AT89C51RC2-3CSIM datasheet(24/127 Pages) ATMEL

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AT89C51RB2/RC2

4180C–8051–12/03

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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).

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

and MOVX instructions. This part of memory that 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 18. This can be

useful if external peripherals are mapped at addresses already used by the internal

XRAM.

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With EXTRAM = 0, the XRAM is indirectly addressed, using the MOVX instruction in

combination with any of the registers R0, R1 of the selected bank or DPTR. An

access to XRAM will not affect ports P0, P2, P3.6 (WR) and P3.7 (RD). For

example, with EXTRAM = 0, MOVX @R0, # data where R0 contains 0A0H,

accesses the XRAM at address 0A0H rather than external memory. An access to

external data memory locations higher than the accessible size of the XRAM will be

performed with the MOVX DPTR instructions in the same way as in the standard

80C51, with P0 and P2 as data/address busses, and P3.6 and P3.7 as write and

read timing signals. Accesses to XRAM above 0FFH can only be done by the use of

DPTR.

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With EXTRAM = 1, MOVX @RI and MOVX @DPTR will be similar to the standard

80C51. MOVX @ Ri will provide an eight-bit address multiplexed with data on Port0

and any output port pins can be used to output higher order address bits. This is to

provide the external paging capability. MOVX @DPTR will generate a sixteen-bit

address. Port2 outputs the high-order eight address bits (the contents of DPH) while

Port0 multiplexes the low-order eight address bits (DPL) with data. MOVX @ RI and

MOVX @DPTR will generate either read or write signals on P3.6 (WR) and P3.7

(RD).

The stack pointer (SP) may be located anywhere in the 256 Bytes RAM (lower and

upper RAM) internal data memory. The stack may not be located in the XRAM.

The M0 bit allows to stretch the XRAM timings; if M0 is set, the read and write pulses

are extended from 6 to 30 clock periods. This is useful to access external slow

peripherals.

AT89C51RC2-3CSIM Datasheet(PDF) 24 Page - ATMEL Corporation