Cirrus Logic EP93xx - Manual (Page 119)

DS785UM1 4-1
Copyright 2007 Cirrus Logic
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Chapter 4
4 Boot ROM
4.1 Introduction
The Boot ROM allows a program or OS to boot from the following devices:
• SPI Flash
• FLASH, SyncFLASH or SyncROM
• UART1
4.1.1 Boot ROM Hardware Operational Overview
The Boot ROM is an AHB slave device containing a 16 kbyte mask-programmed ROM. The
AHB slave always operates with one wait state, so all data reads from the ROM use 2 HCLK
cycles.
On system reset, the ARM920T begins executing code at address zero. The system follows
the Hardware Configuration controls to select the boot device that appears at address zero. If
Internal Boot is selected, the Boot ROM is mapped to address zero and the ARM920T will
execute the Boot ROM code.
4.1.1.1 Memory Map
The normal Boot ROM base address base is 0x8009_0000. It will alias on 16 kbyte intervals.
When internal boot is active, the Boot ROM is double decoded and appears at its normal
address base and at address 0x0000_0000. At address 0x0000_0000 plus the current offset,
the Boot ROM can write the BootModeClr bit to remap itself back to 0x8009_0000 plus the current
offset. Execution t
hen continues with the instruction at the next Boot ROM address in
0x8009_0000 space.
4.1.2 Boot ROM Software Operational Overview
The Boot ROM is a 16 kbyte mask-programmed ROM that controls the source of the first off-
chip code that is executed by the ARM Core. The code within the Boot ROM supports the
following sources for the processor’s initialization program:
• UART1: Code is downloaded through UART1 into an on chip buffer and executed
• SPI Serial Flash: Code is copied from an SPI Serial Flash into an on-chip buffer and
executed
• FLASH: Code present in external FLASH memory is executed directly