MC68340 datasheet(180/441 Pages) FREESCALE

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MOTOROLA

MC68340 USER’S MANUAL

5- 43

5.5.2 Processing of Specific Exceptions

The following paragraphs provide details concerning sources of specific exceptions, how

each arises, and how each is processed.

5.5.2.1 RESET. Assertion of

RESET by external hardware or assertion of the internal

RESET signal by an internal module causes a reset exception. The reset exception has

the highest priority of any exception. Reset is used for system initialization and for

recovery from catastrophic failure. The reset exception aborts any processing in progress

when it is recognized, and that processing cannot be recovered. Reset performs the

following operations:

1. Clears T0 and T1 in the SR to disable tracing

2. Sets the S-bit in the SR to establish supervisor privilege

3. Sets the interrupt priority mask to the highest priority level (%111)

4. Initializes the VBR to zero ($00000000)

5. Generates a vector number to reference the reset exception vector

6. Loads the first long word of the vector into the interrupt SP

7. Loads the second long word of the vector into the PC

8. Fetches and initiates decode of the first instruction to be executed

Figure 5-11 is a flowchart of the reset exception

After initial instruction prefetches, normal program execution begins at the address in the

PC. The reset exception does not save the value of either the PC or the SR.

If a bus error or address error occurs during reset exception processing sequence, a

double bus fault occurs, the processor halts, and the

HALT signal is asserted to indicate

the halted condition.

Execution of the RESET instruction does not cause a reset exception nor does it affect

any internal CPU register. The SIM40 registers and the MCR in each internal peripheral

module (DMA, timers, and serial modules) are not affected. All other internal peripheral

module registers are reset the same as for a hardware reset. The external devices

connected to the

RESET signal are reset at the completion of the RESET instruction.

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