MC68000R Просмотр технического описания (PDF) - Motorola => Freescale

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MC68000R

8-/16-/32-Bit Microprocessors User’s Manual

Motorola => Freescale 

Freescale Semiconductor, Inc.

3.6 SYSTEM CONTROL

The system control inputs are used to reset the processor, to halt the processor, and to

signal a bus error to the processor. The outputs reset the external devices in the system

and signal a processor error halt to those devices. The three system control signals are

described in the following paragraphs.

Bus Error (BERR)

This input signal indicates a problem in the current bus cycle. The problem may be the

following:

1. No response from a device.

2. No interrupt vector number returned.

3. An illegal access request rejected by a memory management unit.

4. Some other application-dependent error.

Either the processor retries the bus cycle or performs exception processing, as

determined by interaction between the bus error signal and the halt signal.

Reset (RESET)

The external assertion of this bidirectional signal along with the assertion of HALT starts

a system initialization sequence by resetting the processor. The processor assertion of

RESET (from executing a RESET instruction) resets all external devices of a system

without affecting the internal state of the processor. To reset both the processor and the

external devices, the RESET and HALT input signals must be asserted at the same

time.

Halt (HALT)

An input to this bidirectional signal causes the processor to stop bus activity at the

completion of the current bus cycle. This operation places all control signals in the

inactive state and places all three-state lines in the high-impedance state (refer to Table

3-4).

When the processor has stopped executing instructions (in the case of a double bus

fault condition, for example), the HALT line is driven by the processor to indicate the

condition to external devices.

Mode (MODE) (MC68HC001/68EC000)

The MODE input selects between the 8-bit and 16-bit operating modes. If this input is

grounded at reset, the processor will come out of reset in the 8-bit mode. If this input is

tied high or floating at reset, the processor will come out of reset in the 16-bit mode.

This input should be changed only at reset and must be stable two clocks after RESET

is negated. Changing this input during normal operation may produce unpredictable

results.

MOTOROLA

M68000 8-/16-/32-BIT MICROPROCESSORS USER'S MANUAL

3-7

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