EM78451AP datasheet(8/54 Pages) EMC

8 / 54 page

EM78451

8-Bit Microcontroller

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Product Specification (V1.2) 05.27.2004

(This specification is subject to change without further notice)

4.1 Operational Registers

4.1.1 R0 (Indirect Address Register)

R0 is not a physically implemented register. It is used as an indirect addressing

pointer. Any instruction using R0 as register actually accesses data pointed by the

RAM Select Register (R4).

4.1.2 R1 (TCC)

It is increased by the instruction cycle clock.

Written and read by program as any other register.

4.1.3 R2 (Program Counter) & Stack

R2 and the hardware stacks are 12 bits wide.

The structure is depicted in Fig. 3.

Generates 4K

× 13 on-chip ROM addresses to the relative programming instruction

codes. One program page is 1024 words long.

All the R2 bits are set to "1"s as RESET condition occurs.

"JMP" instruction allows direct loading of the lower 10 program counter bits. Thus,

"JMP" allows it to jump to any location on one page.

"CALL" instruction loads the lower 10 bits of the PC, and then PC+1 is pushed into

the stack. Thus, the subroutine entry address can be located anywhere within a

page.

"RET" ("RETL k", "RETI") instruction loads the program counter with the contents

at the top of stack.

"MOV R2, A" allows the loading of an address from the "A" register to the lower 8

bits of PC, and the ninth and tenth bits (A8~A9) of PC are cleared.

"ADD R2, A" allows a relative address to be added to the current PC, and the ninth

and tenth bits of PC are cleared.

Any instruction that is written to R2 (e.g. "ADD R2, A", "MOV R2, A", "BC R2,6",

⋅⋅⋅⋅⋅)

(except "TBL") will cause the ninth and tenth bits (A8~A9) of PC to be cleared.

Thus, the computed jump is limited to the first 256 locations of any program page.

"TBL" allows a relative address to be added to the current PC (R2+A

→R2), and

contents of the ninth and tenth bits (A8~A9) of PC are not changed. Thus, the

computed jump can be on the second (or third, 4th) 256th locations on one

program page.

EM78451AP Datasheet(PDF) 8 Page - ELAN Microelectronics Corp