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MSP430 TTL Compatibility at 3.3V

Started by Marc H April 21, 2013
Hello...

I'm new here and to the MSP430 as well as Uc's in general so please be patient.

I have gotten a launch pad and a couple of different Uc's to start using and I have been trying to familiarize myself with some concepts before I do something stupid and ruin some chips.

I see on the data sheet that the devices @3.3vdc will output a signal that is TTL compatible but I cannot find any tolerances on using the pins as input devices. In short I am still unsure if these devices are totally TTL compatible @ 3.3V or whether special considerations need to be made when interfacing them to TTL.

Thank you in advance.

Marc

Beginning Microcontrollers with the MSP430

Five volts on an input is not good; it will forward bias protection diodes on the input. They could be damaged and could cause the pin to act funny even in output.

Below are the Absolute Maximum Ratings from SLAS504f.PDF which is the datasheet for the MSP430F22x4. I am sure you can find similar information in a datasheet available from TI.COM for your processor.

*****************************************
Absolute Maximum Ratings(1)
Voltage applied at VCC to VSS -0.3 V to 4.1 V
Voltage applied to any pin (2) -0.3 V to VCC + 0.3 V
Diode current at any device terminal 2 mA
Unprogrammed device -55C to 150C
Storage temperature, Tstg
(3)
Programmed device -55C to 150C
(1) Stresses beyond those listed under absolute maximum ratingsmay cause permanent damage to the device. These are stress ratings
only, and functional operation of the device at these or any other conditions beyond those indicated under recommended operating
conditionsis not implied. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
(2) All voltages referenced to VSS. The JTAG fuse-blow voltage, VFB, is allowed to exceed the absolute maximum rating. The voltage is
applied to the TEST pin when blowing the JTAG fuse.
(3) Higher temperature may be applied during board soldering process according to the current JEDEC J-STD-020 specification with peak
reflow temperatures not higher than classified on the device label on the shipping boxes or reels.
***********************************************
Emmett Redd Ph.D. mailto:E...@missouristate.edu
Professor (417)836-5221
Department of Physics, Astronomy, and Materials Science
Missouri State University Fax (417)836-6226
901 SOUTH NATIONAL Lab (417)836-3770
SPRINGFIELD, MO 65897 USA Dept (417)836-5131

In statesmanship get the formalities right, never mind about the moralities. -- Mark Twain.
Thanks for posting. Made me look ;-).

That is basically from the data sheet and what was tripping me up except when I read your post, I picked up on this...



@3.3v supply then the max voltage on an input pin would be 3.6vdc correct ? That is much higher than the typical 2.6V TTL logic level. Is this correct or am I missing something ?
Basically, you can "output" from the MSP430 but can't "input" to it at +5V TTL levels. However, you might want to look at the F51xx series. These devices have some ports dedicated to interfacing at +5V TTL levels. There is a separate VCC pin for the +5V for these ports. DigiKey has stock on the MSP430F5172.
Jim
Or look at the large selection of level-shifting ICs from TI. I used one
in a recent project to go the other way -- to/from 3.3 volts at the MSP430
from/to 1.8 volts for a Bluetooth chip (another TI product, a CC2541 if I
remember correctly). The level-shifting chip is supplied by both power
rails, so it "knows" what the logic levels should be on each side.

HTH,

Mike
Marc H wrote:
> Hello...
>
> I'm new here and to the MSP430 as well as Uc's in general so please be patient.
>
> I have gotten a launch pad and a couple of different Uc's to start using and I have been trying to familiarize myself with some concepts before I do something stupid and ruin some chips.
>
> I see on the data sheet that the devices @3.3vdc will output a signal that is TTL compatible but I cannot find any tolerances on using the pins as input devices. In short I am still unsure if these devices are totally TTL compatible @ 3.3V or whether special considerations need to be made when interfacing them to TTL.
>
> Thank you in advance.
From a quick search.


SL sources at .5 ma. I didn't check but I think the 430 will clamp this
fine. As far as output to ttl, I can not see an issue as the threshold
voltage is far below 3.3~3.6 volts. I've never considered it an issue as
I've interfaced to the likes of 5v stuff fine. The specks say it works.
What kind of legacy stuff are you working with? Is this one time or
production?

Best, Dan.

--- In m..., Mike Wirth wrote:
>
> Or look at the large selection of level-shifting ICs from TI. I used one
> in a recent project to go the other way -- to/from 3.3 volts at the MSP430
> from/to 1.8 volts for a Bluetooth chip (another TI product, a CC2541 if I
> remember correctly). The level-shifting chip is supplied by both power
> rails, so it "knows" what the logic levels should be on each side.
>
> HTH,
>
> Mike

Yes, done that too but tradeoff is extra hardware.
Jim
> Yes, done that too but tradeoff is extra hardware.
> Jim

Agreed. But better than a bunch of discreets. With one small chip, I get
4 conversions, and independent direction control ...and a reliable design.

Mike

PS: Having said that, level conversion is still a PITA. Complicated my
life, trying to make a very small wearable wireless device :-)