
Application Note
AN1060 — Rev. 1.0
MOTOROLA
225
without resulting in output driver conflicts. It may be important to 
consider what the existing logic will do with the SCI serial data instead of 
the signals that would have been produced by the PD0 pin. In systems 
where the PD0 pin is used normally as a general-purpose input, the 
driver circuit that drives the PD0 pin must be designed so that the serial 
data can override this driver, or the driver must be disconnected during 
the bootstrap download. A simple series resistor between the driver and 
the PD0 pin solves this problem as shown in 
Figure 5
. The serial data 
from the host computer can then be connected to the PD0/RxD pin, and 
the series resistor will prevent direct conflict between the host driver and 
the normal PD0 driver.
Figure 5. Preventing Driver Conflict 
TxD Pin
The bootloader program uses the PD1/TxD pin to send verification data 
back to the host computer. To minimize the possibility of conflicts with 
circuitry connected to this pin, port D is configured for wire-OR mode by 
the bootloader program during initialization. Since the wire-OR 
configuration prevents the pin from driving active high levels, a pullup 
resistor to V
DD
 is needed if the TxD signal is used. 
In systems where the PD1/TxD pin is normally used as a general-
purpose output, there are no output driver conflicts. It may be important 
to consider what the existing logic will do with the SCI serial data instead 
of the signals that would have been produced by the PD1 pin. 
In systems where the PD1 pin is normally used as a general-purpose 
input, the driver circuit that drives the PD1 pin must be designed so that 
the PD1/TxD pin driver in the MCU can override this driver. A simple 
series resistor between the driver and the PD1 pin can solve this 
problem. The TxD pin can then be configured as an output, and the 
RxD/PD0
(BEING USED
AS INPUT)
EXISTING
CONTROL
SIGNAL
SERIES
RESISTOR
RS232
LEVEL 
SHIFTER
FROM
HOST
SYSTEM
MC68HC11
EXISTING
DRIVER
CONNECTED ONLY DURING
BOOTLOADING
F
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n
.