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kender
Joined: 09 Aug 2004
Posts: 768
Location: Silicon Valley
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| OE# pin on GAL (slightly off topic) |
 Posted: Sun May 22, 2005 5:35 pm |
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Colleagues,
I'm playing with GAL16V8D chip. If I drive the OE# pin high, should the inputs go low or high impedance?
Thanks,
Nick |
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Mark
Joined: 07 Sep 2003
Posts: 2838
Location: Atlanta, GA
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| Posted: Sun May 22, 2005 9:01 pm |
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And what does the datasheet say?
| Quote: |
The GAL16V8 input and I/O pins have built-in active pull-ups. As
a result, unused inputs and I/O's will float to a TTL "high" (logical
"1"). Lattice Semiconductor recommends that all unused inputs
and tri-stated I/O pins be connected to another active input, VCC,
or Ground. Doing this will tend to improve noise immunity and reduce
ICC for the device.
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Mon May 23, 2005 12:38 am |
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| Quote: | I'm playing with GAL16V8D chip. If I drive the OE# pin high,
should the inputs go low or high impedance? |
I assume you're using the GAL in registered mode, so pin 11 is
a dedicated \OE pin.
First, \OE stands for "output enable". So it affects output pins on
the GAL, and not input pins as your question implies.
If you drive \OE high, the GAL outputs will go a Hi-Z state. |
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Mark
Joined: 07 Sep 2003
Posts: 2838
Location: Atlanta, GA
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| Posted: Mon May 23, 2005 6:04 am |
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| PCM programmer wrote: | | Quote: | I'm playing with GAL16V8D chip. If I drive the OE# pin high,
should the inputs go low or high impedance? |
I assume you're using the GAL in registered mode, so pin 11 is
a dedicated \OE pin.
First, \OE stands for "output enable". So it affects output pins on
the GAL, and not input pins as your question implies.
If you drive \OE high, the GAL outputs will go a Hi-Z state. |
Even though the outputs will go to a Hi-Z state the internal active pullups will actually make them high. Correct? |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Mon May 23, 2005 1:09 pm |
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It might be. I interpreted his question as asking if the \OE pin was
an active-low input, or an active-high input.
I know the Lattice data sheet says that it has active pullups on all pins.
All I have (or can quickly find) is an old GAL16V8A made by National
in 1990. I made a small test program with PALASM, that just creates
a divide by 2 circuit which is clocked by Pin 1. If I tie pin 11 (\OE)
to ground, it has output on pin 12 (which is what I selected as the
output pin for the divider). If I tie pin \OE to +5v, then pin 12 goes
low on the oscilloscope. If I then take a 10K pullup to +5v and put it
on pin 12, then it goes high. So the output pin does go to a high-Z
state if \OE is high. But this particular brand and p/n of (ancient) GAL
doesn't appear to have built-in pull-ups on it. I also created an pass-
through circuit, from pin 2 to pin 13, with just a register on it.
Pin 2 (the input) doesn't have a built-in pull-up on it. I can't find
the old data book for this GAL.  It may be that Lattice
added the pull-up feature on their GALs, as an improvement. |
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