MC68HC908GR16 Datasheet, PDF (269/310 Pages) Motorola, Inc – Microcontrollers

Freescale Semiconductor, Inc.
Development Support
Monitor ROM (MON)
• Execution of code in random-access memory (RAM) or FLASH
• FLASH memory security feature⁽¹⁾
• FLASH memory programming interface
• 350 bytes monitor ROM code size ($FE20 to $FF6A)
• Monitor mode entry without high voltage, V_TST, if reset vector is blank
($FFFE and $FFFF contain $FF)
• Normal monitor mode entry if high voltage is applied to IRQ
19.3.1 Functional Description
Figure 19-9 shows a simplified diagram of the monitor mode.
The monitor ROM receives and executes commands from a host computer.
Figure 19-10 and Figure 19-11 show example circuits used to enter monitor mode
and communicate with a host computer via a standard RS-232 interface.
Simple monitor commands can access any memory address. In monitor mode, the
MCU can execute code downloaded into RAM by a host computer while most MCU
pins retain normal operating mode functions. All communication between the host
computer and the MCU is through the PTA0 pin. A level-shifting and multiplexing
interface is required between PTA0 and the host computer. PTA0 is used in a
wired-OR configuration and requires a pullup resistor.
Table 19-1 shows the pin conditions for entering monitor mode. As specified in the
table, monitor mode may be entered after a power-on reset (POR) and will allow
communication at 14,400 baud provided one of the following sets of conditions is
met:
• If $FFFE and $FFFF does not contain $FF (programmed state):
– The external clock is 4.9152 MHz
– PTB4 = low
– IRQ = V_TST
• If $FFFE and $FFFF do not contain $FF (programmed state):
– The external clock is 9.8304 MHz
– PTB4 = high
– IRQ = V_TST
• If $FFFE and $FFFF contain $FF (erased state):
– The external clock is 32.768 kHz
– IRQ = V_SS
The last two conditions are the forced monitor mode.
1. No security feature is absolutely secure. However, Motorola’s strategy is to make reading or
copying the FLASH difficult for unauthorized users.
MC68HC908GR16 — Rev. 1.0
MOTOROLA
Development Support
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Data Sheet
269