Typewriter User Manual

3- 30 MC68340 USER’S MANUAL MOTOROLA

data will be ignored if AVEC is asserted before or at the same time as the DSACK≈

signals. The vector number supplied in an autovector operation is derived from the

interrupt level of the current interrupt. When

AVEC is asserted instead of DSACK≈ during

an interrupt acknowledge cycle, the MC68340 ignores the state of the data bus and

internally generates the vector number (the sum of the interrupt level plus 24 ($18)).

AVEC is multiplexed with CS0. The FIRQ bit in the SIM40 module configuration register

controls whether the

AVEC/CS0 pin is used as an autovector input or as CS0 (refer to

Section 4 System Integration Module for additional information).

AVEC is only sampled

during an interrupt acknowledge cycle. During all other cycles,

AVEC is ignored.

Additionally,

AVEC can be internally generated for external devices by programming the

autovector register. Seven distinct autovectors can be used, corresponding to the seven

levels of interrupt available with signals

IRQ7–IRQ1. Figure 3-16 shows the timing for an

autovector operation.

3.4.4.3 SPURIOUS INTERRUPT CYCLE. Requested interrupts, whether internal or

external, are arbitrated internally. When no internal module (including the SIM40, which

responds for external requests) responds during an interrupt acknowledge cycle by

arbitrating for the interrupt acknowledge cycle internally, the spurious interrupt monitor

generates an internal bus error signal to terminate the vector acquisition. The MC68340

automatically generates the spurious interrupt vector number (24) instead of the interrupt

vector number in this case. When an external device does not respond to an interrupt

acknowledge cycle with

AVEC or DSACK≈, a bus monitor must assert BERR, which

results in the CPU32 taking the spurious interrupt vector. If

HALT is also asserted, the

MC68340 retries the interrupt acknowledge cycle instead of using the spurious interrupt

vector.

Frees

cale Semiconductor,

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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