MC68HC908RK2 Datasheet, PDF (145/158 Pages) Motorola, Inc – Microcontroller Unit

Monitor Module
HIGH BYTE OF INDEX REGISTER
CONDITION CODE REGISTER
ACCUMULATOR
LOW BYTE OF INDEX REGISTER
HIGH BYTE OF PROGRAM COUNTER
LOW BYTE OF PROGRAM COUNTER
SP
SP + 1
SP + 2
SP + 3
SP + 4
SP + 5
SP + 6
SP + 7
Figure 12-12. Stack Pointer at Monitor Mode Entry
12.3.1.6 Baud Rate
With a 9.8304-MHz crystal, data is transferred between the monitor and host at 9600 baud. If a
14.7456-MHz crystal is used, the monitor baud rate is 9600.
NOTE
While in monitor mode with V_TST applied to IRQ, the MCU bus clock is
always driven from the external clock.
12.3.2 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 consecutive 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. If FLASH is
unprogrammed, the eight security byte values to be sent are $00, the
unprogrammed state of the FLASH.
During monitor mode entry, the MCU waits after the power-on reset for the host to send the eight security
bytes on pin PA0.
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
reset occurs. After the host bypasses security, any reset other than a power-on reset requires the host to
send another eight bytes, but security remains bypassed regardless of the data that the host sends.
If the received bytes 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 FLASH locations returns undefined data,
and trying to execute code from FLASH causes an illegal address reset. After the host fails to bypass
security, any reset other than a power-on reset causes an endless loop of illegal address resets.
After receiving the eight security bytes from the host, the MCU transmits a break character signalling 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.
MC68HC908RK2 Data Sheet, Rev. 5.1
Freescale Semiconductor
145