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michaelb
Joined: 28 Nov 2005 Posts: 17
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Posted: Sat Dec 17, 2005 5:08 pm |
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Hi,
Some electronics questions. I'm using a Garmin eTrex to interface with a PIC16F877, using the hardware UART, and it's corresponding RX pin.
1) The GPS provides 4V high, and a -3V(ish) low. If Vdd is 5V and the schmitt triggers require 0.8Vdd = 4Vto go high, why does connecting the serial out of the GPS directly to the PIC RX not work?
2) I've used a 2 resistors and a NPN transistor to interface the GPS to the PIC.
5V - R1 - UART RX
R1 - Collector
0V - Emitter
GPS Out - R2
R2 - Base
As I have the PIC input connected to the collector, it is actually low when the GPS is high. i.e inverted. I saw this method online and never gave it much thought. Why does this work?
These 2 questions may well be linked - i suppose if the GPS output is inverted this would explain both.
Thanks for any help.
Michael |
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UFAnders
Joined: 13 Apr 2005 Posts: 36 Location: Michigan
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Posted: Sat Dec 17, 2005 5:16 pm |
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I imagine that a self-contained handheld module like the eTrex has standard (inverted) serial I/O. The PIC's hardware UART is TRUE LEVEL (not inverted) serial I/O. Perhaps this answers your question? ![Smile](images/smiles/icon_smile.gif) |
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michaelb
Joined: 28 Nov 2005 Posts: 17
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Posted: Sat Dec 17, 2005 5:29 pm |
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Excellent - thanks for that. Didn't realize that standard was inverted. It all becomes clear now.
Thanks again,
Michael |
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