3.3V/5V circuits without Level translators.....anyone done that before

[europus2002]

Hi all,
I have interfaced MSP430F149 microcontroller to a standard TTL gates. Do you guys see any problem with that besides reduced noise immunity?? The VIH (L) for the gates is 2.4V and output from controller should step around 0-3.3V. I am doing this to avoid any use of translators etc..The microcontrroller datasheet shows no provision for TTL inteface....

 

[benta]

Should be OK.
I just wonder why someone would use current-guzzling TTL together with a nice low-power part like the MSP430.

Benta.

 

[europus2002]

I have relays that need some power to turn on and off, so TTL ....that's the reason i used gates for buffering capacity....but there aren;t many gates that gimme Vih level of 2-2.7V........The relays are in the path of high voltage of the order of 400V Dc...and i couldn't find any good replacement for the relay...Any suggestions for this.....since i m stuck.....unless i move over to PIC microchip controllers.....what do u suggest...

 

[Skogsgurra]

I think you are quite OK with the 3.3 V micro feeding the relay drivers. No problems. You don't even need to slow anything down to avoid transients interfering. The relays are slow enough to be their own "filters". Both for "on" tranients and "off" transients. You do have diodes parallel to the coils - I presume?

Gunnar Englund

http://www.gke.org

 

[europus2002]

Hi skog, Thanks for all who replied and tried to help....I just tried with 74LS04 and it drives the output levels to 5/0V all the way down to around 2.1-2.2V of driving signals...i will use same chips but the issue is still with inversion....i will correct in software....Thanks for the tip on the diode....i haven;t used it since i have a transistor (BC847A) having relay coil in collector circuit.....do u think it will be still necessary??? I hardly use relay in my designs ....This is a unique application after about 4-5 years....

 

[Skogsgurra]

Yes. It is needed to take care of the kick-back when you turn the coil off. There are substrate diodes that takes care of the circulating current, but it will mean taking a risk that could cost you a lot. The LS series is not intended to be used that way. The ULN2xxx series (Sprague and others) has everything you need and you can use 24 V coils with them if needed.

Gunnar Englund

http://www.gke.org

 

[Comcokid]

Check the TI web site. They have a application note on level translating for the MSP430. Resistor dividers work fine for inputs. Outputs are more difficult and may require a translator. On my MSP430 designs I use Fairchild tiny logic like the NC7WZ07 and such. For low power design where sections of circuits are OFF, these gates present a high impedance when unpowered and allow the processor to sleep at low current.

 

[europus2002]

Hi skog, thanks....i will update accordingly...I thought of using ULN chips but sometime back i had problem aquiring the basic most of the series ULN2002....and since we don;t have high volumes, i prefer designing with parts that are guranteed to remain off the shelf even 2-3 years from now.
As regards comco's comment, the chip looks exactly what i want ....how long have u been using it?....

 

[europus2002]

Comcokid,
Another thing is, i have 5 ADC channels....i m confused since reference voltage can be set to 2.5V but input swing 0-3V only?? Does it allow 0-5V swing?....I tried looking up the datasheet and user's manual...didn't find clear picture...what say?