More VSS

[jdzura at csc.com]

Bruce,

>The DS-C304 is a ttl output Crystal oscillator. Nearly any frequency will
do, but the slower the better. The frequency of the oscillator defines the
width of the pulse coming out of the Xor gate. At 100 MPH, the VSS freq
should be 2*100*2000, or 400KHz  right?  As long as the oscillator is 500
KHz or above you should be OK.
>I'm pretty sure Digikey has 1 MHz XO43 Style oscillators.
OK, I been thru the Newark, and Digi-key cats, and the 7474 are
SN74LS74AN, is that the same part?.

I can't find a DS-C304A anywhere...
Is a MC74HC86N the right thing?.

The LS defines a logic input level as does the HC in the '86 part. Normally, an HC
part will drive an LS part, but not the other way around. The LS output level is
only 2.1V while an HC input is not guaranteed to switch unless the input is above
2.5V.

The issue is not serious though, you just need to find all LS or all HC parts. The
HC parts are lower power and a little faster than the LS. The other option is to
put a pull up resistor (1K) on the outputs of the LS parts. They will pull the
2.1V up near 5V and the HC parts will be happy.

The DS-C304A was a Motorola part number (I think). All you are looking for is a
ttl or cmos output crystal oscillator. The X0-43 case fits in a 14 pin DIP but has
only 4 pins on it. 1,7,8,&14. They are hermetically sealed in a welded steel case
and hold up well to temperature and will always start. That is a problem in
digital designs, sometimes the clocks don't start and then nothing runs!!

The only thing I have here is an allied catalog, and they list X0-52B, XO-54B, and
several others. Any of these will do, they say they are HCMOS compatible (output
will swing from near 0V to near 5V) and have a 0-70 Degree C temp range. List for
$2.21. I will try look up Digikey part numbers for you this evening.

Joe