DSP56857 datasheet(92/662 Pages) FREESCALE

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Program Boot ROM

5685X Digital Signal Controller User Manual, Rev. 4

3-4

Freescale Semiconductor

3. Two words (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

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 user’s 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 in the SIM Control Register. Some bootstrap routines also make specific

assumptions about the external clock frequency being applied to the part.

3.2.1 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.

3.2.2 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 Slave Select _B (SS). When configured and programmed, the

alternative function 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.

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