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Chapter 2 CPU Basics
II - 42
Register Protection
2-9 Register Protection
2-9-1 Overview
This LSI features a function to protect important register data. When this function is enabled, data is
rewritten only when write is done for several times to a register and other write is disabled. Registers with
this function are as follows.
CPU mode control register (CPUM: x'03F00')
Memory control register (MEMCTR: x'03F01')
2-9-2 Setting of the Register Protection Function
Set the L0CKEN flag of the key control register (KEYCNT) to "1" to enable the register protection func-
tion.
Figure 2-9-1 Key Control Register (KEYCNT: x'03F2B', R/W)
2-9-3 Rewrite Procedure
Write 03 to the CPUM register:
LOOP MOV x'**', (CPUM)
MOV x'03', (CPUM)
CBNE x'03', (CPUM), LOOP
** indicates Don't care
xInterrupts may change the procedure of the program and disable sequence writes. Make
sure that write is done properly or disable interrupts during write.
xWrite to a register is executed even when several writes, which include access to RAM
area (x'00000' to x'02FFF) are done.
LOCKEN
0
Enable
Register protect function selection
Disable
01
24
567
3
(At reset: - - - - - - - 0)
KEYCNT
LOCKEN
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1