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View Full Version : Translator Chips from prop to 5v ttl/cmos



steprog
11-10-2010, 08:24 PM
Hello,
I'm surprizingly having a tough time with translating from prop to a power h-bridge mc33886. Anyway, T chap recommended the cd4050 and that would work but can't get tssop versions.

I just wondered who out there has used any decent translating chips. I got one shot at this board.
Thanks,
Greg

JonnyMac
11-10-2010, 09:05 PM
If it's output only, I just used a 74HCT04 in a design (driving TPIC6A595 output expanders). In my case I used two gates per line (luckily, I only had three) so the 3.3v to 5v translation is not inverted.

Dr_Acula
11-10-2010, 09:41 PM
There are a number of 74HCT gates you can use. You do need to use HCT, not HC or F etc. HCT has a threshold for "High" that is less than 3.3V. (2.0V).

So you could use 74HCT04 gates as suggested above. That gives you six gates and they are inverted. Or you could use, say, a 74HCT244. That gives you 8 gates and they are not inverted.

I have never tried 74LS chips for this purpose, but they do list a 'High' as 2V as well.

I'm not so sure about going straight to a board without breadboarding it first.

Peter Jakacki
11-10-2010, 11:25 PM
Yes, the 74HCT series are the ones to use as they are cheap, readily available and in all kinds of packages. 74LS would not be suitable as any TTL variant will only output around 3.5V high due to the totem pole output which isn't much help is it??

They say that this part has TTL compatible inputs but I think there is an error in the datasheet for this part's Vih(min) which is stated as 3.5V. This is rather funny as that is the top limit of a TTL's Voh whose high output level can be as low as 2.4V.

Hmmm, if the inputs really are TTL compatible which they say they are then the Prop doesn't need any translators at all.

lanternfish
11-10-2010, 11:36 PM
Maxim & ST Electronics have a selection of suitable translators.

steprog
11-11-2010, 04:41 PM
Hi Guys,
I just now saw your responses. I ended up using TI's txb0106. I talked to someone at parallax and they showed me what they used on the robotic board. Looks to be a pretty nice little chip. A hope so anyway.
Thanks,
Greg

Phil Pilgrim (PhiPi)
11-11-2010, 05:31 PM
If you use a bidirectional level translator, like the TI unit, be sure to read the fine print. From the TXB0106 datasheet:


Pullup or Pulldown Resistors on I/O Lines

The TXB0106 is designed to drive capacitive loads of up to 70pF. The output drivers of the TXB0106 have low dc drive strength. If pullup or pulldown resistors are connected externally to the data I/Os, their values must be kept higher than 50kΩ to ensure that they do not contend with the output drivers of the TXB0106.

For the same reason, the TXB0106 should not be used in applications such as I2Cor 1-Wire where an open-drain driver is connected on the bidirectional data I/O. For these applications, use a device from the TI TXS01xx series of level translators.

-Phil

steprog
11-11-2010, 06:36 PM
No pullups, and I plan to drive this at pretty slow frequencies. I will look at the MC33886 data sheet again, but the board is in and the damage done. Thanks, Greg