:

i2c = SoftI2C(scl=Pin(9, Pin.OPEN_DRAIN, value=1), sda=Pin(8, Pin.OPEN_DRAIN, value=1))
eep = EEPROM(i2c, 256, addr=0x50)

A hard I2C interface would also work. At risk of stating the obvious it is not possible to build a filesystem on a chip of this size. Tests eep_i2c.test and eep_i2c.full_test should be run and will work if the driver is correctly configured.