T89C51RD2-3CSIM View Datasheet(PDF) - Atmel Corporation
Part Name
Description
MFG CO.
T89C51RD2-3CSIM Datasheet PDF : 104 Pages
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T89C51RD2
Expanded RAM (XRAM)
The T89C51RD2 provide additional Bytes of random access memory (RAM) space for
increased data parameter handling and high level language usage.
T89C51RD2 devices have expanded RAM in external data space; Maximum size and
location are described in Table 6.
Table 6. Description of Expanded RAM
Port
T89C51RD2
1024
XRAM size
Start
00h
Address
End
3FFh
The T89C51RD2 has internal data memory that is mapped into four separate segments.
The four segments are:
• 1. The Lower 128 bytes of RAM (addresses 00H to 7FH) are directly and indirectly
addressable.
• 2. The Upper 128 bytes of RAM (addresses 80H to FFH) are indirectly addressable
only.
• 3. The Special Function Registers, SFRs, (addresses 80H to FFH) are directly
addressable only.
• 4. The expanded RAM bytes are indirectly accessed by MOVX instructions, and
with the EXTRAM bit cleared in the AUXR register. (See Table 7.)
The Lower 128 bytes can be accessed by either direct or indirect addressing. The Upper
128 bytes can be accessed by indirect addressing only. The Upper 128 bytes occupy
the same address space as the SFR. That means they have the same address, but are
physically separate from SFR space.
Figure 4. Internal and External Data Memory Address
FF or 3FF
FF
FF
FFFF
XRAM
Upper
128 bytes
Internal
Ram
indirect accesses
80
Special
Function
Register
direct accesses
80
External
Data
Memory
Lower
128 bytes
Internal
Ram
direct or indirect
accesses
00
00
0100 or 0400
0000
When an instruction accesses an internal location above address 7FH, the CPU knows
whether the access is to the upper 128 bytes of data RAM or to SFR space by the
addressing mode used in the instruction.
• Instructions that use direct addressing access SFR space. For example: MOV
0A0H, # data ,accesses the SFR at location 0A0H (which is P2).
15
4243G–8051–05/03
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