Exception Processing
MOTOROLA
ColdFire2/2M User’s Manual
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4-7
4.1.4 Fault-on-Fault Halt
If the ColdFire2/2M encounters any type of fault during the exception processing of another
fault, it immediately halts execution with the catastrophic fault-on-fault condition. The
ColdFire2/2M indicates a fault-on-fault condition by continuously driving the processor
status (PST[3:0]) signals with an encoded value of $F until it is reset. Only an external reset
operation can restart a halted ColdFire2/2M. See
information.
Section 7.3.1 CPU Halt
for more
4.2 EXCEPTIONS
The following paragraphs describe the external interrupt exceptions and the different types
of exceptions generated internally by the integer unit. The following exceptions are dis-
cussed:
Reset
Access Error
Address Error
Illegal Instruction
Privilege Violation
Trace
Unimplemented Opcode
Debug Interrupt
Format Error
TRAP Instruction
External Interrupt
4.2.1 Reset Exception
Asserting the reset input signal to the ColdFire2/2M causes a reset exception, vector
number $0. The reset exception has the highest priority of any exception; it provides for
system initialization. Reset also aborts any processing in progress when the reset input is
recognized, and the aborted processing cannot be recovered.
The reset exception places the ColdFire2/2M in the supervisor mode by setting the S-bit and
disables tracing by clearing the T-bit in the SR. This exception also clears the M-bit and sets
the ColdFire2/2M’s interrupt priority mask in the SR to the highest level (level 7). Next, the
VBR, CACR, ACRs, RAMBAR0, and ROMBAR0 are initialized to their reset value. This will
disable the cache, SRAM, and optionally the ROM (see
Model
.)
Section 5.3.2 ROM Programming
Once the ColdFire2/2M is granted the bus, and it does not detect any other alternate
masters taking the bus, the core then performs two longword read bus cycles. Because the
VBR is reset to zero, the first longword is always loaded into the stack pointer from address
zero, and the second longword is always loaded into the program counter from address four.
After the initial instruction is fetched from memory, program execution begins at the address
F
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n
.