Data Sheet
FlashFlex51 MCU
SST89E52RD2/RD / SST89E54RD2/RD / SST89E58RD2/RD
SST89V52RD2/RD / SST89V54RD2/RD / SST89V58RD2/RD
13
©2006 Silicon Storage Technology, Inc. S71255-05-000 5/06
3.2 Program Memory Block Switching
The program memory block switching feature of the device allows either Block 1 or the lowest 8 KByte of Block 0 to be
used for the lowest 8 KByte of the program address space. SFCF[1:0] controls program memory block switching.
3.2.1 Reset Configuration of Program Memory
Block Switching
Program memory block switching is initialized after reset
according to the state of the Start-up Configuration bit SC0
and/or SC1. The SC0 and SC1 bits are programmed via
an external host mode command or an IAP Mode com-
mand. See Table 4-2.
Once out of reset, the SFCF[0] bit can be changed dynam-
ically by the program for desired effects. Changing SFCF[0]
will not change the SC0 bit.
Caution must be taken when dynamically changing the
SFCF[0] bit. Since this will cause different physical memory
to be mapped to the logical program address space. The
user must avoid executing block switching instructions
within the address range 0000H to 1FFFH.
3.3 Data RAM Memory
The data RAM has 1024 bytes of internal memory. The
RAM can be addressed up to 64KB for external data
memory.
3.4 Expanded Data RAM Addressing
The SST89E/V5xRDx both have the capability of 1K of
RAM. See Figure 3-4.
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 3.6,
“Special Function Registers”)
Since the upper 128 bytes occupy the same addresses as
the SFRs, the RAM must be accessed indirectly. 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.
TABLE 3-1: SFCF Values for Program Memory Block Switching
SFCF[1:0] Program Memory Block Switching
10, 11 Block 1 is not visible to the PC;
Block 1 is reachable only via in-application programming from E000H - FFFFH.
01 Both Block 0 and Block 1 are visible to the PC.
Block 0 is occupied from 0000H - 7FFFH. Block 1 is occupied from E000H - FFFFH.
00 Block 1 is overlaid onto the low 8K of the program address space; occupying address locations 0000H - 1FFFH.
When the PC falls within 0000H - 1FFFH, the instruction will be fetched from Block 1 instead of Block 0.
Outside of 0000H - 1FFFH, Block 0 is used. Locations 0000H - 1FFFH of Block 0 are reachable through
in-application programming.
T3-1.0 1255
TABLE 3-2: SFCF Values Under Different Reset
Conditions
SC1
1
1. P = Programmed (Bit logic state = 0),
U = Unprogrammed (Bit logic state = 1)
SC0
1
State of SFCF[1:0] after:
Power-on
or
External
Reset
WDT Reset
or
Brown-out
Reset
Software
Reset
U (1) U (1) 00
(default)
x0 10
U (1) P (0) 01 x1 11
P (0) U (1) 10 10 10
P (0) P (0) 11 11 11
T3-2.0 1255
