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CHAPTER 3 OPERATION
3.2.1 Reset Operation
The reset operation is described below.
■
Reset Operation
When reset conditions occur, the MB89990 series of microcontrollers suspend the currently-
executing instruction to enter the reset state. The contents written at the RAM do not change
before and after reset. However, if a reset occurs during writing of 16-bit long data, data is
written to the upper bytes and may not be written to lower bytes. If a reset occurs around write
timing, the contents of the addresses being written are not assured.
When the reset conditions are cleared, the MB89990 series of microcontrollers are released
from the reset state and start operation after fetching the mode data from address
FFFD
H
, the
upper bytes of the reset vectors from address
FFFE
H
, and the lower bytes from address
FFFF
H
,
in that order. Figure 3.2-1 "Outline of Reset Operation" shows the flowchart for the reset
operation.
Figure 3.2-1 Outline of Reset Operation
Figure 3.2-2 "Reset Vector Structure" indicates the structure of data to be stored in addresses
FFFDH, FFFEH, and FFFFH.
Figure 3.2-2 Reset Vector Structure
Reset clear
Execute the next instruction.
Fetch reset vectors from addresses
FFFE
H
and FFFF
H.
Fetch instruction codes from reset
vectors and execute the instruction.
Fetch mode data from address FFFDH.
Lower 8 bits of reset vector
Upper 8 bits of reset vector
Mode data
FFFF
H
FFFEH
FFFDH
Enter the address where the instruction, which will be executed first
after reset is cleared, is stored.
Reserved; alwa
y
s set 0.
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