MC68HC908MR8 Datasheet, PDF (196/372 Pages) Motorola, Inc – Microcontrollers

Monitor ROM (MON)
10.5 Security
A security feature discourages unauthorized reading of ROM locations
while in monitor mode. The host can satisfy 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 even if they are not used for vectors.
During monitor mode entry, the MCU waits after the power-on reset for
the host to send the eight security bytes on pin PTB0 (R_XD). If the
received bytes match those at locations $FFF6–$FFFD, the host
satisfies the security feature and can read all ROM locations and
execute code from ROM. Security remains satisfied until a power-on
reset occurs. If the reset was not a power-on reset, security remains
satisfied and security code entry is not required. See Figure 10-6.
V_DD
4096 + 32 CGMXCLK CYCLES
RST
24 BUS CYCLES
PA7
256 BUS CYCLES (MINIMUM)
FROM HOST
PA0
1
4
1
1
2
4
1
FROM MCU
Technical Data
196
Notes:
1 = Echo delay, 2 bit times
2 = Data return delay, 2 bit times
4 = Wait 1 bit time before sending next byte.
Figure 10-6. Monitor Mode Entry Timing
Monitor ROM (MON)
MC68HC908MR8 — Rev 4.1
Freescale Semiconductor