
Application Note
AN1060 — Rev. 1.0
MOTOROLA
229
The duplicator program in EEPROM clears the DWOM control bit to 
change port D (thus, TxD) of U3 to normal driven outputs. This 
configuration will prevent interference due to R9 when TxD from the 
target MCU (U6) becomes active. Series resistor R9 demonstrates how 
TxD of U3 can drive RxD of U3[1] and later TxD of U6 can drive RxD of 
U3 without a destructive conflict between the TxD output buffers. 
As the target MCU (U6) leaves reset, its mode pins select bootstrap 
mode so the bootloader firmware begins executing. A break is sent out 
the TxD pin of U6. At this time, the TxD pin of U3 is at a driven high so 
R9 acts as a pullup resistor for TxD of the target MCU (U6). The break 
character sent from U6 is received by U3 so the duplicator program that 
is running in the EEPROM of the master MCU knows that the target 
MCU is ready to accept a bootloaded program. 
The master MCU sends a leading $FF character to set the baud rate in 
the target MCU. Next, the master MCU passes a 3-instruction program 
to the target MCU and pauses so the bootstrap program in the target 
MCU will stop the loading process and jump to the start of the 
downloaded program. This sequence demonstrates the variable-length 
download feature of the MC68HC711E9 bootloader. 
The short program downloaded to the target MCU clears the DWOM bit 
to change its TxD pin to a normal driven CMOS output and jumps to the 
EPROM programming utility in the bootstrap ROM of the target MCU. 
Note that the small downloaded program did not have to set up the SCI 
or initialize any parameters for the EPROM programming process. The 
bootstrap software that ran prior to the loaded program left the SCI 
turned on and configured in a way that was compatible with the SCI in 
the master MCU (the duplicator program in the master MCU also did not 
have to set up the SCI for the same reason). The programming time and 
starting address for EPROM programming in the target MCU were also 
set to default values by the bootloader software before jumping to the 
start of the downloaded program. 
Before the EPROM in the target MCU can be programmed, the V
PP
power supply must be available at the XIRQ/V
PPE
 pin of the target MCU. 
The duplicator program running in the master MCU monitors this voltage 
(for presence or absence, not level) at PE7 through resistor divider 
F
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