SST89E54RD2A(2011) View Datasheet(PDF) - Silicon Storage Technology
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SST89E54RD2A Datasheet PDF : 91 Pages
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A Microchip Technology Company
FlashFlex MCU
SST89E54RD2A/RDA / SST89E58RD2A/RDA
Expanded Data RAM Addressing
Not Recommended for New Designs
The SST89E/V5xRDxA both have the capability of 1K of RAM. See Figure 7.
The device has four sections of internal data memory:
1. The lower 128 Bytes of RAM (00H to 7FH) are directly and indirectly addressable.
2. The higher 128 Bytes of RAM (80H to FFH) are indirectly addressable.
3. The special function registers (80H to FFH) are directly addressable only.
4. The expanded RAM of 768 Bytes (00H to 2FFH) is indirectly addressable by the move
external instruction (MOVX) and clearing the EXTRAM bit. (See “Auxiliary Register
(AUXR)” in Section , “Special Function Registers”)
Since the upper 128 bytes occupy the same addresses as the SFRs, the RAM must be accessed indi-
rectly. The RAM and SFRs space are physically separate even though they have the same addresses.
When instructions access addresses in the upper 128 bytes (above 7FH), the MCU determines
whether to access the SFRs or RAM by the type of instruction given. If it is indirect, then RAM is
accessed. If it is direct, then an SFR is accessed. See the examples below.
Indirect Access:
MOV@R0, #data; R0 contains 90H
Register R0 points to 90H which is located in the upper address range. Data in “#data” is written to
RAM location 90H rather than port 1.
Direct Access:
MOV90H, #data; write data to P1
Data in “#data” is written to port 1. Instructions that write directly to the address write to the SFRs.
To access the expanded RAM, the EXTRAM bit must be cleared and MOVX instructions must be used.
The extra 768 bytes of memory is physically located on the chip and logically occupies the first 768
bytes of external memory (addresses 000H to 2FFH).
When EXTRAM = 0, the expanded RAM is indirectly addressed using the MOVX instruction in combi-
nation with any of the registers R0, R1 of the selected bank or DPTR. Accessing the expanded RAM
does not affect ports P0, P3.6 (WR#), P3.7 (RD#), or P2. With EXTRAM = 0, the expanded RAM can
be accessed as in the following example.
Expanded RAM Access (Indirect Addressing only):
MOVX@DPTR, A; DPTR contains 0A0H
DPTR points to 0A0H and data in “A” is written to address 0A0H of the expanded RAM rather than
external memory. Access to external memory higher than 2FFH using the MOVX instruction will
access external memory (0300H to FFFFH) and will perform in the same way as the standard 8051,
with P0 and P2 as data/address bus, and P3.6 and P3.7 as write and read timing signals.
When EXTRAM = 1, MOVX @Ri and MOVX @DPTR will be similar to the standard 8051. Using
MOVX @Ri provides an 8-bit address with multiplexed data on Port 0. Other output port pins can be
used to output higher order address bits. This provides external paging capabilities. Using MOVX
©2011 Silicon Storage Technology, Inc.
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