DSP56857 datasheet(91/662 Pages) FREESCALE

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Program Boot ROM

Overview, Rev. 4

Freescale Semiconductor

3-3

3.1 Introduction

The 56800E provides separate program and data memory areas. The program area has a 21-bit

address range, while the data area has a 24-bit address range. Each area has a data width of 16

bits. In the 5685x, the active areas in memory include:

•

Up to 40K

× 16 bit Program RAM

•

Up to 24K

× 16 bit Data RAM

•1K

× 16 bit Boot ROM

Off-chip memory expansion capability up to 4MB program and 16MB data with four

programmable chip select signals.

3.2 Program Boot ROM

The 5685x has 1K-word

× 16-bit on-chip Program ROM. The program ROM contains the

bootstrap firmware program able to perform initial loading of the internal program RAM. It is

located in Program memory space at locations $1F0000-$1F03FF. The bootstrap program can

load any Program RAM segment from an external memory or serial EEPROM. On exiting the

reset state the first instruction is fetched from the program ROM ($1F0000) to start execution of

the bootstrap program.

The value on the input pins, MODA, MODB, and MODC, when the last active reset

source—reset pin, power-on reset, or software reset—deasserts, it will determine which bootstrap

mode is entered.

Note:

A COP reset via RST does not alter MOD pins of the SIM register since a COP reset

by definition occurs unexpectedly during system operation, therefore possibly no

longer providing the required MODE inputs.

Note:

A software reset via RST_SW does not alter MOD pins of the SIM register, thereby

allowing users to reboot in a different mode without altering the hardware.

Some boot modes, specifically those transferring code to internal PRAM for execution, require

header data to synchronize the peripheral, define the transfer start address in PRAM, and define

the number of words to load. The following four points describe the required format for available

boot modes:

1. The four byte ASCII sequence BOOT (mode 1 only)

2. Two words (4 bytes) defining the number of program words to be loaded (modes 0, 1, 4, 5,

and 6 only)

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