Wednesday, January 18, 2017

Building a 68000 Single Board Computer - Interrupt Control Circuitry (schematic page 9)


The interrupt circuitry being tested on a breadboard
before it was constructed on the wirewrap board.

The interrupt control circuitry surrounding the 68000 is conventional. A 74LS148 eight line to three line priority encoder, U28, converts the seven levels of interrupt request input into a 3-bit code on IPL0* to IPL2*. Note that each interrupt request input must have a pull-up resistor, except IRQ7* which is always driven by U35A.

The function code from the 68000 is decoded by U32, a 74LS138, and the resulting IACK* output is used to enable a second decoder, U33. U33 is also strobed by AS* and converts the information on A01 to A03 during an IACK cycle into one of seven levels of interrupt acknowledge output (IACK1* to IACK7*). Other function code information supplied by U32 that may be useful in debugging the system is the "user/supervisor" memory access codes and the "program/data" bus cycle codes.

The ABORT switch can generate a level 7 interrupt. This is present on the ECB and with the TUTOR firmware can be used to interrupt program execution. The switch is debounced by cross-coupled NAND gates U34A and U34B and then clocks flip-flop U35A high (it has its data input tied high). The Q* output of the flip-flop goes low, and is connected to the IRQ7* input. Note that, unlike interrupts 1 through 6, a level 7 interrupt cannot be masked or disabled. During an interrupt acknowledge cycle for a level 7 interrupt, IACK7* will go low and clear the flip-flop. The IACK7* signal will also ripple through U7D, U34C and U4F, driving VPA* low to indicate to the 68000 to perform an autovectored interrupt operation. The CPU will then use the level 7 autovector interrupt address in RAM.

Interrupts 5 and 6 are connected to the two ACIAs, allowing interrupt driven i/o to be performed if desired (the TS2 and TUTOR monitor programs do not make use of this). These interrupts are used in the same way on the Motorola ECB board. Like level 7, the interrupts are also configured for autovectored operation. The relevant IACK5* or IACK6* signal will go low and in turn drive VPA* low to perform autovectored interrupt handling.

IRQ4* is also available and configured for autovectored operation. Interrupts IRQ1* through IRQ3* are not -- they could be used with vectored interrupts if external circuitry is added for this.

The interrupt circuitry can be tested from the TUTOR monitor. Pressing the ABORT switch should generate a level 7 interrupt which will produce a "SOFTWARE ABORT" message and register dump.

Pulling IRQ5* or IRQ6* low should cause TUTOR to produce "AV#5 TRAP ERROR" and "AV#6 TRAP ERROR" messages, respectively. In order for this to happen the interrupt mask in the status register must be set to enable these interrupts.

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