EM78P257
OTP ROM
• Depending on the device type, R2 and hardware stack are 11-bits wide. The structure is depicted in
Fig. 4.
• Generates 2K´ 13 on-chip ROM addresses to the relative programming instruction codes. One
program page is 1K words long.
• R2 is set as all "0"s when under RESET condition.
• "JMP" instruction allows direct loading of the lower 10 program counter bits. Thus, "JMP" allows PC
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" ("RETL k", "RETI") instruction loads the program counter with the contents of the top-level
stack.
• "ADD R2,A" allows the contents of ‘ A’ to be added to the current PC, and the ninth and tenth bits of
the PC are cleared.
• "MOV R2,A" allows to load an address from the "A" register to the lower 8 bits of the PC, and the ninth
and tenth bits of the PC are cleared.
• Any instruction that is written to R2 (e.g. "ADD R2,A", "MOV R2,A", "BC R2,6",×××××) will cause the
ninth and tenth bits (A8~A9) of the PC to be cleared. Thus, the computed jump is limited to the first
256 locations of a page.
• In case of EM78P257A/B, the second most significant bit(A10) will be loaded with the content of bit
PS0 in the status register (R3) upon the execution of a "JMP", "CALL", or any other instructions
which write to R2.
• All instructions are single cycle (fclk/2 or fclk/4), except for the instructions that would change the
contents of R2. This instruction will need one more instruction cycle.
This specification is subject to change without prior notice.
11
07.27.2004 (V1.4)
ELAN [ ELAN MICROELECTRONICS CORP ]
