When connecting a 5V <-> 3.3V communcation, do we need transciver or level translator

[matrixofdynamism]

When connecting a 5V <-> 3.3V communcation, do we need transciver or level translator

Transceiver is a device that can be used for transmit and recieving of data. Thus, two components that do not have a tri-state output can be connected together and commmunicate without contention on a bus. If level translation is required since the two devices work at different power supply voltages, this must be done to prevent device damage.

I need to connect a 5V device to a 3.3V device. Do I need a "voltage level translator" IC or a "trasceiver" IC or a "transceiver with voltage translation". In other words, is a voltage level translator also a trasceiver, what is the difference? What do I need?

 

[Easyrider83]

Re: When connecting a 5V <-> 3.3V communcation, do we need transciver or level transl

If the both devices located near each other on one pcb, you can use this connection:
5V <-----> 3.3V
OUT -----> Resistive devider
IN <------- Direct connection

 

[matrixofdynamism]

Re: When connecting a 5V <-> 3.3V communcation, do we need transciver or level transl

There are multiple pins, using resistor divider is a good idea but does not appear "proper". I guess if something works, it is acceptable.
So you mean to say that the transceiver has to do with things off the board, more like communication through backplane or across devices. However, the voltage level translator is for things on the same board and that really either could be used in my case? I shall be connecting a NES controller to a CPLD. I intend to use a 3.3V CPLD. The voltage conversion is required here since the NES controller takes 5V supply whereas the CPLD will not accept such a big input.

hmm, perhaps I can have 5V VCCIO and not use the voltage translator but that shall require looking into the design in more detail.

 

[Easyrider83]

Re: When connecting a 5V <-> 3.3V communcation, do we need transciver or level transl

Many CPLD allow you to choose I/O voltage level. Sure, if you planing to trace a lot of them, you will need a voltage shifter. Like TXS0108E

 

[spudboy488]

Re: When connecting a 5V <-> 3.3V communcation, do we need transciver or level transl

You would need to evaluate your design. If you have lines going in only 1 direction, the chart Easyrider83 provided would work. If you have bi-directional lines, a level translator would be required. Also, check for a CPLD that has 5V tolerant I/O. That helps to simplify.

 

[KlausST]

Re: When connecting a 5V <-> 3.3V communcation, do we need transciver or level transl

Hi,

It depends on so much details..

Some 3V3 supplied logic families are 5V input tolerant, like AHC.
Some 3V3 supplied PLDs are 5V input tolerant, like XC95..XL.

But usually 5V supplied HC family input high levels are 0.7 x VCC wich is 3.5V. So with a 3V3 level you arenot on the safe side. Often it works, but is is not guaranteed.

Then it depends on interface type.

For a bidirectional I2C data line (open drain with pullup) you can not use resistive dividers, nor the typical transceivers. You need special transceiver circuits.

There are special ICs for 3V3/5V SD card applications.

There are 8 bit wide transceiver circuits for bidirectional data bus with two supply pins: 3V3 and 5V. Like 74LVX4245.

And and and..

Maybe you should give us more informations..

Klaus

 

[matrixofdynamism]

Re: When connecting a 5V <-> 3.3V communcation, do we need transciver or level transl

I finally decided on octal transceiver

 

[FlapJack]

Re: When connecting a 5V <-> 3.3V communcation, do we need transciver or level transl

I always found that a series resistor worked fine. If you limit the current you can not burn out an input.

 

[matrixofdynamism]

Re: When connecting a 5V <-> 3.3V communcation, do we need transciver or level transl

Dear FlapJack, using a resistor in series seems to be a good idea as does using resistors in voltage divider configuration. So since this can potentially work if communication is 5V device -> 3.3V device but not necessarily the other way around due to not meeting the voltage thresholds, I assume that using resistors or not depends on the direction of data. In this case 2 signals goto the 5V device and 1 comes back.

I read the datasheet of a CPLD that I can potentially use in final version of my project. It works at 3.3V but can be made to work with 5V range as well. Basically we first enable an internal clamp diode, that clamps the voltage in the CPLD pin *inside* the CPLD at 3.3V if it rises above it. Then we setup this input as a open gate/collector input and use a pull up resistor to 5V *outside* the CPLD.

The problem with this approach is that according to the datasheet, the CPLD may not support a continuous 5V input for very long and gives duty cycle values for different >3.3V inputs applied to the input with this internal clamp diode enabled and external pull resistor present. This makes me feel uneasy since it gives the impression that even though the CPLD can work this way with 5V inputs, it potentially creates problem of reliability.

Now, I can see that you do have a point since if the current is kept below the maximum permitted for the input buffer, the buffer should live ...to see another input. I shall keep this in mind.