UART level translation, what type of translation IC is required?

Community General Discussions
1

I have two devices, and need to figure out how to translate between them for 2 wire UART.

U-Blox NEO-5G, 1.8V
Input low: 0.22V (MAX)
Input high: 0.91V (MIN)
Output low: 0.4V (MAX)
Output high: 1.4V (MIN)

Telit GC864
Input low: 0 - 0.5V
Input high: 2.1 - 3.3V
Output low: 0 - 0.35V
Output high: 2.2 - 3.0V

UBlox -> Telit
LOW: .4V (MAX) -> 0-0.5V (WORKS)
HIGH: 1.4V (MIN) -> 2.1-3.3V (DOES NOT WORK)

Telit -> UBlox
LOW: 0-0.35V -> 0.22V (MAX) (DOES NOT WORK)
HIGH: 2.2-3.0V -> .91V (MIN) (WORKS)

So i obviously need a level converter of some sorts.
What type of converter is needed? CMOS? If so how do I select the voltages?

Thanks!

2

have you connected the devices yet

How do you know " (DOES NOT WORK) ".
Have you connected the devices and tried it.

U-Blox Ouput High will never be less than 1.4V. It may actually be 3V.
Have you measured the output voltages ?

You are getting the specifications confused. You may not need a converter.

3

Well the U-Blox is a 1.8v

Well the U-Blox is a 1.8v device so I don’t think it would be possible for the high to be 3V. I assume connections won’t work because their ranges differ too much (by specification anyway).

I previously tried a direct connection between the devices but did not have any luck. This, of course, could be a software issue.

I will test the output voltages. I have a basic volt meter, will see what kind of numbers it gives me.

Still if the spec claims such a variation of voltages, even if the setup works once how am I guaranteed it will work with a different batch of the chips (ie same components but bought at different times or bought in volume)? I feel like there should be a more concrete solution.

4

What you need is the ST232
What you need is the ST232 (for 5VDC) or the MAX232 (for 3.3vdc). Just grab a ST232 and call it good.

5

Not at RS232 levels, but at 1.8 and 3.3 volt logic levels
A Google search for 1.8 volt to 3.3 volt level shifter turned up Fairchild Bidirectional Level Translators.

The 74VCX164245 does appear to be a part that can be sampled, best in a TSSOP package that would be a little challenging to solder. Might look for a Schmart board to prototype it with.

Also might look for other devices available, or possible level translation circuits. (involving a couple transistors as has been discussed in a recent I2C thread.)

6

used an I2C translator for

used an I2C translator for 1.8 to 3.3 V

works great. thanks guys!