s908qc16g0cdte Freescale Semiconductor, Inc, s908qc16g0cdte Datasheet - Page 146

• Manufacturer Part Number: s908qc16g0cdte
• Description: M68hc08 Family Of 8-bit Microcontroller Units Mcus
• Manufacturer: Freescale Semiconductor, Inc

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15.3.2 Security
A security feature discourages unauthorized reading of ROM 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.
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 ROM locations and execute code from ROM. Security remains bypassed
until a power-on reset occurs. If the reset was not a power-on reset, security remains bypassed and
security code entry is not required. See
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 ROM location returns an invalid value and trying to execute code from ROM 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.
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 ROM 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.
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Notes:
1 = Echo delay, approximately 2 bit times
2 = Data return delay, approximately 2 bit times
3 = Wait 1 bit time before sending next byte
4 = Wait until clock is stable and monitor runs
Do not leave locations $FFF6–$FFFD blank. For security reasons, program
locations $FFF6–$FFFD even if they are not used for vectors.
The MCU does not transmit a break character until after the host sends the
eight security bytes.
V
RST
PA0
DD
Figure 15-16. Monitor Mode Entry Timing
FROM HOST
MC68HC08QY/QT Family Data Sheet, Rev. 2
4096 + 32 CGMXCLK CYCLES
FROM MCU
4
Figure
15-16.
NOTE
NOTE
1
3
1
1
2
Freescale Semiconductor
3
1