NSAM265SR Datasheet, PDF (7/58 Pages) National Semiconductor (TI) – CompactSPEECH Digital Speech Processors

1 0 Theory of Operation (Continued)
When a message is created with the WRAM command the
data to be recorded is provided by the microcontroller and
not the codec The data is transferred directly to the mes-
sage memory It is not compressed by the Compact-
SPEECH voice compression algorithm
The WRAM command together with the RRAM (Read
RAM) command which enables the microcontroller to read
data from the CompactSPEECH can be used to store data
other than compressed voice in the message memory For
example in the NSAM265SF the AFLASH memory can be
used to store a telephone directory
A message can be played back (P command) and deleted
(DM command) Redundant data (e g trailing tones or si-
lence) can be removed from the message tail with the CMT
(Cut Message Tail) command
The PA and RES (Resume) commands respectively tem-
porarily suspend the P and R commands and then allow
them to resume execution from where they were suspend-
ed
Current Message
Most message handling commands e g P DM RRAM op-
erate on the current message The GTM (Get Tagged Mes-
sage) command selects the current message
Deleting the current message does not cause a different
message to become current The current message is unde-
fined If however you issue the GTM command to skip to the
next message the first message that is newer than the just
deleted message will be selected as the current message
1 5 1 Message Tag
Each message has a 2-byte message tag which you can
use to categorize messages and implement such features
as OutGoing Messages mailboxes and different handling
of old and new messages
In the NSAM265SR bits 0–6 are application definable bits
7 – 15 are reserved
In the NSAM265SF bits 0–14 are application definable bit
15 is the MESSAGE SAFE bit The MESSAGE SAFE bit
should be used to record safe (non-volatile) messages (e g
OGMs) when the NSAM265SF is configured to memory-in-
tensive mode (see Section 1 7 3)
For memory-management reasons the NSAM265SF must
keep at least one FLASH block without safe messages If
there is no such block available during recording of a safe
message or when the microcontroller tries to create a new
message with the R or WRAM command recording stops
and the EV MEMFULL bit in the status word is set
The GMT (Get Message Tag) and SMT commands may be
used to handle message tags
Note For the NSAM265SF message tag bits can only be cleared Message
tag bits are set only when a message is first created This limitation is
inherent in FLASH memories which only allow bits to be changed
from 1 to 0 (changing bits from 0 to 1 requires a special erasure
procedure see Section 1 7 1) However the main reason for updating
an existing tag is to mark a message as old and this can be done by
using one of the bits as a new old indicator setting it to 1 when a
message is first created and clearing it when necessary
1 6 ARAM SUPPORT
The NSAM265SR supports up to two 4-Mbit (1M c 4)
ARAM devices or one 16-Mbit (4M x 4) device for storing
messages An ARAM device is actually a bad DRAM device
i e it may contain bad bits The NSAM265SR can use such
devices for message recording without noticeable effect on
voice quality if they conform to the specifications described
in Section 2 6
After an ARAM mapping process (see the AMAP command)
which marks bad ARAM rows which can not be used for
recording the NSAM265SR can use the rest of the ARAM
space for message recording A single 1-Mbit c 4 ARAM
device holds an average of 15 minutes of recording time
(actual time may vary because of environmental conditions
e g speech attributes background noise etc )
1 7 FLASH SUPPORT
The NSAM265SF CompactSPEECH supports 4-Mbit and
8-Mbit byte wide AFLASH devices for storing messages
A FLASH device is organized in 64 Kbytes blocks An
AFLASH device is a FLASH device with one or more bad
blocks which can not be used for message recording The
NSAM265SF can use such devices for message recording
without any affect on voice quality if they conform to the
specifications described in Section 2 9
There are two major problems imposed by current FLASH
technology block erasure time and FLASH endurance
Both these limitations are handled by the CompactSPEECH
firmware
1 7 1 Block Erasure
The basic software interface to a DRAM device includes
read and write operations Writing a value to a memory loca-
tion simply replaces its contents
In a FLASH environment an erase operation is also re-
quired You must ensure that a memory location which was
previously written is erased prior to writing The basic unit
that can be read or written is a byte the basic unit that can
be erased is an entire 64 Kbytes block
Block erasure takes time The following erasure times are
quoted from AMD and INTEL datasheets for devices sup-
ported by the NSAM265SF
INTEL 28F008SA 1 6 sec (typical) 10 sec (max)
AMD
AM29F040 1 5 sec (min)
30 sec (max)
A FLASH memory can not be written while erasure is in
progress During erasure access to the FLASH is not al-
lowed The CompactSPEECH however accepts com-
mands which do not require FLASH access (e g Get
Status) during erasure
1 7 2 Flash Endurance
FLASH memories may be erased a limited number of times
Currently FLASH manufacturers do not guarantee more
than 100 000 erase cycles
To reduce the effect of this limitation the memory manager
utilizes FLASH blocks evenly i e each block is erased
more or less the same number of times to ensure that all
blocks have the same lifetime
Consider the following extensive usage of all FLASH blocks
1 Record 15 minutes of messages (until the memory is full)
2 Playback 15 minutes (all the recorded messages)
3 Delete all messages
Assuming a 4-Mbit FLASH device is used in this manner 24
times a day the expected life time of the FLASH is
Flash Lifetime e 100 000 (24 365) e 11 4 years
Thus the FLASH device will last for over ten years even
when used for six hours of recording per day
Note that if two 4-Mbit devices are used then under the
same conditions each device will last for more than 20
years
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