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Forum Index : Microcontroller and PC projects : 3v input problem....

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Phil23
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Joined: 27/03/2016
Location: Australia
Posts: 1664
Posted: 11:46am 02 Jun 2017
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So what's the disadvantage of doing the hardware solution like Geoff uses in most of his GPS projects? It was what I had in mind for all my HC-12's, which I'd rather run on 5V, so there's less load on the 3.3V supply.





Phil.
 
JohnS
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Joined: 18/11/2011
Location: United Kingdom
Posts: 3678
Posted: 12:10pm 02 Jun 2017
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Do you mean to have GPS Vdd at 5V?

What is the max voltage drop A-K when TxD is high i.e. what is the max volts on pin 22?

(And is it within PIC32 spec?)

I take it that the 1k is intended to provide a current limit? But as there are apparently no clamping diodes in the PIC32 is the 1k guaranteed to work?

JohnEdited by JohnS 2017-06-03
 
WhiteWizzard
Guru

Joined: 05/04/2013
Location: United Kingdom
Posts: 2794
Posted: 02:45am 03 Jun 2017
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  Grogster said   Hey there.

There is a 10k external pullup on the receiver board. . . This pulls up the data line to 5v at any time that the receiver module is NOT pulling the line down, so it stays idle high via the 10k resistor, even if the receiver is outputting 0v, if you see what I mean. This is simply to prevent the line floating and being susceptible to rouge data, I expect - just about any receiver you look at, has some form of pullup on the RXD line.


Perhaps I've missed something - but why not remove this pull-up on the Dout, and then simply add your own to 3v3. Then can feed into MM input.
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piclover
Senior Member

Joined: 14/06/2015
Location: France
Posts: 134
Posted: 12:46pm 03 Jun 2017
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@Phil23

In your schematic, the 1N4004 diode (200V/1A rectifier diode) is an overkill, and being a Silicium diode, it will only clamp at 0.7V or so above Vdd, which is beyond specifications for a PIC 3.3V inputs.

I use small BAT85 Schottky diodes for such a clamping (will clamp at around 0.3V above Vdd, i.e. within specs).
 
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