MC68HC11E0CFNE3 Datasheet, PDF (197/242 Pages) Freescale Semiconductor, Inc – M68HC11E Family

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