The AC3 can use any of the following interrupts: IRQ3, IRQ5, IRQ7, IRQ9, IRQ10, IRQ11, IRQ12 and IRQ15. These are selected by bits ‘7’, ‘6’ and ‘5’ of the acquisition mode select register. An interrupt is only generated when acquisition mode 3 is selected.
One of two interrupt methods is selected by bit ‘4’ of the acquisition mode select register. When previously written with a ‘1’, shared interrupt operation will occur. The host PC (bus master) holds the interrupt line high with a passive pull up resistor. When the AC3 requests an interrupt it generates an active low pulse on the interrupt line selected using a driver that generates an active low or high impedance output. This pulse lasts for approximately 500 ns. The bus master determines that the AC3 is the source of the interrupt by polling the PICS and interrupt status register.
NOTE: Reading any of the AC3 registers resets the interrupt bit, therefore it is essential that the polling routine reads this register only.
After power on, reset, or after a ‘0’ is written to bit ‘4’ of the acquisition mode select register, level interrupt operation will occur. This is described in the IBM PC-AT technical reference manual. In this mode, the AC3 drives the interrupt line to logic ‘0’ when the interrupt mode has been selected. When an interrupt is needed, the AC3 asserts it by driving the selected interrupt line to logic ‘1’. The interrupt line returns to logic ‘0’ and the interrupt bit is cleared when any register in the AC3 is read by the bus master.
All unselected interrupt lines present a high impedance to the ISA bus, except when the ΔT bus interface is selected. When this occurs, all interrupts will be set to logic ‘0’.
In either mode, after an interrupt has occurred, bit ‘0’ of the PICS and interrupt status register is set. This bit is cleared by reading any register in the AC3.
Simulating the use of interrupts in both modes using the ACQUIRE bit of the command register described in section Command register (base address + 13) is possible. Writing 1 to this register causes the AC3 to respond as though it has received an interrupt signal when the AC3 has been set to acquisition modes 4 and 5. It causes the acquisition of data and the generation of an interrupt when the AC3 has been set to acquisition mode 3.