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Evaluating and Programming the 29K RISC Family
DRAM memory is modelled with the RAM* modelling commands. A default
DRAM memory section is established at address 0x4000,0000. Unless a
ROMBANK command is used to allocate a ROM/SRAM memory bank at this
address range, all accesses to high memory will be satisfied by the default DRAM
memory.
The default linker command files used with the High C 29K tool chain, typically
links programs for execution according the the above default memory regions. How-
ever, older release of the compiler tool chain (or other tool chains) may link for differ-
ent memory models. This would require the use of RAMBANK–type commands to
establish the correct memory model. Alternatively, a compiler command file could
be used to ensure a program is linked for the default simulator memory mode (see
section 2.3.6).
Am29200 and Am29205
The simulator does not maintain different memory access parameters for
instruction and data access when modeling microcontroller members of the 29K
family. However, it does support separate memory modeling parameters for DRAM
and ROM address regions (see Table 1-11). Each of these two memory regions has its
own memory controller supporting up to four banks. A bank is a contiguous range of
memory within the address range accessed via the region controller. The DRAM re-
gion controller is a little more complicated than the ROM region controller. The pa-
rameters shown in Table 1-11 are for the older simulator, but they are accepted by the
new simulator. For a list of alternative parameters, which are only accepted by the
newer simulator, see the following Am29240 section.
The DRAM access is fixed at four cycles (1 for precharge + 3 for latency), it can
not be programmed. Subsequent accesses to the same page take four cycles unless
pagemode memories are supported. Note the first access is only three cycles rather
than four, as the
RAS
will already have met the precharge time. Basically, to prechar-
ge the
RAS
bit lines, all
RAS
lines need to be taken high in between each change of the
row addresses. A separate cycle is needed for precharge when back–to–back DRAM
accesses occurs. Use of pagemode memories is indicated by the PAGEMODE pa-
rameter being set; when used, the processor need not supply
RAS
memory strobe sig-
nals before page
CAS
strobes for same page accesses. This reduces subsequent page
access latency to three cycles. Additionally, when pagemode is used and a data burst
is attempted within a page, access latency is two cycles. The DRAM memory width
can be set to 16 or 32–bits. Of course when an Am29205 is used, all data memory
accesses are restricted by the 16–bit width of the processor data bus.
To explain further, access times to DRAM for none pagemode memories follow
the sequence:
X,3,4,4,4,X,3,4,4,4,X,X,3,X,3,...