
Application Note
AN1060 — Rev. 1.0
246
MOTOROLA
Original M68HC11 
Versions Required 
Exactly 256 Bytes 
to be Downloaded 
to RAM
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. 
Variable-Length 
Download
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. 
EPROM/OTP 
Versions 
of M68HC11 
Have an EPROM 
Emulation Mode
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. 
F
Freescale Semiconductor, Inc.
For More Information On This Product,
  Go to: www.freescale.com
n
.