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Chapter 4 Memory
MC9S08JS16 MCU Series Reference Manual, Rev. 4
Freescale Semiconductor
45
4.5.2
Modes of Operation
After any reset, the MCU jumps to bootloader ROM address, where several qualification factors are
examined to determine whether to jump to the bootloader code or to the user code. The bootloader ROM
can be accessed in bootloader mode or user mode. This section describes the valid operations and
protection mechanism in these two modes.
The following four items will be examined after each reset of the MCU.
•
BLMS pin
•
SIGNATURE semaphore byte
•
Flash block CRC checksum
•
CRC BYPASS byte
4.5.2.1
Bootloader Mode
Bootloader mode can be entered in the following 4 conditions:
1. When BLMS pin is low and BKGD/MS is not pulled low during power-on-reset (POR), the
bootloader mode is entered directly with no other qualifications.
2. When BLMS pin and BKGD/MS are high during power-on-reset (POR), a CHECKSUM BYPASS
flash location is examined. If it is not equal to 0x00 or 0xFF, then the bootloader mode is entered.
3. When BLMS pin and BKGD/MS are high during power-on-reset (POR), a CHECKSUM BYPASS
flash location is examined. If it is equal to 0xFF, a flash CRC is calculated for the flash array and
compared with a FLASHCRC 16-bit word. If the result does not match, then the bootloader mode
is entered.
4. After a reset (other than a power-on reset), the SIGNATURE semaphore byte is examined. If it is
equal to 0xC3, then the bootloader mode is entered.
4.5.2.2
User mode
User mode can be entered in the following 3 conditions:
1. When BLMS pin and BKGD/MS are high during power-on-reset (POR), and the CHECKSUM
BYPASS byte is equal to 0x00, the user mode is entered.
2. When BLMS pin and BKGD/MS are high during power-on-reset (POR), and the CHECKSUM
BYPASS byte is equal to 0xFF, a flash CRC is calculated for the flash array and compared with a
FLASHCRC 16-bit word. If the result matches, the user mode is entered.
3. When a reset occurs (other than a power-on reset), if the SIGNATURE semaphore is not equal to
0xC3, the user mode is entered.
Table 4-5. Signal Properties
Signal
Function
I/O
BLMS
Bootloader mode selection
I
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