AT89C51ID2-RDTIM datasheet(29/159 Pages) ATMEL

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AT89C51ID2

4289A–8051–09/03

Expanded RAM

(XRAM)

The AT89C51ID2 provides additional Bytes of random access memory (RAM) space for

increased data parameter handling and high level language usage.

AT89C51ID2 devices have expanded RAM in external data space configurable up to

1792bytes (see Table 23.).

The AT89C51ID2 has internal data memory that is mapped into four separate

segments.

The four segments are:

The Lower 128 bytes of RAM (addresses 00h to 7Fh) are directly and indirectly

addressable.

The Upper 128 bytes of RAM (addresses 80h to FFh) are indirectly addressable

only.

The Special Function Registers, SFRs, (addresses 80h to FFh) are directly

addressable only.

The expanded RAM bytes are indirectly accessed by MOVX instructions, and

with the EXTRAM bit cleared in the AUXR register (see Table 23).

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 7. Internal and External Data Memory Address

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).

•

Instructions that use indirect addressing access the Upper 128 bytes of data RAM.

For example: MOV @R0, # data where R0 contains 0A0h, accesses the data byte

at address 0A0h, rather than P2 (whose address is 0A0h).

•

The XRAM bytes can be accessed by indirect addressing, with EXTRAM bit cleared

and MOVX instructions. This part of memory which is physically located on-chip,

logically occupies the first bytes of external data memory. The bits XRS0 and XRS1

are used to hide a part of the available XRAM as explained in Table 23. This can be

XRAM

Upper

128 bytes

Internal

Ram

Lower

128 bytes

Internal

Ram

Special

Function

80h

0FFh to 6FFh

0FFh

External

Data

Memory

0000

00FFh up to 06FFh

0FFFFh

indirect accesses

direct accesses

direct or indirect

accesses

7Fh

AT89C51ID2-RDTIM Datasheet(PDF) 29 Page - ATMEL Corporation

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