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Generating pulse wave

J

Joerg

Jan 1, 1970
0
Slater said:
I have this chip:
http://www.gamesx.com/rgbadd/cxa1645.pdf
As explained on page 2, pin 6 needs a "0.4 to 0.5 Vp-p sine wave or
pulse". Also explained on page 12, paragraph #3. The duty cycle must
be 50%.
For PAL operation, the frequency is 4.433619 MHz. I already have a
crystal of such frequency. How do I interface it to the chip?
I'm trying to build this circuit:
http://www.nexusuk.org/projects/rgb2svid/circuit


With that circuit pin 6 will get a lot more than 0.5Vpp.

But I can't find a 74HCT04 anywhere!


Wow, you must really live in the boonies :)

Anyhow, it's just six inverters in there. You could also use a 74HC04,
or a 74HCT00 or 74HC00 wired up a bit differently. If you can't get
those I suggest to move closer to civilization :)
 
S

Slater

Jan 1, 1970
0
Wow, you must really live in the boonies :)

Exactly!!
Anyhow, it's just six inverters in there. You could also use a 74HC04,
or a 74HCT00 or 74HC00 wired up a bit differently. If you can't get
those I suggest to move closer to civilization :)

I know where to get a 74HC04. Could you please tell me how to wire it,
in such a way that specifications (0.4 to 0.5 Vp-p) are not exceeded?
A little drawing would be wonderful!

Thank you!
 
A

Andrew Holme

Jan 1, 1970
0
Slater said:
I know where to get a 74HC04. Could you please tell me how to wire it,
in such a way that specifications (0.4 to 0.5 Vp-p) are not exceeded?

Page 2 says "0.4 to 0.5" but note 3 on page 12 says "0.4 to 5.0 Vpp" and
talks about it going outside the GND/Vcc supply rails if the duty cycle is
not 50% (due to the internal 1/2 Vcc biasing and ac coupling) which confirms
the 5.0 Vpp. Also, the 74HCT04 output is 5.0 Vpp. So that 0.5 on page 2 is
a typo.
 
J

Joerg

Jan 1, 1970
0
Slater said:
Exactly!!

Must be east of Tromso then :)

I know where to get a 74HC04. Could you please tell me how to wire it,
in such a way that specifications (0.4 to 0.5 Vp-p) are not exceeded?
A little drawing would be wonderful!

You just plop it in, has the same pin configuration. Looking at the
datasheet it appears that the first part on page 2 may not be accurate.
Note 3 on page 12 says, quote "The SC input (Pin 6) can be either a sine
wave or a pulse in the range from 0.4 to 5.0Vp-p". 5Vpp is what will
come out of your 74HC04, or ion reality a few hundred millivolts less
but that's fine. The 74HC04 generates pulses, not sinewaves.

So you shold be fine without attenuation. If you ever need to attenuate
it would be easy: 4.7k resistor from U3 pin 4 to U1 pin 6, then 560ohms
from U1 pin 6 to ground. But I don't think you need to do that here.

Thank you!


You are welcome :)
 
S

Slater

Jan 1, 1970
0
Joerg said:
So you shold be fine without attenuation. If you ever need to attenuate
it would be easy: 4.7k resistor from U3 pin 4 to U1 pin 6, then 560ohms
from U1 pin 6 to ground.

How much attenuation will those values give?
 
S

Slater

Jan 1, 1970
0
Jan Panteltje said:
I am not sure what you want to do,
but that will NOT convert your PC monitor's RGB out to S-video.
So hopefully it was for something else :)

Yes, it's for converting the RGB signal from a videotape to S-Video
:)
 
J

Joerg

Jan 1, 1970
0
Slater said:
How much attenuation will those values give?


It's a simple voltage divider, resulting in roughly a factor of 9.4, so
a 5V signal would become 530mV. Realistically less because logic chips
never do a full swing into any load. But according to page 12 of the
datasheet I don't think you need to do this.

--
Beste hilsen, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
 
J

Joerg

Jan 1, 1970
0
Jim said:
Neeerp! For a crystal oscillator you want 74HCU04... note the "U"!

Sure, but tough luck finding one of those in the woods of Norway ;-)

It's not nice and harder on the crystal but a 74HC04 usually works ok
for crystals in the low meggeehoitz range.
 
J

Joerg

Jan 1, 1970
0
Huh? What kind of VTR gives out RGB? Is this the SCART connector?
Interesting.


It seems the OP lives in Norway. In Europe many VCRs had (have?) RGB,
via the SCART-Connector. We still have our old European VCR, to be able
to watch some of the classic movie from there. Until a while ago we
could occasionally find a foreign movie at the video rental place but
that went belly-up.
 
R

Rich Grise

Jan 1, 1970
0
Sure, but tough luck finding one of those in the woods of Norway ;-)

It's not nice and harder on the crystal but a 74HC04 usually works ok for
crystals in the low meggeehoitz range.

Harder on the crystal? Use either a smaller cap, or a bigger resistor. ;-)

Cheers!
Rich
 
J

Joerg

Jan 1, 1970
0
Rich said:
Harder on the crystal? Use either a smaller cap, or a bigger resistor. ;-)

Then, eventually it oscillateth no more :)
 
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