Interfacing 3.3 Volt and 5 Volt Devices

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I am looking to use a pulse output generator from Al Williams with my Butterfly. The problem is that the output generator operates at 5 volts and the Butterfly at 3.3 volts.

I have searched for a thread on this but did not find something that I thought would work. Maxim has a Logic Level Translator chips. Are these a good answer or is there something else that will work.

The pules output chip takes serial data with a ShiftIn and ShiftOut system ( I believe SPI will work) for control and outputs a pulse stream as required.

I have used this before and it works very well, I just forgot about it. For production type projects it is likely too expensive but for my hobby type stuff it is easier, I just need to bridge between 3.3 volts and 5 volts.

Thanks

Bob Parry

Bob Parry

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I wonder where you have searched...there are many threads about it...

for interfacing from 3.3V to 5V every HCT type logic is good.

Klaus
********************************
Look at: www.megausb.de (German)
********************************

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If you're only talking about one or two pulse inputs/outputs to/from the Butterfly, why not use an NPN transistor with the pull up resistor from the collector to 3.3V. Or, you can use a PNP transistor with a pull down resistor betweem the collector and GND, with the emitter connected to 3.3V

Both of these configurations will invert the pulse. If that is a problem, connect the NPN transistor as an emitter follower, with the collector connected to 3.3V and a resistor between the emitter and GND.

If the pulse comes from the Butterfly, use the same logic for the 5V side as was used for the 3.3V side.

And of course, you will have to drive the base of the transistor via a resistor between the signal source and the base.

You can avoid reality, for a while.  But you can't avoid the consequences of reality! - C.W. Livingston

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Carl:
What do you gain by using
* 2 resistors and a transistor (inverted) or
* 5 resistors and 2 resistors (non inverted) instead of
a single gate logic/pico gate logic?
* HCTxx/AHCTxx for 3.3V to 5V
* AHCxx or NC7SZxx for 5V to 3.3V
for about Euro 0.20 @ 100pcs at Farnell.

I like the small footprint, low part count, low power, high speed with symmetrical(predictable) fall- and risetimes...
Look at the datasheets and your dealer...

For sure a single transistor is cheaper, but if you take layout, pcb area, assembling and the increased fail rate into account....

Klaus
********************************
Look at: www.megausb.de (German)
********************************

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If the 5V device is TTL, simply putting a 50 ohm resistor between the input to one and the output to the other will be fine.

Regards,
Paul

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MegaUSBFreak wrote:
Carl:
What do you gain by using
* 2 resistors and a transistor (inverted) or
* 5 resistors and 2 resistors (non inverted) instead of
a single gate logic/pico gate logic?
* HCTxx/AHCTxx for 3.3V to 5V
* AHCxx or NC7SZxx for 5V to 3.3V
for about Euro 0.20 @ 100pcs at Farnell.

I like the small footprint, low part count, low power, high speed with symmetrical(predictable) fall- and risetimes...
Look at the datasheets and your dealer...

For sure a single transistor is cheaper, but if you take layout, pcb area, assembling and the increased fail rate into account....

Imediate use, if the OP had the transistors & and resistors in his/her possesion and was breadboarding. I too would use a standard level shifting if it were say, the full 8 bits of an I/O port.

But for one input or output signal, I'd probably use the transistor and two resistors. In fact, I am finishing up a project where I made just such a decision regarding the Rx line for RS232 command input. THe realestate was available and the two 1206 resistors and SOT23 transistor fit nicely on the PCB.

So, what did I gain? Thats subject to opinion, for sure. But, for me, it was the convienience of not having to put a full blown RS232 converter and 4 capacitors on the PCB.

And, while you question my judgement in the use of transistors, (I get that continually by the morons at work) rather then a full blown level shifting IC, it provided the OP with another alternative that might have better fit his requirement.

You can avoid reality, for a while.  But you can't avoid the consequences of reality! - C.W. Livingston