MC68HC711xxxx Motorola, MC68HC711xxxx Datasheet - Page 246

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

Application Note
Original M68HC11
Versions Required
Exactly 256 Bytes
to be Downloaded
to RAM
Variable-Length
Download
EPROM/OTP
Versions
of M68HC11
Have an EPROM
Emulation Mode
246
Even users that know about the 256 bytes of download data sometimes
forget the initial $FF that makes the total number of bytes required for the
entire download operation equal to 256 + 1 or 257 bytes.
When on-chip RAM surpassed 256 bytes, the time required to serially
load this many characters became more significant. The variable-length
download feature allows shorter programs to be loaded without
sacrificing compatibility with earlier fixed-length download versions of
the bootloader. The end of a download is indicated by an idle RxD line
for at least four character times. If a personal computer is being used to
send the download data to the MCU, there can be problems keeping
characters close enough together to avoid tripping the end-of-download
detect mechanism. Using 1200 as the baud rate rather than the faster
default rate may help this problem.
Assemblers often produce S-record encoded programs which must be
converted to binary before bootloading them to the MCU. The process of
reading S-record data from a file and translating it to binary can be slow,
depending on the personal computer and the programming language
used for the translation. One strategy that can be used to overcome this
problem is to translate the file into binary and store it into a RAM array
before starting the download process. Data can then be read and
downloaded without the translation or file-read delays.
The end-of-download mechanism goes into effect when the initial $FF is
received to set the baud rate. Any amount of time may pass between
reset and when the $FF is sent to start the download process.
The conditions that configure the MCU for EPROM emulation mode are
essentially the same as those for resetting the MCU in bootstrap mode.
While RESET is low and mode select pins are configured for bootstrap
mode (low), the MCU is configured for EPROM emulation mode.
AN1060 — Rev. 1.0
MOTOROLA