Motorola USB08 - Manual (Page 33)

Hardware Description
Circuit Description
USB08 Evaluation Board Designer Reference Manual
MOTOROLA Hardware Description 33
The I/O pins of the MCU are accessible on the expansion connector X1.
User specific peripheral circuits can be attached to X1.
NOTE: It may be that not all functions of the demo board may be used with
user-specific peripheral circuits attached to X1.
2.4.3 Monitor Mode Interface
For FLASH programming and software debugging, the MC68HC908JB8
uses a special operating mode, monitor mode. The difference between
monitor mode and normal user mode is that firmware out of the read-only
memory (ROM) is executed instead of the user program. First, this
firmware examines a set of I/O pins and specifies the concrete operating
parameters. Finally, this firmware establishes an asynchronous serial
interface function on the port pin PTA0. This interface works
bidirectionally (half duplex) and corresponds to the usual RS232
conventions. The baud rate equals 9600 baud. An additional
requirement, besides the quartz clock (6 MHz), is the allocation of
certain logic levels to some port pins as listed in
Table 2-1 .
The monitor mode circuitry on the evaluation board produces the levels
shown in
Table 2-1 using four pullup or pulldown resistors. These
resistors are connected to the MCU using the jumpers JP1-C–JP1-F.
After removing these jumpers, a previously loaded user program can
access the four port A pins without restrictions.
Apart from the above requirements, to enter monitor mode it is
necessary to apply a voltage of approximately 7–10 V to the IRQ pin of
the MCU. This voltage is generated by the RS232 transceiver’s (IC2)
charge pump and limited to 8.2 V using the breakdown diode D7. JP1-A
Table 2-1. Port A Monitor Mode Entry Levels
Port Pin Level
PTA0 High
PTA1 High
PTA2 Low
PTA3 High