Interfacing bi-direction serial 5V devices to the Prop !?!

ChrisE

Hello all!
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Firstly, I would like to wish everybody a Merry Christmas and a happy new year for this holiday season.
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Now back to the topic at hand, I have read through the sticky thread,·HOW TO SAFELY INTERFACE A 5V SIGNAL TO THE PROPELLER?, and although it is·a little 'cloudy' at points, I understand the parts about one-way communication between different devices.
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But would a simple resistor·be suitable for bi-directional communication between·a prop and Parallax's GPS module,·and·which value is best? 4.7K?··
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Thanks,
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Chris

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Beau Schwabe

ChrisE,
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"would a simple resistor be suitable for bi-directional communication between a prop and Parallax's GPS module" -· for receiving the RAW signals yes, a 4.7K resistor would be fine.
For sending it depends on what the input threshold voltage is for the 5V device. (<--This applies to any 5V device connected to a lower voltage interface.)·· ... I could not find this
information on the GPS datasheet, and I no longer have my setup connected.· But if I recall, the input voltage threshold is around 4V.· You could try this trick using a diode to the
GND connection of the 3.3V regulator supplying the Propeller.· Effectively this level shifts the Propeller I/O so that it outputs .6V for a logic LOW and 3.9V for a logic HIGH.
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I haven't tried this with the Propeller, but I have used this method with other processors running at 2V.· It might be just enough to get you within the input trigger threshold.








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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

ChrisE

Thanks Beau,·I will give it a try when I receive my Prop and·GPS module.·
I'm in the process of designing a·PCB·for a hand-held device so·I·need·some idea of the layout to accommodate·the additional components. I will include a·jumper (and pull-down resistor) to manually select RAW mode·if I have problems after proto-typing.
Thanks again.



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T Chap

Chris although what Beau described works, I always just use any of the Cmos 4050 devices as a buffer. For example if you want to get the Propeller out to a 5 volts device, then set 5 volts on the 4050 Vdd and it will turn on directly from the Propellers 3.3 output, producing 5 volts with higher current available as well. Likewise, to safely get 5 volt signals into the Propeller, simply put 3.3 volts as Vdd to the 4050, and it handles over voltage inputs by design, ouputting only 3.3v to the Prop. A 4050 has 6 i/o's. Obviously you would need one 4050 for inputs to the Prop set at 3.3Vdd, and one 4050 on the output set at 5 volts. I use 1k's on the Prop inputs from 5 volts all the time and have never seen a problem. Your device may turn on with 3.3volts, the products data sheet usually will show the minimum logic level to turn it on relative to the Vdd.