Application Note
AN1060 — Rev. 1.0
224
MOTOROLA
Allowing for Bootstrap Mode
Since bootstrap mode requires few connections to the MCU, it is easy to
design systems that accommodate bootstrap mode.
Bootstrap mode is useful for diagnosing or repairing systems that have
failed due to changes in the CONFIG register or failures of the expansion
address/data buses, (rendering programs in external memory useless).
Bootstrap mode can also be used to load information into the EPROM or
EEPROM of an M68HC11 after final assembly of a module. Bootstrap
mode is also useful for performing system checks and calibration
routines. The following paragraphs explain system requirements for use
of bootstrap mode in a product.
Mode Select Pins
It must be possible to force the MODA and MODB pins to logic 0, which
implies that these two pins should be pulled up to V
DD
through resistors
rather than being tied directly to V
DD
. If mode pins are connected directly
to V
DD
, it is not possible to force a mode other than the one the MCU is
hard wired for. It is also good practice to use pulldown resistors to V
SS
rather than connecting mode pins directly to V
SS
because it is
sometimes a useful debug aid to attempt reset in modes other than the
one the system was primarily designed for. Physically, this requirement
sometimes calls for the addition of a test point or a wire connected to one
or both mode pins. Mode selection only uses the mode pins while
RESET is active.
RESET
It must be possible to initiate a reset while the mode select pins are held
low. In systems where there is no provision for manual reset, it is usually
possible to generate a reset by turning power off and back on.
RxD Pin
It must be possible to drive the PD0/RxD pin with serial data from a host
computer (or another MCU). In many systems, this pin is already used
for SCI communications; thus no changes are required.
In systems where the PD0/RxD pin is normally used as a general-
purpose output, a serial signal from the host can be connected to the pin
F
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n
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