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-   -   3.3v to 5v CMOS interfacing (http://www.diyaudio.com/forums/digital-source/79559-3-3v-5v-cmos-interfacing.html)

Zodiac 14th May 2006 07:55 PM

3.3v to 5v CMOS interfacing
 
Hi

I'm trying to interface a 3.3v logic output (from a PCM2706 USB receiver) to a 5v TTL input (HC4024 counter). Is there any easy way to do this for example using a transistor switch? Googling hasn't brought any easy answers... or would swapping the HC for a HCT do the trick?

Any help greatly appreciated :)

Real_Macgyver 14th May 2006 09:03 PM

Yes, you can use a single transistor as a level converter. Use a CE stage, 3.3V input on the base and output on the collector.
The collector needs a resistor on a few kohm and the emitter is grounded. The collector is connected to +5V and the output signal is taken after the collector resistor. This makes it swing between +5V and 0V. Notice that the signal will be inverted. Don't forget the base resistor.

BlackCatSound 14th May 2006 10:01 PM

You should be able to find the input switching thresholds for the TTL gate, but an HC is not TTL, its CMOS.

Its very likely the 3v3 output will drive it directly, no need for any 'glue'.

Dave 14th May 2006 10:27 PM

HCT logic will work with 3.3v input signals.

Zodiac 14th May 2006 10:47 PM

Quote:

Originally posted by Real_Macgyver
Yes, you can use a single transistor as a level converter. Use a CE stage, 3.3V input on the base and output on the collector.
The collector needs a resistor on a few kohm and the emitter is grounded. The collector is connected to +5V and the output signal is taken after the collector resistor. This makes it swing between +5V and 0V. Notice that the signal will be inverted. Don't forget the base resistor.

Thanks, I kinda thought that would do the trick....sounds straightforward enough! I guess 100r for the base and 2k for the collector would be ok?

Zodiac 14th May 2006 10:49 PM

Quote:

Originally posted by BlackCatSound
You should be able to find the input switching thresholds for the TTL gate, but an HC is not TTL, its CMOS.

Its very likely the 3v3 output will drive it directly, no need for any 'glue'.

Yeah I made a mistake - it's CMOS not TTL therefore I was having voltage threshold problems, figured that bit out with a scope after tracing the circuit from the master clock output of the receiver

Thanks for the help :)

Zodiac 15th May 2006 10:43 PM

no luck it seems... inserted an NPN transistor switch in the circuit, base connected via 470r resistor to the output of the receiver chip (high state 2.8v, low 0.4v), connected emitter to gnd and collector to 5v via 2k47 resistor. Output to circuit from collector / resistor junction.

Looked at it with a scope and multimeter - looks like the collector is saturated permanently (hanging at 15mv or so), scope shows signal on one side of the 470r resistor, not on the other

Any ideas? Why not working!!!

darkfenriz 15th May 2006 10:47 PM

Try open collector or open drain buffer.

Zodiac 15th May 2006 11:03 PM

Quote:

Originally posted by darkfenriz
Try open collector or open drain buffer.
Whats an open drain buffer when its at home!?

Also what should I be looking for?

Dave 15th May 2006 11:11 PM

Just swap to an HCT device, the transistor and pull resistor will be probably be too slow on the rising edge anyway.


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