M68HC08 datasheet(135/332 Pages) FREESCALE

135 / 332 page

Monitor ROM (MON)

MC68HC908JB16 — Rev. 1.1

Technical Data

Freescale Semiconductor

Monitor ROM (MON)

135

9.5 Security

A security feature discourages unauthorized reading of FLASH locations

while in monitor mode. The host can bypass the security feature at

monitor mode entry by sending eight security bytes that match the bytes

at locations $FFF6–$FFFD. Locations $FFF6–$FFFD contain user-

defined data.

NOTE:

Do not leave locations $FFF6–$FFFD blank. For security reasons,

program locations $FFF6–$FFFD.

During monitor mode entry, the MCU waits after the power-on reset for

the host to send the eight security bytes on pin PTA0. If the received

bytes match those at locations $FFF6–$FFFD, the host bypasses the

security feature and can read all FLASH locations and execute code

from FLASH. Security remains bypassed until a power-on or an LVI

reset occurs. If the reset was not a power-on or an LVI reset, security

remains bypassed and security code entry is not required.

(See Figure 9-8.)

Figure 9-8. Monitor Mode Entry Timing

PTA0

RST

VDD

4096 + 32 OSCDCLK CYCLES

256 BUS CYCLES (MINIMUM)

NOTES:

2 = Data return delay, 2 bit times.

4 = Wait 1 bit time before sending next byte.

FROM HOST

FROM MCU

1 = Echo delay, 2 bit times.

M68HC08 Datasheet(PDF) 135 Page - Freescale Semiconductor, Inc