User`s manual

24 rabbit.com Rabbit Memory Organization
5.1.2 SRAM
Static RAM may or may not be battery-backed. If the SRAM is battery-backed it retains its data when pri-
mary power is disconnected. SRAM chips typically used for Rabbit systems are 128 KB, 256 KB, 512 KB,
or 1 MB. With the configurable physical memory of the Rabbit 4000 and support in Dynamic C 10.21 and
later versions, static RAM chips of 1 MB and larger may also be used.
When the memory is battery-backed, power is supplied at 2 V to 3 V from a battery. While preserving
memory contents with battery power, the shutdown circuitry must keep the chip select line high.
5.1.3 Basic Memory Configuration
A basic Rabbit system typically contains two or three static memory devices: one flash memory device and
one or two RAM devices. Additional static memory devices may be added. If an application requires stor-
ing a lot of data in flash memory, it is recommended that a mass storage flash device be added such as
NAND or serial flash. Dynamic C contains drivers for both NAND and serial mass storage devices. Alter-
natively, another parallel flash memory device could be added, although these devices tend to be smaller
and more expensive and are not as suitable for larger amounts of data. Note that some board designs may
only contain a serial boot flash and SRAM. On these boards, the program is copied into the SRAM at boot
time from the serial flash. The program is then executed from static RAM.
Trying to use a single, parallel flash memory chip to store both a program and live data that must be fre-
quently changed can create software latency problems. When data is written to a small-sector flash mem-
ory, the memory is inoperative during the 5 to 20 ms that it takes to write a sector. If the same memory is
used to hold data and the program, then the execution of code must cease during this write time. The 5-20
ms is timed out by a small routine executing from root RAM while system interrupts are disabled, effec-
tively freezing the system for 5-20 ms. The 5-20 ms lockup period can adversely affect real-time opera-
tion.