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Chapter 7 ROM
7.1
Introduction
There are several classes of functions stored in ROM:
? A boot program, including ROM-based, slave-port command interpreter
? A collection of utilities that can be invoked via the ROM-based slave port interpreter
? ROM functions that are callable from user code using the call_trap() function
ROM code can only be executed when the CPU is in Supervisor mode. Any attempt to access the ROM
while in User mode will result in a privilege violation exception. Error exceptions arising from User-mode
attempts to access Supervisor-only resources will result in a reset of the device.
7.2
Boot ROM
The MMA955xL platform boots from a standard routine in ROM. This boot function (shown in
Figure 7-1 ) is responsible for a number of initialization steps before transferring control to user code in
flash memory. The ROM also contains a simple command interpreter capable of running a number of
utility and test functions for programming and erasing flash memory, as well as a limited set of other
functions.
Individual steps shown in Figure 7-1 are described in more detail in subsequent sections. One common
theme is the use of the Flash Options Register (FOPT). This register is not visible to software operating in
User mode on the ColdFire core. Normally, it is accessed only by supervisor code operating out of the
on-chip ROM.
One of the functions of FOPT is to configure boot options for the device. These are normally fetched once
at power-up from the locations 0x3FFE and 0x3FFF. FOPT bits control the security state of the device,
such as whether or not a mass-erase operation is pending (required to clear device security) and whether
the part is to boot to flash, RAM or the slave-port command interpreter. For a flash boot, the FOPT also
controls whether a checksum is calculated prior to transferring control to flash and determines what is done
if a checksum fails.
Because FOPT[15:8] is initialized only at power-up, it can be manipulated by the slave-port command
interpreter and BDM to reconfigure device operation on subsequent reset operations.
For fielded applications, the normal control flow for the boot function is 1-2-3-4-9. (See Figure 7-1 .) Other
options are intended primarily for debug and development purposes.
MMA955xL Intelligent, Motion-Sensing Platform Hardware Reference Manual, Rev. 1.0
Freescale Semiconductor, Inc.
71
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