MC68HC711xxxx Motorola, MC68HC711xxxx Datasheet - Page 218

• Manufacturer Part Number: MC68HC711xxxx
• Description: Microcontrollers
• Manufacturer: Motorola

Application Note
The SCI receiver and transmitter are enabled. The receiver is required
by the bootloading process, and the transmitter is used to transmit data
back to the host computer for optional verification. The last item in the
initialization is to set an intercharacter delay constant used to terminate
the download when the host computer stops sending data to the
MC68HC711E9. This delay constant is stored in the timer output
compare 1 (TOC1) register, but the on-chip timer is not used in the
bootloader program. This example illustrates the extreme measures
used in the bootloader firmware to minimize memory usage. However,
such measures are not usually considered good programming technique
because they are misleading to someone trying to understand the
program or use it as an example.
After initialization, a break character is transmitted [3] by the SCI. By
connecting the TxD pin to the RxD pin (with a pullup because of port D
wired-OR mode), this break will be received as a $00 character and
cause an immediate jump [4] to the start of the on-chip EEPROM ($B600
in the MC68HC711E9). This feature is useful to pass control to a
program in EEPROM essentially from reset. Refer to
Common
Bootstrap Mode Problems
before using this feature.
If the first character is received as $FF, the baud rate is assumed to be
the default rate (7812 baud at a 2-MHz E-clock rate). If $FF was sent at
1200 baud by the host, the SCI will receive the character as $E0 or $C0
because of the baud rate mismatch, and the bootloader will switch to
1200 baud [5] for the rest of the download operation. When the baud rate
is switched to 1200 baud, the delay constant used to monitor the
intercharacter delay also must be changed to reflect the new character
time.
At [6], the Y index register is initialized to $0000 to point to the start of
on-chip RAM. The index register Y is used to keep track of where the
next received data byte will be stored in RAM. The main loop for loading
begins at [7].
The number of data bytes in the downloaded program can be any
number between 0 and 512 bytes (the size of on-chip RAM). This
procedure is called "variable-length download" and is accomplished by
ending the download sequence when an idle time of at least four
character times occurs after the last character to be downloaded. In
AN1060 — Rev. 1.0
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