MC68HC908GT16 Datasheet, PDF (224/412 Pages) Motorola, Inc – Microcontrollers

Monitor ROM (MON)
V_DD
4096 + 32 CGMXCLK CYCLES
RST
FROM HOST
PA0
256 BUS CYCLES
(MINIMUM)
FROM MCU
1
4
1
1
2
4
1
Notes:
1 = Echo delay, 2 bit times
2 = Data return delay, 2 bit times
4 = Wait 1 bit time before sending next byte.
Figure 15-10. Monitor Mode Entry Timing
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 $40 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).
Technical Data
224
MC68HC908GT16 • MC68HC908GT8 — Rev. 2
Monitor ROM (MON)
MOTOROLA