Digi RCM4000 - Manual (Page 45)

User’s Manual 39
4.6 Memory
4.6.1 SRAM
RCM4000 modules have 512K of data SRAM installed at U16.
4.6.2 Flash EPROM
All RCM4000 modules also have 512K of flash EPROM installed at U3.
NOTE: Rabbit Semiconductor recommends that any customer applications should not be
constrained by the sector size of the flash EPROM since it may be necessary to change
the sector size in the future.
Writing to arbitrary flash memory addresses at run time is discouraged. Instead, define a
“user block” area to store persistent data. The functions writeUserBlock and
readUserBlock
are provided for this. Refer to the Rabbit 4000 Microprocessor
Designer’s Handbook for additional information.
4.6.3 NAND Flash
The RCM4000 model has a NAND flash to store data and Web pages. The NAND flash is
particularly suitable for mass-storage applications, but is generally unsuitable for direct
program execution. The NAND flash differs from parallel NOR flash (the type of flash
memory used to store program code on Rabbit-based boards and RabbitCore modules
currently in production) in two respects. First, the NAND flash requires error-correcting
code (ECC) for reliability. Although NAND flash manufacturers do guarantee that block 0
will be error-free, most manufacturers guarantee that a new NAND flash chip will be
shipped with a relatively small percentage of errors, and will not develop more than some
maximum number or percentage of errors over its rated lifetime of up to 100,000 writes.
Second, the standard NAND flash addressing method multiplexes commands, data, and
addresses on the same I/O pins, while requiring that certain control lines must be held sta-
ble for the duration of the NAND flash access. The software function calls provided by
Rabbit Semiconductor for the NAND flash take care of the data-integrity and reliability
attributes.
Sample programs in the SAMPLES\RCM4000\NANDFlash folder illustrate the use of
the NAND flash. These sample programs are described in Section 3.2.1, “Use of NAND
Flash.”