==============================================
:TEP: 103
-:Group: Core Working Group
+:Group: Core Working Group
:Type: Documentary
-:Status: Draft
+:Status: Final
:TinyOS-Version: 2.x
:Author: David Gay, Jonathan Hui
-:Draft-Created: 27-Sep-2004
-:Draft-Version: $Revision$
-:Draft-Modified: $Date$
-:Draft-Discuss: TinyOS Developer List <tinyos-devel at mail.millennium.berkeley.edu>
-
-
.. Note::
This memo documents a part of TinyOS for the TinyOS Community, and
1. Introduction
====================================================================
-Flash chips are a form of EEPROM (electrically-eraseable, programmable
+Flash chips are a form of EEPROM (electrically-erasable, programmable
read-only memory), distinguished by a fast erase capability. However,
erases can only be done in large units (from 256B to 128kB depending on the
flash chip). Erases are the only way to switch bits from 0 to 1, and
programming operations can only switch 1's to 0's. Additionally, some
chips require that programming only happen once between each erase,
-or that it be in relatively large units (e.g., 256B).
+or that it be in relatively large units (e.g., 256B).
In the table below, we summarise these differences by categorising
flash chips by their underlying technology (NOR vs NAND). We also
include a column for Atmel's AT45DB flash chip family, as it has
-significantly different tradeoffs than other flash chips: ::
-
-
- NOR AT45DB NAND
- (ex: ST M25P40, (ex: Samsung
- Intel PXA27x) K9K1G08R0B)
-
- Erase : Slow (seconds) Fast (ms) Fast (ms)
- Erase unit : Large (64KB-128KB) Small (256B) Medium (8KB-32KB)
- Writes : Slow (100s kB/s) Slow (60kB/s) Fast (MBs/s)
- Write unit : 1 bit 256B 100's of bytes
- Bit-errors : Low Low High (requires ECC,
- bad-block mapping)
- Read : Bus limited* Slow+Bus limited Bus limited
- Erase cycles : 10^4 - 10^5 10^4 ** 10^5 - 10^7
- Intended use : Code storage Data storage Data storage
- Energy/byte : 1uJ 1uJ .01uJ
-
- * M25P40 reads are limited by the use of a 25MHz SPI bus. The PXA27x flash is
- memory mapped (reads are very fast and can directly execute code).
- ** Or infinite? Data sheet just says that every page within a sector
- must be written every 10^4 writes within that sector
+significantly different tradeoffs than other flash chips:
+
+
+ ============= ================== ================= ===================
+ X NOR AT45DB NAND
+ (ex: ST M25P40, (ex: Samsung
+ Intel PXA27x) K9K1G08R0B)
+ ============= ================== ================= ===================
+ Erase Slow (seconds) Fast (ms) Fast (ms)
+ Erase unit Large (64KB-128KB) Small (256B) Medium (8KB-32KB)
+ Writes Slow (100s kB/s) Slow (60kB/s) Fast (MBs/s)
+ Write unit 1 bit 256B 100's of bytes
+ Bit-errors Low Low High (requires ECC,
+ bad-block mapping)
+ Read Bus limited [*]_ Slow+Bus limited Bus limited
+ Erase cycles 10^4 - 10^5 10^4 [*]_ 10^5 - 10^7
+ Intended use Code storage Data storage Data storage
+ Energy/byte 1uJ 1uJ .01uJ
+ ============= ================== ================= ===================
+
+.. [*] M25P40 reads are limited by the use of a 25MHz SPI bus. The PXA27x flash
+ is memory mapped (reads are very fast and can directly execute code).
+.. [*] Or infinite? Data sheet just says that every page within a sector
+ must be written every 10^4 writes within that sector
The energy/byte is the per-byte cost of erasing plus programming. It is
derived from the timing and power consumption of erase and write operations
(for NOR flash, values are for the STMicroelectronics M25P family, for NAND
-flash, values are from a Samsung datasheet). Energy/byte for reads appears
+flash, values are from the Samsung datasheet). Energy/byte for reads appears
to depend mostly on how long the read takes (the power consumptions are
comparable), i.e., on the efficiency of the bus + processor.
- ``ByteEEPROMC`` allows arbitrary rewrites of sections of the flash.
This is not readily implementable on a flash chip with large erase units.
- The ``Matchbox`` implementation was AT45DB-specific. It was not
- reimplemented for these other chips, in part because it does not
+ reimplemented for these other chips, in part because it does not
support some applications (e.g., network reprogramming) very well.
-
+
+2. Storage in TinyOS 2.x
+====================================================================
+
One approach to hiding the differences between different flash chips is to
provide a disk-like, block interface (with, e.g., 512B blocks). This is the
approach taken by compact flash cards. However, in the context of TinyOS,
in a free, open-source operating system.
- To support arbitrary block writes where blocks are smaller than the
erase unit, and to deal with the limited number of erase cycles/block
- requires remapping blocks. We believe that maintaining this remapping
+ requires remapping blocks. We believe that maintaining this remapping
table is too expensive on many mote-class devices.
-Another approach to supporting multiple flash chips is to build a
-file system (like Matchbox) which can be implemented for multiple
-flash chips. However, TinyOS is currently targeted at running a
-single application, and many applications know their storage needs
-in advance: for instance, a little space for configuration data, and
-everything else for a log of all sampled data. In such cases, the
-flexibility offered by a filing system (e.g., arbitrary numbers of
-files) is overkill, and may come at the expense of implementation
-and runtime complexity.
-
-Instead, TinyOS 2.x, divides flash chips into separate volumes (with
-sizes fixed at compile-time). Each volume provides a single storage
-abstraction (the abstraction defines the format). So far there are three
-such abstractions: large objects written in a single session,
-small objects with arbitrary reads and writes, and logs. This approach
-has two advantages:
-
+A second approach is to provide a generic low-level interface providing
+operations (read, write, erase) corresponding to the basic flash
+operations, along with information describing the flash chip's layout
+(minimum write and erase unit, timing information, etc). However,
+we believe that the large differences between NAND and NOR flash (see the
+table above), in particular the differences in reliability, write and
+erase units, make the design of a useful generic low-level interface
+tricky at best.
+
+We thus believe it is best, for now at least, to define high-level
+storage abstractions that are useful for sensor network applications,
+and leave their implementation up to each flash chip - such abstractions
+will be necessary anyway. We leave open the possibility that a future
+TEP may define portable lower-level flash interfaces (either for all
+flash chips, or, e.g., for NOR-family flashes). Such low-level
+interfaces would allow implementations of the storage abstractions
+defined in this TEP to be used for multiple flash chips.
+
+This TEP describes three high-level storage abstractions: large objects
+written in a single session, small objects with arbitrary reads and
+writes, and logs. TinyOS 2.x, divides flash chips into separate volumes
+(with sizes fixed at compile-time). Each volume provides a single
+storage abstraction (the abstraction defines the format).
+
+We prefer the use of single abstractions over fixed-size volumes over
+the use of a more general filing system (like Matchbox) for several
+reasons:
+
+- TinyOS is currently targeted at running a single application, and many
+ applications know their storage needs in advance: for instance, a
+ little space for configuration data, and everything else for a log of
+ all sampled data. In such cases, the flexibility offered by a filing
+ system (e.g., arbitrary numbers of files) is overkill,
- Each abstraction is relatively easy to implement on a new flash chip, and
has relatively little overhead.
- The problem of dealing with the limited number of erase cycles/block
"wear levelling" (ensuring that each block of the flash is erased
the same number of time, to maximise flash chip lifetime).
-New abstractions (including a filing system) can easily be added to this
-framework, or can be built on top of these abstractions.
+New abstractions (including a filing system) can easily be added to
+this framework.
The rest of this TEP covers some principles for the organisation of
-flash chips (Section 2), then describes the flash volumes and
-storage abstractions in detail (Section 3).
+flash chips (Section 3), then describes the flash volumes and
+storage abstractions in detail (Section 4).
-2. HPL/HAL/HIL Architecture
+3. HPL/HAL/HIL Architecture
====================================================================
The flash chip architecture follows the three-layer Hardware
Abstraction Architecture (HAA), with each chip providing a presentation
-layer (HPL, Section 2.1), adaptation layer (HAL, Section 2.2) and
-platform-independent interface layer (the storage abstractions described in
-Section 3) [2_]. The implementation of these layers SHOULD be found in the
+layer (HPL, Section 3.1), adaptation layer (HAL, Section 3.2) and
+platform-independent interface layer (HIL, Section 3.3) [2_].
+The implementation of these layers SHOULD be found in the
``tos/chips/CHIPNAME`` directory. If a flash chip is part of a larger
family with a similar interface, the HAA SHOULD support all family members
by relying, e.g., on platform-provided configuration information.
and ST M25P chip families.
-2.1 Hardware Presentation Layer (HPL)
+3.1 Hardware Presentation Layer (HPL)
--------------------------------------------------------------------
The flash HPL has a chip-dependent, platform-independent interface. The
implementation of this HPL is platform-dependent. The flash HPL SHOULD be
-stateless.
+stateless.
To remain platform independent, a flash chip's HPL SHOULD connect to
platform-specific components
providing access to the flash chip; these components
SHOULD be placed in the ``tos/platforms/PLATFORM/chips/CHIPNAME``
-directory. If the flash chip implementation supports a family of
+directory. If the flash chip implementation supports a family of
flash chips, this directory MAY also contain a file describing the
particular flash chip found on the platform.
-2.2 Hardware Adaptation Layer (HAL)
+3.2 Hardware Adaptation Layer (HAL)
--------------------------------------------------------------------
The flash HAL has a chip-dependent, platform-independent interface and
programmer (see Section 3). This allows users to build new flash
abstractions that interact cleanly with the rest of the flash system.
+3.3 Hardware Interface Layer (HIL)
+--------------------------------------------------------------------
-3. Non-Volatile Storage Abstracitons in TinyOS 2.x
+Each flash chip MUST support at least one of the storage abstractions
+described in Section 4. These abstractions SHOULD be presented in
+components named ``ChipAbstractionC``, e.g., ``At45dbLogStorageC``.
+Additionally, a flash chip implementation MAY support platforms
+with multiple instances of the same storage chip. The way in which
+this is achieved is not specified further in this TEP.
+
+Each platform MUST have ``AbstractionC`` components (e.g.,
+``LogStorageC``) implementing the storage abstractions of Section 4
+supported by its flash chip(s). On platforms with multiple storage chips
+SHOULD redirect uses of ``AbstractionC`` to the appropriate storage
+chip, based on the requested volume.
+
+4. Non-Volatile Storage Abstractions
===================================================================
The HIL implementations are platform-independent, but chip (family)
dependent. They implement the three storage abstractions and
volume structure discussed in the introduction.
-3.1. Volumes
+4.1. Volumes
-------------------------------------------------------------------
The division of the flash chip into fixed-size volumes is specified by
location for a volume. If there is any reason that the physical allocation
cannot be satisfied, an error should be given at compile time. The tool
SHOULD be named ``tos-storage-CHIPNAME`` and be distributed with the other
-tools supporting a platform.
+tools supporting a platform. The XML file SHOULD be named
+``volumes-CHIPNAME.xml``.
The compile-time tool MUST prepend 'VOLUME\_' to each volume name in
the XML file and '#define' each resulting name to map to a unique
-integer.
+integer.
The storage abstractions are accessed by instantiating generic
components that take the volume macro as argument: ::
A volume MUST NOT be used with more than one storage abstraction instance.
-3.2 Large objects
+4.2 Large objects
------------------------------------------------------------------
-The motivating example for large objects is the transmission or long-term
-storage of large pieces of data. For instance, programs in a network-reprogramming
-system, or large data-packets in a reliable data-transmission system. Such
-objects have two interesting characteristics: each byte in the object is
-written at most once, and a full object is written in a single "session"
-(i.e., without the mote rebooting).
+The motivating example for large objects is the transmission or
+long-term storage of large pieces of data. For instance, programs in a
+network-reprogramming system, or large data-packets in a reliable
+data-transmission system. Such objects have an interesting
+characteristic: each byte in the object is written at most once.
This leads to the definition of the ``BlockStorageC`` abstraction for storing
-large objects:
+large objects or other "write-once" objects:
- A large object ranges from a few kilobytes upwards.
- A large object is erased before the first write.
-- A commit ensures that a large object survives a reboot or crash
+- A sync ensures that a large object survives a reboot or crash
- Reads are unrestricted
-- Writes can only be performed between an erase and a commit
-- Each byte can only be written once between an erase and a commit
+- Each byte can only be written once between two erases
Large objects are accessed by instantiating a BlockStorageC component
which takes a volume id argument: ::
}
} ...
-The ``BlockRead`` and ``BlockWrite`` interfaces contain the following
-operations (all split-phase, except ``BlockRead.getSize``):
+The ``BlockRead`` and ``BlockWrite`` interfaces (briefly presented in
+Appendix B) contain the following operations (all split-phase, except
+``BlockRead.getSize``):
- ``BlockWrite.erase``: erase the volume. After a reboot or a commit, a
volume MUST be erased before it can be written to.
- ``BlockWrite.write``: write some bytes starting at a given
- offset. Each byte MUST NOT be written more than once between an erase
- and the subsequent commit. Writes MUST NOT be performed between a
- commit and the subsequent erase.
+ offset. Each byte MUST NOT be written more than once between two erases.
-- ``BlockWrite.commit``: commit all writes to a given volume. Writes
- MUST NOT be performed after a commit until a subsequent erase. Commit
- MUST be called to ensure written data survives a reboot or crash.
-- ``BlockRead.verify``: verify that the volume contains the results of a
- successful commit.
+- ``BlockWrite.sync``: ensure all previous writes are present on a given
+ volume. Sync MUST be called to ensure written data survives a reboot
+ or crash.
- ``BlockRead.read``: read some bytes starting at a given offset.
For full details on arguments and other considerations, see the comments in
the interface definitions.
+Note that these interfaces contain no direct support for verifying the
+integrity of the BlockStorage data, but such support can easily be built
+by using the ``computeCrc`` command and storing the result in a
+well-defined location, and checking this CRC when desired.
+
-3.3 Logging
+4.3 Logging
------------------------------------------------------------------
Event and result logging is a common requirement in sensor
record. On failure (crash or reboot), the log MUST only lose whole
records from the end of the log. Additionally, once a circular log wraps
around, calls to ``LogWrite.append`` MUST only lose whole records from
-the beginning of the log.
+the beginning of the log.
Logs are accessed by instantiating a LogStorageC component which takes a
volume id and a boolean argument: ::
If the ``circular`` argument is TRUE, the log is circular; otherwise
it is linear.
-The ``LogRead`` and ``LogWrite`` interfaces contain the following
-operations (all split-phase except ``LogWrite.currentOffset``,
-``LogRead.currentOffset`` and ``LogRead.getSize``):
+The ``LogRead`` and ``LogWrite`` interfaces (briefly presented in
+Appendix B) contain the following operations (all split-phase except
+``LogWrite.currentOffset``, ``LogRead.currentOffset`` and
+``LogRead.getSize``):
- ``LogWrite.erase``: erase the log. A log MUST be erased (possibly in
some previous session) before any other operation can be used.
- ``LogWrite.append``: append some bytes to the log. In a circular log,
- this may overwrite the current read position. In this case, the
+ this may overwrite the current read position. In this case, the
read position MUST be advanced to the log's current beginning
(i.e., as if ``LogRead.seek`` had been called with ``SEEK_BEGINNING``).
Additionally, the ``LogWrite.appendDone`` event reports whenever, in a
- circular log, an append erases old records.
+ circular log, an append MAY have erased old records.
Each append creates a separate record. Log implementations may have a
maximum record size; all implementations MUST support records of up
- ``LogRead.seek``: set the read position to a value returned by
a prior call to ``LogWrite.currentOffset`` or ``LogRead.currentOffset``,
or to the special ``SEEK_BEGINNING`` value. In a circular log, if
- the specified position has been overwritten, behave as if
+ the specified position has been overwritten, behave as if
``SEEK_BEGINNING`` was requested.
``SEEK_BEGINNING`` positions the read position at the beginning of
incomplete or inconsistent log entries.
-3.4 Small objects:
+4.4 Small objects:
------------------------------------------------------------------
Sensor network applications need to store configuration data, e.g.,
}
} ...
-A small object MUST be mounted (via the ``Mount`` interface) before
+A small object MUST be mounted (via the ``Mount`` interface) before
the first use.
-The ``Mount`` and ``ConfigStorage`` interfaces contain the following
-operations (all split-phase except ``ConfigStorage.getSize`` and
-``ConfigStorage.valid``):
+The ``Mount`` and ``ConfigStorage`` interfaces (briefly presented in
+Appendix B) contain the following operations (all split-phase except
+``ConfigStorage.getSize`` and ``ConfigStorage.valid``):
- ``Mount.mount``: mount the volume.
For full details on arguments, etc, see the comments in the interface
definitions.
-4. Implementations
+5. Implementations
====================================================================
An AT45DB implementation can be found in tinyos-2.x/tos/chips/at45db.
An ST M25P implementation can be found in tinyos-2.x/tos/chips/stm25p.
-5. Authors' Addresses
+6. Authors' Addresses
====================================================================
| David Gay
| phone - +1 415 692 0828
| email - jhui@archedrock.com
-6. Citations
+7. Citations
====================================================================
.. [1] David Gay. "Design of Matchbox, the simple filing system for
A.1 AT45DB
------------------------------------------------------------------
-
+
The Atmel AT45DB family HPL is: ::
configuration HplAt45dbC {
The ``HplAt45db`` interface has flash->buffer, buffer->flash, compare
buffer to flash, erase page, read, compute CRC, and write operations. Most
of these operations are asynchronous, i.e., their completion is signaled
-before the flash chip has completed the operation. The HPL also includes
+before the flash chip has completed the operation. The HPL also includes
operations to wait for asynchronous operations to complete.
A generic, system-independent implementation of the HPL
Note that the AT45DB HAL resource management is independent of the
underlying HPL's power management. The motivation for this is that
individual flash operations may take a long time, so it may be desirable to
-release the flash's bus during long-running operations.
+release the flash's bus during long-running operations.
The ``At45db`` interface abstracts from the low-level HPL operations by:
A.2 ST M25P
------------------------------------------------------------------
-
+
The ST M25P family HPL is: ::
configuration Stm25pSpiC {
volume-relative addresses. Clients of the ST M25P HAL must implement the
``getVolumeId`` event of the ``Stm25pVolume`` interface so that the HAL can
obtain the volume id of each of its clients.
+
+Appendix B. Storage interfaces
+====================================================================
+
+These interfaces are presented briefly here for reference; please refer
+to the TinyOS documentation for a full description of the commands,
+events and their parameters.
+
+B.1 BlockStorage interfaces
+------------------------------------------------------------------
+
+The BlockStorage interfaces are: ::
+
+ interface BlockRead {
+ command error_t read(storage_addr_t addr, void* buf, storage_len_t len);
+ event void readDone(storage_addr_t addr, void* buf, storage_len_t len,
+ error_t error);
+
+ command error_t computeCrc(storage_addr_t addr, storage_len_t len,
+ uint16_t crc);
+ event void computeCrcDone(storage_addr_t addr, storage_len_t len,
+ uint16_t crc, error_t error);
+
+ command storage_len_t getSize();
+ }
+
+ interface BlockWrite {
+ command error_t write(storage_addr_t addr, void* buf, storage_len_t len);
+ event void writeDone(storage_addr_t addr, void* buf, storage_len_t len,
+ error_t error);
+
+ command error_t erase();
+ event void eraseDone(error_t error);
+
+ command error_t sync();
+ event void syncDone(error_t error);
+ }
+
+B.2 ConfigStorage interfaces
+------------------------------------------------------------------
+
+The ConfigStorage interfaces are: ::
+
+ interface Mount {
+ command error_t mount();
+ event void mountDone(error_t error);
+ }
+
+ interface ConfigStorage {
+ command error_t read(storage_addr_t addr, void* buf, storage_len_t len);
+ event void readDone(storage_addr_t addr, void* buf, storage_len_t len,
+ error_t error);
+
+ command error_t write(storage_addr_t addr, void* buf, storage_len_t len);
+ event void writeDone(storage_addr_t addr, void* buf, storage_len_t len,
+ error_t error);
+
+ command error_t commit();
+ event void commitDone(error_t error);
+
+ command storage_len_t getSize();
+ command bool valid();
+ }
+
+B.3 LogStorage interfaces
+------------------------------------------------------------------
+
+The LogStorage interfaces are: ::
+
+ interface LogRead {
+ command error_t read(void* buf, storage_len_t len);
+ event void readDone(void* buf, storage_len_t len, error_t error);
+
+ command storage_cookie_t currentOffset();
+
+ command error_t seek(storage_cookie_t offset);
+ event void seekDone(error_t error);
+
+ command storage_len_t getSize();
+ }
+
+ interface LogWrite {
+ command error_t append(void* buf, storage_len_t len);
+ event void appendDone(void* buf, storage_len_t len, bool recordsLost,
+ error_t error);
+
+ command storage_cookie_t currentOffset();
+
+ command error_t erase();
+ event void eraseDone(error_t error);
+
+ command error_t sync();
+ event void syncDone(error_t error);
+ }
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