MC68000 interrupts

MC68000 interrupts

The MC68000 interrupt and exception processing is based on using an external stack to store the processor’s context informa-tion. This is very common and similar methods are provided on the 80x86 family and many of the small 8 bit microcontrollers.

Seven interrupt levels are supported and are encoded onto three interrupt pins IP0–IP2. With all three signals high, no exter-nal interrupt is requested. With all three asserted, a non-maskable level 7 interrupt is generated. Levels 1–6, generated by other combinations, can be internally masked by writing to the appro-priate bits within the status register.

The interrupt cycle is started by a peripheral generating an interrupt. This is usually encoded using a LS148 seven to three priority encoder. This converts seven external pins into a 3 bit binary code. The appropriate code sequence is generated and drives the interrupt pins. The processor samples the levels and requires the levels to remain constant to be recognised. It is recommended that the interrupt level remains asserted until its interrupt acknowledgement cycle commences to ensure recognition.

Once the processor has recognised the interrupt, it waits until the current instruction has been completed and starts an interrupt acknowledgement cycle. This starts an external bus cycle with all three function code pins driven high to indicate an interrupt acknowledgement cycle.

The interrupt level being acknowledged is placed on ad-dress bus bits A1–A3 to allow external circuitry to identify which level is being acknowledged. This is essential when one or more interrupt requests are pending. The system now has a choice over which way it will respond:

•                                                                         If the peripheral can generate an 8 bit vector number, this is placed on the lower byte of the address bus and DTACK* asserted. The vector number is read and the cycle com-pleted. This vector number then selects the address and subsequent software handler from the vector table.

•                                                                         If the peripheral cannot generate a vector, it can assert VPA* and the processor will terminate the cycle using the M6800 interface. It will select the specific interrupt vector allocated to the specific interrupt level. This method is called auto-vectoring.

To prevent an interrupt request generating multiple ac-knowledgements, the internal interrupt mask is raised to the interrupt level, effectively masking any further requests. Only if a higher level interrupt occurs will the processor nest its interrupt service routines. The interrupt service routine must clear the interrupt source and thus remove the request before returning to normal execution. If another interrupt is pending from a different source, it can be recognised and cause another acknowledgement to occur.

A typical circuit is shown. Here, level 5 has been allocated as a vectored interrupt and level 3 auto-vectored. The VPA* signal is gated with the level 3 interrupt to allow level 3 to be used with vectored or auto-vectored sources in future designs.