1 (edited by jacubilloro 2006-08-30 03:36:27)

Topic: "Worlds cheesiest inverter" WTH?

Why would anyone in the world use a 74xx139 as a single inverter??? I know there has to be a HEAVY reason for doing this, but I can't seem to find one just by looking at the schematics. Specially when you already have a 74hct14 IC in there and use only 2 gates...
Can any chumbyan designer please tell me why the weird selection?

Re: "Worlds cheesiest inverter" WTH?

The 74HCT14 IC is on a 5V rail for compatibilty issues with 5V/higher voltage logic.

The 74xx139 is on a 3.3V rail. I didn't want to add yet another 74HCT14 device on the 3.3V, or throw in a single logic gate device (they usually come in really small packages with tight pitch). Space was tight and I also wanted to use wide-pitch packages where possible so that most hobbyists can solder onto the board with a standard soldering iron.

So...I had a free mux, turned it into an inverter! (the other half of the device is used to demux the chip select line from the chumbilical)

7BAA 2E53 01C1 DCFF 497B  E7F0 9699 A303 78F0 D9B9

Re: "Worlds cheesiest inverter" WTH?

Reason seems good enough for me smile
Another thing, Isn't it kind of dangerous to have the 2 used outputs from the 74VHC244 buffer connected toghether? Aren't you concerned about creating a "short"???. I know that at 3.3V there is probably no danger to burn the chip, but still...

Re: "Worlds cheesiest inverter" WTH?

Haha, I just caught this myself just a few minutes ago, but I realized that you were using the other half of the chip and I thought it made some sense after thinking through the logic diagram for 10 minutes (though honestly, it still didn't make a lot of sense, but I guess it is cheaper than sticking another piece of logic on there ;P).

As for the two outs being connected together.. *shrug*. smile

Re: "Worlds cheesiest inverter" WTH?

bunnie wrote:

... I also wanted to use wide-pitch packages where possible so that most hobbyists can solder onto the board with a standard soldering iron.

Regarding this, Is there any advice you might give us about how to use the MX21 easily? There are breakout boards, but most of them are $400 evaluation kits...
Any suggestions?

Re: "Worlds cheesiest inverter" WTH?

The 74VHC244 has two independent output enables. OE1 controls Y1-4, OE2 controls Y5-8. CS2 and CS3 are mutually exclusive (guaranteed by the 2-4 demultiplexer logic function) so this should effectively behave as a mulitplexed tri-state bus free of contention, which is how I believe the master in/slave out line of the SPI bus should be wired up.

Good questions though!

7BAA 2E53 01C1 DCFF 497B  E7F0 9699 A303 78F0 D9B9

Re: "Worlds cheesiest inverter" WTH?

The MX21 is not very easy to use, unfortunately. We have some eval kits that we bought for a lot of mulah. Your best bet is...probably to use a chumby. tongue Or suck up to a Freescale sales rep.

7BAA 2E53 01C1 DCFF 497B  E7F0 9699 A303 78F0 D9B9

Re: "Worlds cheesiest inverter" WTH?

Our best bet is to use the very hardware we want, but can't obtain.. ahh life, how cruel.

Re: "Worlds cheesiest inverter" WTH?

doh! sad yes, ramping up to production is a lot of work. I'm working on it!

7BAA 2E53 01C1 DCFF 497B  E7F0 9699 A303 78F0 D9B9

Re: "Worlds cheesiest inverter" WTH?

And me too smile

Joe

Re: "Worlds cheesiest inverter" WTH?

Thnkz guys
Keep the hard work!
smile