CMOS/TTL Interfacing
![Timothy D. Swieter](https://forums.parallax.com/uploads/userpics/482/n6OIB3WNWVK0M.jpg)
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
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
Comments
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.
...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
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
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Timothy D. Swieter
www.tdswieter.com
One little spark is all it takes for an idea to explode
(7 - 9 nS propagation delay, instead of 15nS for the HCT)
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