Freescale Semiconductor, Inc.
Operating Modes and On-Chip Memory
Operating modes
3.3.4 Special bootstrap mode
When the MCU is reset in special bootstrap mode, a small on-chip ROM
is enabled at address $BE40–$BFFF. The ROM contains a reset vector
and a bootloader program. The MCU fetches the reset vector, then
executes the bootloader.
For normal use of the bootloader program, send a synchronization byte
$FF to the SCI receiver at either E clock ÷256, or E clock ÷1664 (7812
or 1200 baud respectively, for an E clock of 2MHz). Then download up
to 1024 bytes of program data (which is put into RAM starting at $0080).
These characters are echoed through the transmitter. The bootloader
program ends the download after a timeout of four character times or
1024 bytes. When loading is complete, the program jumps to location
$0080 and begins executing the code. Use of an external pull-up resistor
is required when using the SCI transmitter pin (TXD) because port D pins
are configured for wired-OR operation by the bootloader. In bootstrap
mode, the interrupt vectors point to RAM. This allows the use of
interrupts through a jump table.
Further baud rate options are available on the MC68HC11P2 by using a
different value for the synchronization byte, as shown in Table 3-1. Refer
also to Motorola application note AN1060, M68HC11 Bootstrap Mode
(the bootloader mode is similar to that used on the MC68HC11K4).
Table 3-1. Example bootloader baud rates
Baud rates for an E clock of:
Sync.
byte
Timeout
delay
2.00MHz
7812
2.10MHz
8192
3.00MHz
11718
1800
3.15MHz
12288
1890
4.00MHz
15624
2400
$FF
$FF
$F0
$FD
$FD
4 char.
4
1200
1260
4.9
9600
10080
5461
14400
7812
15120
8192
19200
10416
7812
17.3
13
5208
3906
4096
5859
6144
MC68HC11P2 — Rev 1.0
Technical Data
Operating Modes and On-Chip Memory
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