From: "Ralph Wade Phillips"
Subject: Re: ISA IRQ signal active how long?
Date: Fri, 4 Oct 2002 11:40:29 -0500
Organization: Phillips Enterprises
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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
Tri-State or Totem-Pole. IRQs are active HIGH on the ISA bus, and
open collector logic won't work there.
You can tri-state so that if you don't init the device, the IRQ is
available for other devices. But that's not even a good choice - something
else can camp on it, and you then get two totem pole devices arguing over
which one's high and which one's low.
> 2. When ISA card request interrupt. It must assert the IRQ signal. But
> how long
> does IRQ signal be asserted(active high)?
It should be until cleared by the interrupt handler, IIRC.
> 3. Should I care the 8259 PIC act as edge-trigger or level-sensitive
PC standard is edge-triggered, not level-sensitive, for ISA card,
IIRC. This (and the active-high) I've ranted about since, oh, about 1981