
Application Note
AN1060 — Rev. 1.0
MOTOROLA
215
Automatic Selection of Baud Rate
The bootloader program in the MC68HC711E9 accommodates either of 
two baud rates. 
The higher of these baud rates (7812 baud at a 2-MHz E-clock 
rate) is used in systems that operate from a binary frequency 
crystal such as 2
23
 Hz (8.389 MHz). At this crystal frequency, the 
baud rate is 8192 baud, which was used extensively in automotive 
applications. 
The second baud rate available to the M68HC11 bootloader is 
1200 baud at a 2-MHz E-clock rate. Some of the newest versions 
of the M68HC11, including the MC68HC11F1 and 
MC68HC117K4, accommodate other baud rates using the same 
differentiation technique explained here. Refer to the reference 
numbers in square brackets in 
Figure 2
 during the following 
explanation. 
NOTE:
Software can change some aspects of the memory map after reset.
Figure 2
 shows how the bootloader program differentiates between the 
default baud rate (7812 baud at a 2-MHz E-clock rate) and the alternate 
baud rate (1200 baud at a 2-MHz E-clock rate). The host computer 
sends an initial $FF character, which is used by the bootloader to 
determine the baud rate that will be used for the downloading operation. 
The top half of 
Figure 2
 shows normal reception of $FF. Receive data 
samples at [1] detect the falling edge of the start bit and then verify the 
start bit by taking a sample at the center of the start bit time. Samples 
are then taken at the middle of each bit time [2] to reconstruct the value 
of the received character (all 1s in this case). A sample is then taken at 
the middle of the stop bit time as a framing check (a 1 is expected) [3]. 
Unless another character immediately follows this $FF character, the 
receive data line will idle in the high state as shown at [4]. 
The bottom half of 
Figure 2
 shows how the receiver will incorrectly 
receive the $FF character that is sent from the host at 1200 baud. 
Because the receiver is set to 7812 baud, the receive data samples are 
taken at the same times as in the upper half of 
Figure 2
. The start bit at 
1200 baud [5] is 6.5 times as long as the start bit at 7812 baud [6]. 
F
Freescale Semiconductor, Inc.
For More Information On This Product,
  Go to: www.freescale.com
n
.