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Flash Memory Operation
5-14
Flash Memory Controller
5.3.4 Flash Memory Access During Write or Erase
When any write or any erase operation is initiated from RAM and while
BUSY=1, the CPU may not read or write to or from any flash location.
Otherwise, an access violation occurs, ACCVIFG is set, and the result is
unpredictable. Also if a write to flash is attempted with WRT=0, the ACCVIFG
interrupt flag is set, and the flash memory is unaffected.
When a byte/word write or any erase operation is initiated from within flash
memory, the flash controller returns op-code 03FFFh to the CPU at the next
instruction fetch. Op-code 03FFFh is the JMP PC instruction. This causes the
CPU to loop until the flash operation is finished. When the operation is finished
and BUSY=0, the flash controller allows the CPU to fetch the proper op-code
and program execution resumes.
The flash access conditions while BUSY=1 are listed in Table 5−3.
Table 5−3.Flash Access While BUSY = 1
Flash
Operation
Flash
Access
WAIT Result
Read 0 ACCVIFG = 0. 03FFFh is the value read
Any erase, or
Byte/word write
Write 0 ACCVIFG = 1. Write is ignored
Any erase, or
Byte/word write
Instruction
fetch
0 ACCVIFG = 0. CPU fetches 03FFFh. This
is the JMP PC instruction.
Any 0 ACCVIFG = 1, LOCK = 1
Read 1 ACCVIFG = 0, 03FFFh is the value read
Block write
Write 1 ACCVIFG = 0, Write is ignored
Instruction
fetch
1 ACCVIFG = 1, LOCK = 1
All interrupt sources should be disabled before initiating any flash operation.
If an enabled interrupt were to occur during a flash operation, the CPU would
fetch 03FFFh as the address of the interrupt service routine. The CPU would
then execute the JMP PC instruction while BUSY=1. When the flash operation
finished, the CPU would begin executing code at address 03FFFh, not the
correct address for interrupt service routine.