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EM78P159NAS

8-Bit Microcontroller with OTP ROM

ELAN Microelectronics 

EM78P159N

8-Bit Microcontroller with OTP ROM

4.1.3 R2 (Program Counter) & Stack

„ Depending on the device type, R2 and hardware stack are 10-bit wide. The

structure is depicted in the following figure.

PC (A9 ~ A0)

Reset Vector

Interrupt Vector

000H

008H

Stack Level 1

Stack Level 2

Stack Level 3

Stack Level 4

Stack Level 5

On-chip Program

Memory

3FFH

Figure 4-2 Program Counter Organization

„ Generating 1024×13 bits on-chip OTP ROM addresses to the relative

programming instruction codes. One program page is 1024 words long.

„ R2 is set as all "0" when under RESET condition.

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

Thus, "JMP" allows PC to go to any location within a 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" ("RETLk", "RETI") instruction loads the program counter with the contents

of the top-level stack.

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

and succeeding bits of the PC will increase progressively.

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

the PC, and the ninth and tenth bits (A8 ~ A9) of the PC will remain unchanged.

„ Any instruction (except “ADD R2,A”) that is written to R2 (e.g., "MOV R2, A", "BC

R2, 6",⋅⋅⋅⋅⋅) will cause the ninth bit and the tenth bit (A8 ~ A9) of the PC to remain

unchanged.

„ All instructions are single instruction cycle (fclk/2 or fclk/4) except for the

instruction that would change the contents of R2. Such instruction will need one

more instruction cycle.

Product Specification (V1.0) 03.10.2006

•7

(This specification is subject to change without further notice)

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