20x2

johnlong

Senior Member
Hi All
Just a quick one
I have a 20x2 on a board I have made for it for a project I am playing with
I make up my own breadboard adapters for the down load side of things
when the adaptor is attached the board runs as expected, but when disconnected
from the 3 header pins the board does not run at all.
I use this approch with my 28x2 and 40x2 they both run with the download circuit disconnected.
So does the 20x2 need a perminate attachment to the download circuit or if not any ideas
what may be amiss.
regards
john
 

Hemi345

Senior Member
Yes, you should include the download circuit. Without it, the serial pin will float and issues running the program will result. I have seen others pull the SERIN pin low using a 100K resistor as this doesn't cause any issues when the 10K/22K download circuit is connected and allows the PICAXE to function correctly while it's disconnected.

I usually just include the 10K and 22K resistors in every design and use a 3 pin header as it doesn't add much to the PCB anyway.
 
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johnlong

Senior Member
Thanks
just seemed strange to me as the 28x2 and 40x2 does not display this
issue will happlyrun the program without any of the above
 

lbenson

Senior Member
Depending on what your board and your download cable are like, you may or you may not need to have it attached. The download circuit is not required, but the Serial In pin must be pulled low. As Hemi suggests, if you have a 10K-100K resistor on your board doing that, you shouldn't need the download cable.

Can you post photos of your 20x2 and 28x2 boards?
 

westaust55

Moderator
All PICAXE require as a minimum a pull-down resistor on the Serial in pin to start the BASIC program.
Once running you can use the DISCONNECT command which will allow you to use the SERIAL-IN pin as another digital input.

If you do not RECONNECT then a hard reset will likely be required to reprogram.

I am one of those who use a 100 kOhm resistor when not having a permanent download circuit on a board - my cases are when there are multiple PICAXE chips on one board.
 

inglewoodpete

Senior Member
Thanks
just seemed strange to me as the 28x2 and 40x2 does not display this
issue will happlyrun the program without any of the above
In low humidity and/or electrically 'noisy' environments, any PICAXE will display problems if the minimum circuit requirements (Manual 1) are not met.
 

hippy

Senior Member
just seemed strange to me as the 28x2 and 40x2 does not display this
issue will happlyrun the program without any of the above
Some silicon will tend towards floating high while other silicon will tend towards floating low. There is usually no guarantee of what behaviour any input pin will have. You seem to have got lucky with your 28X2 and 40X2 tending to float low, unlucky with the 20X2 which doesn't.

The 22K/10K to 0V, or a 100K, or across that, ensures that a floating input will see a low so takes luck out of the equation.
 

Buzby

Senior Member
Floating inputs can be very unpredictable !.

A few years ago I had a 40X2 circuit which didn't start properly when I pressed the reset button on the board, but ran perfectly if I switched off the PSU and then powered up.

I spent two days trying to find a software or wiring issue, looking for some difference between power-up and reset, but found nothing.

Eventually, after lots of debug sertxds, I found that a floating input, partially coded for future use, was reading 'high' when I pressed the reset button, but 'low' when the PSU was cycled.

More detailed diagnostics showed that the capacitance of my hand could affect the floating input, but because my hand was only near the 40X2 when I was pressing the reset button that's the only time it had an effect.

Floating pins can lead to tricky problems.

Will I ever learn ?

Cheers,

Buzby
 

johnlong

Senior Member
Thanks folks
There is always something to learn with this little piece of black magic
it makes it such fun
Will try the 100K aproach as there is space and a convient ground near by
if not it wont hurt to leave the download circuit attached.
Without a shadow of a dout this forum is superb
regards john
 

johnlong

Senior Member
Thanks folks
100k worked lovely will have to remember to attach 1 on everyboard
I intend to remove the download circuit from
regards john
 

RNovember

Well-known member
Resistors are cheep.

Especially if you intend to reprogram that circuit in the future, it might be easier to just leave the resistors there, and not have to bother with the hassle of changing the circuit between programming sessions.

That just seems easier to me.
 
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