UPD17708 datasheet(312/356 Pages) NEC

312 / 356 page

312

µPD17704, 17705, 17707, 17708, 17709

Example 2.

MEM

0.01H

; 1st digit of key input data of security code

MEM

0.02H

; 2nd digit of key input data of security code

MEM

0.03H

; 1st digit data when security code is changed

MEM

0.04H

; 2nd digit data when security code is changed

MEM

0.11H

; 1st digit of current security code

MEM

0.12H

; 2nd digit of current security code

CHANGE

FLG

0.13H.0

; “1” while security code is changed

START:

Key input processing

← contents of key A

; Security code input wait mode

← contents of key B

; Substitutes contents of pressed key to R1 and R2

SKT1

CHANGE

; <4> ; If CHANGE flag is “1”

SECURITY_CHK

M1, R3

; Rewrites M1 and M2

M2, R4

CLR1

CHANGE

SECURITY_CHK:

SET2

CMP, Z

; <1> ; Compares security code and input data

SUB

R1, M1

SUB

R2, M2

SKT1

ERROR

; Input data differs from security code

MAIN:

Key input processing

← contents of key C

; Security code rewriting mode

← contents of key D

; Substitutes contents of pressed key to R3 and R4

SET1

CHANGE

; <5> ; Until security code is changed,

; Sets CHANGE flag to “1”

M1, R3

; <2> ; Rewrites security code

M2, R4

; <3>

CLR1

CHANGE

; If security code has been changed,

; Sets CHANGE flag to “0”

MAIN

ERROR:

Must not operate

The program in Example 2 sets the CHANGE flag to “1” in <5> before the security code is rewritten in <2> and

<3>.

Therefore, even if CE reset is effected before <3> is executed, the security code is rewritten in <4>.

UPD17708 Datasheet(PDF) 312 Page - NEC

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