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Memory
2.2.2 CPU Memory Space
XDATA memory space. The XDATA memory map is given in
The SRAM is mapped into address range of 0x0000 through (SRAM_SIZE
–
1).
The XREG area is mapped into the 1-KB address range (0x6000
–
0x63FF). These registers are additional
registers, effectively extending the SFR register space. Some peripheral registers and most of the radio
control and data registers are mapped in here.
The SFR registers are mapped into address range (0x7080
–
0x70FF).
The flash information page (2 KB) is mapped into the address range (0x7800
–
0x7FFF). This is a read-only
area and contains various information about the device.
The upper 32 KB of the XDATA memory space (0x8000
–
0xFFFF) is a read-only flash code bank (XBANK)
and can be mapped to any of the available flash banks using the MEMCTR.XBANK[2:0] bits.
The mapping of flash memory, SRAM, and registers to XDATA allows the DMA controller and the CPU
access to all the physical memories in a single unified address space.
Writing to unimplemented areas in the memory map (shaded in the figure) has no effect. Reading from
unimplemented areas returns 0x00. Writes to read-only regions, i.e., flash areas, are ignored.
CODE memory space. The CODE memory space is 64 KB and is divided into a common area
(0x0000
–
0x7FFF) and a bank area (0x8000
–
0xFFFF) as shown in
. The common area is
always mapped to the lower 32 KB of the physical flash memory (bank 0). The bank area can be mapped
to any of the available 32-KB flash banks (from 0 to 7). The number of available flash banks depends on
the flash size option. Use the flash-bank-select register, FMAP, to select the flash bank. On 32-KB
devices, no flash memory can be mapped into the bank area. Reads from this region return 0x00 on these
devices.
To allow program execution from SRAM, it is possible to map the available SRAM into the lower range of
the bank area from 0x8000 through ( SRAM_SIZE
–
1). The rest of of the currently selected bank
is still mapped into the address range from ( SRAM_SIZE) through 0xFFFF). Set the
MEMCTR.XMAP
bit to enable this feature.
DATA memory space. The 8-bit address range of DATA memory is mapped into the upper 256 bytes of
the SRAM, i.e., the address range from (SRAM_SIZE
–
256) through (SRAM_SIZE
–
1).
SFR memory space. The 128-entry hardware register area is accessed through this memory space. The
SFR registers are also accessible through the XDATA address space at the address range
(0x7080
–
0x70FF). Some CPU-specific SFR registers reside inside the CPU core and can only be
accessed using the SFR memory space and not through the duplicate mapping into XDATA memory
space. These specific SFR registers are listed in
2.2.3 Physical Memory
RAM. All devices contain static RAM. At power on, the content of RAM is undefined. RAM content is
retained in all power modes.
Flash Memory. The on-chip flash memory is primarily intended to hold program code and constant data.
The flash memory has the following features:
•
Page size: 1 KB or 2 KB (details are given in the data sheet of the device.)
•
Flash-page erase time: 20 ms
•
Flash-chip (mass) erase time: 20 ms
•
Flash write time (4 bytes): 20
μ
s
•
Data retention (at room temperature): 100 years
•
Program/erase endurance: 20,000 cycles
The flash memory is organized as a set of 1- or 2-KB pages. The 16 bytes of the upper available page
30
8051 CPU
SWRU191C
–
April 2009
–
Revised January 2012
Copyright
©
2009
–
2012, Texas Instruments Incorporated