Automatic ranging level convertor?

Question

I'm designing a PCB based on an ATxmega microcontroller (max. 3.6v tolerant). I plan to connect a range of external 3.3v devices. However, I do have a few 5v devices that I want to connect.

There are many designs and ICs for 3.3v <-> 5v bidirectional level convertors already out there. But here's the caveat: I want to allow both 3.3v and 5v voltages on the same pin. So, if the connected device is 5v, the level convertor will kick in and convert the voltage to 3.3v. If the device is already 3.3v, the level convertor does not convert the voltage.

For example, if a 5v device was connected, the circuit would look like this:

|------------|         |----------------------------|         
| 5v device  | <-----> | 5v <-> 3.3v level convert. | <-----> ATxmega pin
|------------|         |----------------------------|

If a 3.3v device was connected, the circuit would be connected directly:

|-------------|                  
| 3.3v device | <-----> ATxmega pin
|-------------|

What circuit will bypass the level convertor when the voltage is already 3.3v, and convert the voltage when a 5v device is connected?

In other words, can I design an automatic-ranging 3.3v<-->3.3v/5v level convertor (without jumpers)?

Rocketmagnet · Accepted Answer · 2012-03-31T18:59:41.397

3

The question is: How will your device know what voltage to use? The attached device will have to tell it some how. Will your device have a sense pin so it can test the attached device's supply voltage?

If so, the answer is easy. There are many voltage translators out there, e.g. the TI SN74LVC8T245. The input power supply will be your Atmel's 3.3v, while the output voltage supply can come straight from your device. This way, you will be able to connect a device with any voltage requirement from 1.65 to 5.5v, and it should just work.

Schematic

edited Mar 31 '12 at 18:59

answered Mar 31 '12 at 18:06

Rocketmagnet

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Awesome, that's exactly what I'm looking for. Although I have no idea how to make it have auto-sensing voltage. – mr_schlomo Mar 31 '12 at 18:35
Does the schematic help? – Rocketmagnet Mar 31 '12 at 18:59
Aaaah, that's how you do it! Thanks for the diagram. :D – mr_schlomo Mar 31 '12 at 19:07

score 0 · Answer 2 · answered Mar 31 '12 at 18:07

Just use a level-converting buffer that can accept a wide input voltage range. There are many companies that make them. The TI SN74ALVC164145EP is one of them, but there are hundreds more with different speeds, capabilities, voltage ranges, and number of bits.

Automatic ranging level convertor?

2 Answers2