AT87251G2D-3CSUM【PDF 3/77 页】IC 8051 MCU 32K OTP 24MHZ 40DIP

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AT/TSC8x251G2D

4135F–8051–11/06

Address Spaces

The TSC80251G2D derivatives implement four different address spaces:

On-chip ROM program/code memory (not present in ROMless devices)

On-chip RAM data memory

Special Function Registers (SFRs)

Configuration array

Program/Code Memory

The TSC83251G2D and TSC87251G2D implement 32 KB of on-chip program/code

memory. Figure 4 shows the split of the internal and external program/code memory

spaces. If EA# is tied to a high level, the 32-Kbyte on-chip program memory is mapped

in the lower part of segment FF: where the C251 core jumps after reset. The rest of the

program/code memory space is mapped to the external memory. If EA# is tied to a low

level, the internal program/code memory is not used and all the accesses are directed to

the external memory.

The TSC83251G2D products provide the internal program/code memory in a masked

ROM memory while the TSC87251G2D products provide it in an EPROM memory. For

the TSC80251G2D products, there is no internal program/code memory and EA# must

be tied to a low level.

Figure 4.

Program/Code Memory Mapping

Note:

Special care should be taken when the Program Counter (PC) increments:

If the program executes exclusively from on-chip code memory (not from external mem-

ory), beware of executing code from the upper eight bytes of the on-chip ROM

(FF:7FF8h-FF:7FFFh). Because of its pipeline capability, the TSC80251G2D derivative

may attempt to prefetch code from external memory (at an address above FF:7FFFh)

and thereby disrupt I/O Ports 0 and 2. Fetching code constants from these 8 bytes does

not affect Ports 0 and 2.

When PC reaches the end of segment FF:, it loops to the reset address FF:0000h (for

On-chip ROM/EPROM

Code Memory

Program/code

Segments

Program/code

External Memory Space

32 KB

EA# = 0

EA# = 1

32 KB

Reserved

64 KB

128 KB

FF:FFFFh

FF:8000h

FF:7FFFh

FF:0000h

FE:FFFFh

FE:0000h

FD:FFFFh

01:FFFFh

01:0000h

02:0000h

00:FFFFh

00:0000h

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