M68HC08 datasheet(136/332 Pages) FREESCALE

136 / 332 page

Monitor ROM (MON)

Technical Data

MC68HC908JB16 — Rev. 1.1

136

Monitor ROM (MON)

Freescale Semiconductor

Upon power-on reset, if the received bytes of the security code do not

match the data at locations $FFF6–$FFFD, the host fails to bypass the

security feature. The MCU remains in monitor mode, but reading a

FLASH location returns an invalid value and trying to execute code from

FLASH causes an illegal address reset. After receiving the eight security

bytes from the host, the MCU transmits a break character, signifying that

it is ready to receive a command.

NOTE:

The MCU does not transmit a break character until after the host sends

the eight security bytes.

To determine whether the security code entered is correct, check to see

if bit 6 of RAM address $80 is set. If it is, then the correct security code

has been entered and FLASH can be accessed.

If the security sequence fails, the device should be reset by a power-on

reset and brought up in monitor mode to attempt another entry. After

failing the security sequence, the FLASH module can also be mass

erased by executing an erase routine that was downloaded into internal

RAM. The mass erase operation clears the security code locations so

that all eight security bytes become $FF (blank).

9.5.1 Extended Security

To further disable monitor mode functions, the monitor commands can

be disabled by writing $7B to the FLASH location $FFD1 and $87 to the

FLASH location $FFD0. Table 9-10 shows the security settings that

affect monitor mode operations.

Table 9-10. Monitor Mode Security

Extended Security

Monitor Mode Entry Security

Monitor Functions Available

NOT SET

BYPASSED

Read/write of RAM and FLASH.

FAILED

Read/write of RAM.

Read of FLASH disabled. FLASH can only

be mass erased.

SET

BYPASSED

Read/write of RAM and FLASH disabled.

FAILED

Read/write of RAM.

Read of FLASH disabled. FLASH can only

be mass erased.

M68HC08 Datasheet(PDF) 136 Page - Freescale Semiconductor, Inc

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