SIM Register Descriptions (SYS_BASE = $1FFF08)
MOTOROLA
System Integration Module (SIM)
4-9
Preliminary
4
The four-byte string BOOT loads B first in boot mode one. The bytes/words for the
remaining data are loaded least significant byte/word first. The boot code is
general-purpose, and assumes the number of program words and starting address are valid
for the users system. If the values are invalid, unpredictable results will occur. If a
reserved mode is specified, the debughlt instruction will be executed causing the software
to enter an infinite loop.
Once the bootstrap program completes loading the specified number of words, if
applicable to that boot mode, the bootstrap program jumps to the starting address and
executes the loaded program. Some of the bootstrap routines reconfigure the memory map
by setting the PRAM DISABLE field. Some bootstrap routines also make specific
assumptions about the external clock frequency being applied to the part.
4.6.1.2.1 Boot Mode 0: Bootstrap From Byte-Wide External Memory
The PRAM DISABLE remains zero, leaving both internal program and data RAM
enabled. The bootstrap program loads program memory from a byte-wide memory located
at $040000 using CS0 as the chip select, before jumping to the start of the user code.
4.6.1.2.2 Boot Mode 1: Bootstrap From SPI
The PRAM DISABLE remains zero, leaving both internal program and data RAM
enabled. The bootstrap program loads program memory from a serial EEPROM via the
SPI. GPIOF3 is an alternative function of the SS, which when configured and
programmed, can be used as the SS output. This mode is compatible with ATMEL
AT25xxx and AT45xxx series serial EEPROMs. In order to determine the correct SPI
configuration, the first four bytes in the serial memory must be the string BOOT in ASCII.
They are: $42, $4F, $4F and $54. If, after trying all three configurations, BOOT is not
read, the part will move to the DEBUG HALT state. After the string BOOT, the data
should continue as described in the data sequence above. After loading the user program,
GPIOF3 is returned to its power-on reset state—input under peripheral control—and the
bootstrap program jumps to the start of the user code. This boot loader assumes the
external clock is being applied at a frequency between 2MHz and 4MHz. For some
external devices, it enables the PLL during boot loading but always leaves the PLL off
when complete.
4.6.1.2.3 Boot Mode 2: Normal Expanded Mode
The PRAM DISABLE remains at zero, leaving both internal program and data RAM
enabled. No code is loaded. The bootstrap program simply vectors to external program
memory location P:$040000 using CS0 as the chip select.