From: John Hudak
Subject: Re: ISA IRQ signal active how long?
Date: Tue, 08 Oct 2002 09:57:20 -0400
Organization: The Software Engineering Institute
NNTP-Posting-Date: Tue, 8 Oct 2002 13:59:31 +0000 (UTC)
To: Ru-Chin Tsai
X-Mailer: Mozilla 4.73 [en] (Windows NT 5.0; U)
Errrr, with respect to question #2, I suggest getting a 8259 data sheet
and looking at the IRQ timing waveform and the table. It is spelled out
very clearly. I don't remember - thats why there are data sheets.
WRT question 3, get the ISA bus definition 'standard'...if you can't
find it on the web, there are many good books that have reprints in the
Appendix, or in the main text. Also, there are (were?) chip sets that
made interfacing to the ISA bus a snap - perhaps one of these may be an
option to consider.
WRT question 1, either totem-pole or OC will work (I am not sure about
TS - its been a while). If you use OC, good engineering practice
encourges the use of a pull up resistor to limit the current through the
transistor. Usual values range from 330 to 1K ohms, depending on how
much current (and what the output voltage off) should be. Draw the
output transistor configuration coupled to the input transistor config,
look up the TTL characteristics (refer to the TI book "Designing with
TTL ICs" circa 1975 for a good documentation on TTL signal and noise
margins, and design characteristics.)
Sorry I can't cite the numbers, its been way to long and I've move on to
Ru-Chin Tsai wrote:
> 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
> 2. When ISA card request interrupt. It must assert the IRQ signal. But
> how long
> does IRQ signal be asserted(active high)?
> 3. Should I care the 8259 PIC act as edge-trigger or level-sensitive
> Thank you for your help sincerely. These problem delay my project for
> long time.