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Register Descriptions (SYS_BASE = $1FFF08)
5685X Digital Signal Controller User Manual, Rev. 4
4-10
Freescale Semiconductor
3. Two word (4 bytes) starting address where loaded in the program memory (modes 0, 1, 4,
5, and 6 only)
4. The user program (two bytes for each 16-bit program word) on all boot modes
In Boot mode 1, 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.