CMOS/TTL Interfacing

Timothy D. Swieter · 2006-06-05 01:20

First let me say, I love the Propeller!!· I have been playing with it for the past week and just love the power it has - I am looking forward to using it in a real project above just experimenting (maybe even for a project at work).
One area that I am having trouble with is the TTL/CMOS interface.· I have read else where in the forum about the 1k resistor between the Propeller input and the TTL devices output - no problem, I have seen this used before.· The problem I am running into is the logic high level of the TTL device is greater then the 3.3V that the Propeller provides.· One device I am working with is the Hitachi HM55B Compass module that Parallax sells.· I believe my problem is with the logic·levels, any advice for interfacing the output of the Propellers to the inputs of the HM55B?· I worked with a coworker on a project a while ago with interfacing 3V CMOS to 12V CMOS, I could dig out that schematic; I don't remember what we did.· I thought I would check with you guys as you often have a quick, simple solution.
I will also be interfacing to the·Hitachi H48C, but have not studied the data sheet to know if I will run into the same problems.
I would recommend that Parallax create a guide (PDF) that explains the CMOS, TTL, and interfacing.· I see this as a possible resource for the many intermediate hobbyists who are going for a BASIC STAMP to the Propeller.

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Timothy D. Swieter
www.tdswieter.com

One little spark is all it takes for an idea to explode

Beau Schwabe · 2006-06-05 04:30

According to the spec's, the HM55B wants to see something like 4V as a logic HIGH.

You can try something like this simple level shifter below that will give you about
900mV for a logic LOW and about 4.12V for a logic HIGH.


                                    4.7K
                                 o--/\/\--> +5V
                                 |
                      D1   D2    |
0V to 3.3V Input >----|<---|<----o--------> 900mV to 4.12V Output

...If you want to use a transistorized Level-Up-Shifter then the attached schematic should work.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Post Edited (Beau Schwabe (Parallax)) : 6/5/2006 4:56:16 AM GMT

David B · 2006-06-05 14:55

A quick and easy way to do this is to add a 5V 74HCT14 schmidt trigger hex buffer chip to your board. The 3.3V propeller is able to switch its inputs, and you get 0-5V on the outputs. The logic level is inverted, but that can be accounted for in your Spin program. Plus with a single inexpensive part, you get 6 level converters.

Probably there are other chips that would also work; I used the 74HCT14 because I happened to have a bunch of them lying around my workbench.

I used the 74HCT14 on my Propeller board to both power and drive the data of my LCD display, and it works great.

David

Timothy D. Swieter · 2006-06-05 16:13

Thanks guys. I will check into both of these options. I like the simplicity of the 74HCT14 - I will have to order these as I am sure I don't have any laying around. I will stop by Radio Shack tonight and grab a few componets to try Bau's sugestions.

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Timothy D. Swieter
www.tdswieter.com

One little spark is all it takes for an idea to explode

Gadgetman · 2006-06-06 06:57

You may also want to look at the 74AHC14 as it is slightly faster.
(7 - 9 nS propagation delay, instead of 15nS for the HCT)

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Don't visit my new website...

T Chap · 2006-06-06 07:42

Not sure about switching times, but I use optocouplers all the time to change 5v to 24v, 24v to 5v etc. Maybe use a 4v Zener to modify the output downwards from 5v if you like the idea. Everything stays non-inverted, but you gotta pull the photo tranny output low with a 10k. Very few parts per channel to build.

CMOS/TTL Interfacing

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