Re: 2 leg crystal on FPGA: Lattice vs Xilinx
- From: Phil Hays <invalid@xxxxxxxxx>
- Date: Sat, 29 Sep 2007 19:08:36 GMT
Hal Murray wrote:
Well, no. Back when I was quite a bit younger, I tried to build a stable
oscillator with a unused gate on a TTL 7414, with is an inverter with
hysteresis. Tried all sorts of values of capacitors and resisters, and
the best I was able to do was to usually generate the third harmonic.
Sometimes the fifth, sometimes some other multiple of the crystal
frequency, sometimes not a stable frequency, and sometimes even the
frequency of the crystal. Switched to using a 7404, a plain inverter,
and then making a stable oscillator was easy.
Did you put it into production and ship a million of them?
No, but a commercial video display card in my S100 system that I still
have uses the same scheme, and I seem to remember seeing the circuit in a
data book in that timeframe. This scheme did seem to work, at least for
the copies I used all the time, unlike the 7414, which did the most
amusing other things, other than oscillate at the intended frequency. I
don't recall seeing oscillators until the late 1970's.
They used to make 74U04, U for unbuffered. It was intended for hacks
like this.
CMOS gates came in both buffered and unbuffered versions, the unbuffered
version for things like this. TTL gates gave you schematics. No buffer on
this inverter, at least in the plain TTL version. Maybe on the 74F or
74ALS or later versions.
http://focus.ti.com/lit/ds/symlink/sn7404.pdf
--
Phil Hays
.
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