How do I keep the RX line from powering the rest of my circuit. As soon as I plug in the rx line from a usart to usb adapter, it back feeds through the pic and powers up everything else on the board. The actual pic is not running, just other items which often messes up the pic initialization when I actually apply power to the board.
edit: actually, sometimes the pic also starts running when rx is plugged in.
Last edit: Jim giordano 2019-12-03
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Thanks Kent. This is such an irritation, I figured someone would have a solution. This is only the second pic project I've tried that receives characters through RX so I just haven't connected it in the past.
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P.S. you would need a pulldown resistor on the output to keep the RX line from floating when the opto output goes low. Also, if this is a low voltage (3.3V) supply then there would be a concern due to the Vbe drop of the npn.
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Well, I found that a 10 cent 2n7000 mosfet works, but it seems wrong to waste a pin for that. I thought there would be a jumper or something on the board. While this is a cheapie ebay cp2102 board, I have the same problem with the ten times more expensive microchip usb to usart adapter.
Edit: I tested just tying the gate to the plus rail and it seems to work, saving a pin. I'm almost happy :)
Last edit: Jim giordano 2019-12-04
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This is a very common issue with CMOS logic devices that have internal protection diodes on the I/O pins. Check the link below to see another method of eliminating this ghost powering.
Thank you, I knew someone had to have figured this out before. I don't understand how that could work, but I'll give it a try. Bat85's will be on my next electronics order :)
edit: I dawned on me later how it works. Thanks!
Last edit: Jim giordano 2019-12-06
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How do I keep the RX line from powering the rest of my circuit. As soon as I plug in the rx line from a usart to usb adapter, it back feeds through the pic and powers up everything else on the board. The actual pic is not running, just other items which often messes up the pic initialization when I actually apply power to the board.
edit: actually, sometimes the pic also starts running when rx is plugged in.
Last edit: Jim giordano 2019-12-03
A 4n35 optocoupler? You could power the npn supply with an output pin and output to the RX. Never tried it, just a thought.
Thanks Kent. This is such an irritation, I figured someone would have a solution. This is only the second pic project I've tried that receives characters through RX so I just haven't connected it in the past.
P.S. you would need a pulldown resistor on the output to keep the RX line from floating when the opto output goes low. Also, if this is a low voltage (3.3V) supply then there would be a concern due to the Vbe drop of the npn.
Jim,
I've seen a series resistor limit the current. like 1K or maybe even higher.
73
mike w9ys
Well, I found that a 10 cent 2n7000 mosfet works, but it seems wrong to waste a pin for that. I thought there would be a jumper or something on the board. While this is a cheapie ebay cp2102 board, I have the same problem with the ten times more expensive microchip usb to usart adapter.
Edit: I tested just tying the gate to the plus rail and it seems to work, saving a pin. I'm almost happy :)
Last edit: Jim giordano 2019-12-04
This is a very common issue with CMOS logic devices that have internal protection diodes on the I/O pins. Check the link below to see another method of eliminating this ghost powering.
https://sourceforge.net/p/gcbasic/discussion/629990/thread/a3553abe/
Bill
Thank you, I knew someone had to have figured this out before. I don't understand how that could work, but I'll give it a try. Bat85's will be on my next electronics order :)
edit: I dawned on me later how it works. Thanks!
Last edit: Jim giordano 2019-12-06