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From: "Meindert Sprang"
Subject: Re: ISA IRQ signal active how long?
Date: Fri, 4 Oct 2002 12:54:56 +0200
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"Ru-Chin Tsai" wrote in message
> I design a ISA card with IRQ support. We know 8259's IRQ signal is
> active high. I have read many posts about ISA IRQ pin and remain
> feeling some confusions. Please help me solve these problems.
> 1. The ISA IRQ will be totem-pole or open-collector or tr-state output
> It seem to have no standard about this issue. Many answers were
> posted on
> newsgroup and I can't identify which one is correct output
You can use a normal totem-pole output.
> 2. When ISA card request interrupt. It must assert the IRQ signal. But
> how long
> does IRQ signal be asserted(active high)?
Until you service the interrupt. I have done it like this:
An interrupt is requested by setting a flip-flop (FF). The FF output is
connected to the ISA INTx signal. This FF is reset when the hardware is
accessed, by combining the output of the address selection logic and (IORD
or IOWR). So whenever my hardware is read from or written to, is clears the
> 3. Should I care the 8259 PIC act as edge-trigger or level-sensitive
Level mode. That's standard in PC world.
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