Interfacing with external hardware

Dan,

With regards to digital signals (GIO) the IOs on the MCU are generally referred to as 3.3V LVCMOS.   So they can drive your 5V TTL inputs directly, but coming in the other direction you need to introduce a level shifter or if the signal is slow enough you can use a voltage divider.

You might take a look here at this application report scea035 or this one scea040.    These are old but the shift from 5V logic to 3.3V logic was made a long time ago so they are the most relevant I think, to start with - without getting overly complex.       Once you get your head around the 3.3V LVCMOS <> 5V TTL case then you might look at the newer app reports on ti.com under the category TI Home > Interface > Voltage Level Translation.   

You didn't actually ask but the Analog inputs on the LAUNCHXL-TMS57004 launchpad are 3.3V only because that is all that is supported on the lower end TMS570LS0432 device.   If you were to move up in the family say to the TMS570LS1224 part that is on the new LAUNCHXL2-TMS570012 then there is an option to take 5V inputs into the ADC pins (but GIO still need translators).

Thanks, Anthony. Do you happen to know of any TI videos that show one how to do voltage translation? My 5V input signal to the GIO port of the LaunchPad will be a rectangular pulse of approximately 10 microsec duration. I need to edge trigger an hardware interrupt off the leading edge of that signal.

Dan

The video is here:

The concept is explained in the datasheet for TI FET BUS switch SN74CBT16245DGGR
(note: it's key to use this particular ic - I learned that the hard way)
The switch's primary use is not level shifting, but it fits the purpose in a fast and good way.

The schema design is from Jack Gasset from gadgetfactory.net . He designed it for his Papilio boards - biut it is working for the Hercules series too.

Jack's log on this board: https://github.com/GadgetFactory/Papilio-Wings/tree/master/BPW5015-Buffer-Wing

Hi Jan

That's a nice solution. What was your application?

I wont be needing that many IO pins in my application so I will probably use a SN74CBTD1G125 instead. I ordered and received these ICs but they are really small so now I need to find a similar IC that I can mount on a breadboard.

Dan

I am using this solution for level translation, where the frequency is high, and there is no need to buffer the signal.

I'm quoting Jack Gasset here, because I have not tested the solution up to that frequency:
The benefit of this is that it is bi-directional but also very, very fast. It can operate at speeds up to 2GHz.

This use as level shifter is documented in TI's specifications - so you are not hacking, you are using the IC as documented.
It is not doing buffering - that means that the current consumed by the receiver has to be provided by the sending device -
and I haven't checked its behavior in an open-drain scenario.

I'll have a look at the data sheet of the IC you are proposing. If it mentions the use as level shifter you should be fine.

Hey, the chip you selected has the level shift built in. The only difference with Jack's design is that it is uni-directional.
If you need input-only or output-only level shifting, you're good to go.