interfacing 5v stuff.

buffler

OK, I'm taking the Great Leap Forward to the Propeller chip from all those hopelessly inadequate pic thingys etc.
I suddenly find ALL MY I/O STUFF AND SUPPORT CHIPS ARE 5 VOLT AND I CAN'T FIND ANY INFO ON HOW TO HOOK THEM TO THE &%^#&**&(()& 3.3 VOLT I/O PORTS ON THE PROPELLER.
Now that I've vented due no doubt to too much caffeine, where the heck can I get a simple primer. I assume there's going to be a whole bunch of glue junk and resistors and diodes and a transistor as big as the Propeller chip for every output and stuff like that. Too depressing for words. Thanks for putting up with this rant, hope there is something SIMPLE to do. I admit that I tried the search on the forum, but I did not get anything even remotely useful...
Oh, BTW, the development board I have has a 5 volt regulator with a 7 volt absolute maximum input voltage. The power connector says 6-9 volts. I want to hook it to 12 volts like almost all other magic development boards. If 9 v is OK, is 12 volts?
A hopeless and bewildered noob.

Don

Oldbitcollector (Jeff)

Hi Don,

First of all, welcome to the Propeller.

Yes, the 3v nature of the Propeller is something that you need to consider when you are interfacing sensors and other semiconductors. Here's some tips to chew on.

Invest in some 1K resistors. Generally 1K resistors will be perfect when communicating with sensors that operate at 5v. Placing one of these on the signal line between the sensor and the Propeller will protect it from damage.

Many semiconductors have 3v versions. Example, the MAX 3232 communication IC is a perfect substitute chip for the MAX 232 communications IC. Many ICs are able to read the output I/O from the Propeller as they generally operate with an input range of 3v-5v. (Check the datasheets.)

We've all hit the 5v issue, related threads and advice are common here. In the end, the power of the Propeller is worth the effort. You'll unlocked the door to amazing capabilities compared to the PICs you've been using.

OBC

Phil Pilgrim (PhiPi)

This should help: http://forums.parallax.com/showthread.php?p=585920

OBC wrote:

Invest in some 1K resistors. Generally 1K resistors will be perfect when communicating with sensors that operate at 5v. Placing one of these on the signal line between the sensor and the Propeller will protect it from damage.

Actually, 1K is too small. Series protection resistors need to be at least 2.2K to meet the Prop's 500uA protection diode maximum current rating:

(5V - 3.3V - 0.6V) / 500uA = 2.2K

IIRC, the 1K thing got started before the max spec was characterized and has taken on a life of its own, despite the more recent indications to the contrary.

-Phil

Oldbitcollector (Jeff)

Even as I typed that I *knew* that Phil will jump in with the 2.2K recommendation.

I've developed my habit of 1K resistors thanks to the many Parallax sensor demos which employ them.
(To date I've not had issues, bit Phil is far wiser than I am with this stuff... )

OBC

Phil Pilgrim (PhiPi)

Man, I hate to be the troll. But the thing about exceeding the max rating is that damage can be slow and unnoticeable, but cumulative until sometime down the road, things start to flake out. I've heard at least one Parallax engineer recommend 3.3K, just to be safe.

-Phil

Oldbitcollector (Jeff)

This is an issue which deserves some consensus.

1K, 2.2K, 3.3K... Correct answers depending on who you ask.

OBC

Dr_Acula

I assume there's going to be a whole bunch of glue junk and resistors and diodes and a transistor as big as the Propeller chip for every output and stuff like that.

1) For a 5V device into the propeller, use a 2.2k resistor.
2) For bidirectional interface to a 5V device, use a 2.2k resistor.
3) For output only, you may not need anything. For instance, the logic level High for some TTL, eg 74HC is not quite high enough at 3.3V. But if you use chips from the 74HCT logic family they all work fine with 3.3V as the input.

There should be a solution for most of the parts you have. There are solutions for keyboard, mouse, LCD displays, analog and digital. And some things work in your favour - eg SD cards all work at 3.3V.

buffler

Thanks! Once I had my snit, I found a nice pdf doc with a bunch of interface info, just what I sought, and just what i thought, lotsa annoying glue stuff, what a drag. Is there a way to include a pdf file for others to download? I found ways to put in almost everything else, including videos...I also found recommended 3.7k resistors.Fairchild has a nice app note to use a 4 pole data switch that has essentially no delay.
Don

Oldbitcollector (Jeff)

While Phil and I are tearing off in the distance on the best way to protect your Propeller from 5v interfacing damage, I see somehow we both missed part of your original question regarding power input.

Many of the Propeller boards are labeled 6v-9v DC input. I've even seen boards which are labeled 5.5v-12v DC input which use the same regulation circuits. Generally I've found that 7.5v DC is about perfect for what I'm doing.

The more "current" you pull through the regulator circuit of your Propeller board, you will increase the heat of the regulators, and possibly damage them as they are not heat-sinked. If you look at the datasheet for the 7808 & LM2937 5v regulators used by many Propeller boards, you'll find them forgiving about a range of voltage inputs. I've only run into issues when trying to operate a bunch of power hungry servos or other high current devices.

I can only relate my own experiences again here, and I'm sure other more qualified answers will be presented as well.

OBC

Phil Pilgrim (PhiPi)

buffler wrote:

Is there a way to include a pdf file for others to download?

Yes, you can attach a PDF file to your posts ("Manage Attachments" under "Go Advanced"); or, if the file is already on the web somewhere, you can include a link to it in the body of your post.

-Phil

jazzed

Oldbitcollector wrote: »

This is an issue which deserves some consensus.

1K, 2.2K, 3.3K... Correct answers depending on who you ask.

OBC

Depends on the interface. A "pull-up" interface like I2C may not need series resistors at all except for paranoia if outputs are never driven high. The demo board's keyboard interface seems to violate all the rules, but the pins are not supposed to be ever driven high. With an output that is really 5V to Propeller, at least 2.2K should be used.

buffler

Gosh, I hope this works. It's useful, so might make a sticky.

3.3Vtipsandtricksen026368.pdf

also, google Fairchild AN-996 for the fet switch approach.

Re the power question, the regulator on my board is a UTC UZ1086L, which data sheet from the mfg. says is a low dropout regulator absolute maximum input 7.5 volts. Hence my confusion. A good-ol' 7805 is good for 12 v easily; I usually put a heatsink on them when operating that way. I do have a glob of 9 volt wall warts, but this board is going in a 12v unit. I'll pull the 1086 and put on a 7805 although, horreurs, I might alter the Pb free status of the board and destroy what's left of my once brilliant brain.
Thanks
Don