Freescale Semiconductor MC68HC908MR16, MC68HC908MR32 Security, Monitor Baud Rate Selection

Monitor ROM (MON)

18.3.1.8 Baud Rate

With a 4.9152-MHz crystal and the PTC2 pin at logic 1 during reset, data is transferred between the monitor and host at 4800 baud. If the PTC2 pin is at logic 0 during reset, the monitor baud rate is 9600. See Table 18-9.

Table 18-9. Monitor Baud Rate Selection

		VCO Frequency Multiplier (N)
	1	2	3	4	5	6
Monitor baud rate	4800	9600	14,400	19,200	24,000	28,800

18.3.2 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 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 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 reset occurs. If the reset was not a power-on reset, security remains bypassed and security code entry is not required. (See Figure 18-13.)

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 $60 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 can be reset (via power-pin reset only) and brought up in monitor mode to attempt another entry. After failing the security sequence, the FLASH mode can also be bulk erased by executing an erase routine that was downloaded into internal RAM. The bulk erase operation clears the security code locations so that all eight security bytes become $FF.

MC68HC908MR32 • MC68HC908MR16 Data Sheet, Rev. 6.1

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