Automatic Selection of Baud Rate
M68HC11 Bootstrap Mode, Rev. 1.1
Freescale Semiconductor
197
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].
Figure 1. MC68HC711E9 Composite Memory Map
$0000
512-BYTE
RAM
$01FF
$1000
$103F
$B600
$B7FF
$BF00
$BFFF
$FFFF
$BFC0
$FFC0
SINGLE
CHIP
EXPANDED
MULTIPLEXED
EXTERNAL
SPECIAL
BOOTSTRAP
SPECIAL
TEST
EXTERNAL
64-BYTE
REGISTER
BLOCK
512-BYTE
EEPROM
BOOT
ROM
12K USER
EPROM
(or OTP)
(MAY BE REMAPPED
TO ANY 4K BOUNDARY)
(MAY BE REMAPPED
TO ANY 4K BOUNDARY)
(MAY BE DISABLED
BY AN EEPROM BIT)
(MAY BE DISABLED
BY AN EEPROM BIT)
SPECIAL
MODE
VECTORS
NORMAL
MODE
VECTORS
$BFC0
$BFFF
$FFC0
$FFFF
MODA = 0
MODB = 1
MODA = 1
MODB = 1
MODA = 0
MODB = 0
MODA = 1
MODB = 0
EXTERNAL
NOTE: Software can change some aspects of the memory map after reset.
$D000