simple question about the minimum operating circuit

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chipset

Senior Member
Hey, just was wondering, is it necessary to have the download circuit hooked up to the PICAXE? Im thinking not but am I going to do something potentially damaging to it in the process? I figure as long as the PICAXE has 5v and 0v then it should function happily all day long. Am I wrong in this?

Reason I ask is that I would liike to program Pics on one board and transfer them to other components that dont have a download circuit.

If anything I though it may be a bit of a typo to say that the picaxe require the serial download circuit in order to function, or does it?

I await your responses.
 
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BradenBurton

New Member
Correct me if i'm wrong, but i'm pretty sure that it only needs the 2 resistor download circuit if you are trying to download data to it.
Once data has been loaded into it, the 2 resistors are no longer needed - if there is no longer any need for it to communicate to another chip or computer
It will act on it own,
Just my 2c worth - i'm new to this so don't take me 100% on that
 
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ylp88

Senior Member
You must keep the SERIN line pulled down with a resistor. If you don't keep it low, rather floating, then the PICAXE may erroneously reset as it unexpectedly expects a new program to be downloaded onto it.

Thus, if you want to program off board, then you can simplify the connection to a single resistor on the SERIN pin.

ylp88
 
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BradenBurton

New Member
Ah yes, if it is left floating it may get the new program about to be downloaded signal (by random left floating stuff happening :). right ?
 
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chipset

Senior Member
thats what I was wondering about and I got my answer. I will be adding a pull down resisitor to my projects and nothing else.

It would be nice if the manual explained that a little clearer. I suppose it is in fact necessary to have a pull down resistor so saying its required isnt technically wrong although saying the serial cable port is necessary is a bit of a misnomer.

Either way I now know and knowing is half the battle lol
 
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ylp88

Senior Member
chipset said:
It would be nice if the manual explained that a little clearer. I suppose it is in fact necessary to have a pull down resistor so saying its required isnt technically wrong although saying the serial cable port is necessary is a bit of a misnomer.
Click to expand...
From the PICAXE FAQ document:
I've built a second pcb (without the download circuit) and the PICAXE program will not run!
If you program a PICAXE chip in a different board, and then move the chip to a board without the download circuit, you must ensure that the 'serial in' pin is tied to ground (0V) on the second board for reliable operation.
Click to expand...
ylp88
 
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chipset

Senior Member
no Im saying that the schematic they show includes the serial port, obviously that doesnt need to be there but what IS necessary to make sure the pin isnt floating?? It doesnt actually say that part... Does it need to be pulled to ground or hooked to a pull down resistor? What size pull down should it be.

For being in the manual, more or less the bible of the Picaxe, it should be clear about what is necessary to be present on the pins in order to have aproperly working circuit.

Maybe it was assumed if you put the suggested serial circuit on it would work properly, and it will, however it claims that that is the MINIMUM operating circuit when in fact it is not (all it needs is one resistor or maybe none at all, just grounding serin)..
 
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ylp88

Senior Member
chipset said:
no Im saying that the schematic they show includes the serial port, obviously that doesnt need to be there but what IS necessary to make sure the pin isnt floating?? It doesnt actually say that part... Does it need to be pulled to ground or hooked to a pull down resistor? What size pull down should it be.
Click to expand...
Well, without the serial cable connected, the SERIN pin sees the 22k and 10k programming resistors in series, thus replacing it with a 33k resistor is about the closest you'd get for a direct substitute, although you'll probably find (but I've not tried) that you'll be able to get away with a very wide range (even connecting it to ground, but its best to avoid that practice...). "Note 1" in my manual (which is old, but I'm sure that the quote is still there in the latest version) seems to link the "10k/22k resistors must be included ... DO NOT leave the serial in pin floating".

chipset said:
For being in the manual, more or less the bible of the Picaxe, it should be clear about what is necessary to be present on the pins in order to have aproperly working circuit.

Maybe it was assumed if you put the suggested serial circuit on it would work properly, and it will, however it claims that that is the MINIMUM operating circuit when in fact it is not (all it needs is one resistor or maybe none at all, just grounding serin)..
Click to expand...
I think it is clear what is necessary "to have aproperly circuit" - from the manual: "DO NOT leave the serial in pin floating...". Now, from my point of view, what's the fun of operating a PICAXE without being able to program it! ;) The programing circuit it part of the minimum requirement if you ask me! :p But surely we are now dealing with semantics... haha...

ylp88
 
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chipset

Senior Member
semantics for sure lol, and lets hope we dont include my lack of spell check haha. Obviously some people would like to make the P.A. do a certain thing and stay like that as I do and not have to keep on tweeking it.

IMO having to constantly tweek it means one of two things..

1. you have too much time on your hands

2. You shoulda done it right the first time.


I know Ill get shot for the second one but its just a joke ok lol.
 
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inglewoodpete

Senior Member
Tying The serial in pin to groud is probably not best practice. If you have a look at the specs for the PIC12F683, SI is on the GP5 pin, which is a both-way port. This means that it can be an output pin and it's output could be 'high'. There is a possibility that, should the pin go high as an output and, if it is shorted to ground, the high current could destroy the port.

Having said all that, you would have to know what Rev-Ed's firmware does in all circumstances to determine whether the firmware could possibly damage the shorted port.

On the other hand, I'm sure there is someone out there who would try (or otherwise) to write to the appropraite registers and 'blow it up'.
 
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chipset

Senior Member
so unless I purposely code the chip to short the ser out, Ill be ok taking it straight to ground right? Im gonna ground it unless there is a drastic reason not to>>>
 
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Dippy

Moderator
Do it if you want to chippy. I wouldn't, but if there is a board space issue , then you may have no choice.
If its the cost, then I can lend you a couple of pennies ;)

As you know, if you tie it directly to ground then you lose any option of In-circuit programming.
So, up to you.
Of course, if in a few days time you post that your PICAXE keeps failing then it will give me a chortle :) :) (Just kidding).

As a general thing, concerning pull-up / down, I would always say use resistors esp for newbies and where the pin I/O can be changed. For more experienced users and where cost/space is a problem the direct connection can, in many cases, be fine. For prototyping I would never do it just in case you want to strap some 'extra' to the board or if you have a coding slip of the finger.
 
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SilentScreamer

Senior Member
chipset said:
so unless I purposely code the chip to short the ser out, Ill be ok taking it straight to ground right? Im gonna ground it unless there is a drastic reason not to>>>
Click to expand...
You have no control over the serial in pins state, it is used by the firmware to detect when a download is started and to download the program. Many users here have tied it straight to ground and I've never heard of any issues, though why not just put a 10K resistor in and do it the "proper way".
 
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ylp88

Senior Member
chipset said:
so unless I purposely code the chip to short the ser out, Ill be ok taking it straight to ground right? Im gonna ground it unless there is a drastic reason not to>>>
Click to expand...
Note, that it is the SERIN line which should be pulled low. The SEROUT line can be left floating if you don't need to use it (for programming or SERTXD, for example). The concern is not so much that you have to be careful not to do anything which might destroy the chip (which you could prevent if you are careful), it is that you have to be careful that the PICAXE firmware doen't do anything which might destroy the chip (which is a problem since you don't actually have control over the SERIN pin and you can't control this aspect).

As SilentScreamer says, why not just throw in a 10k resistor? Surely a couple of cents is a cheap price to pay to follow good design practice?

ylp88
 
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chipset

Senior Member
I will use the resistor, I can definitely afford it :p Regardless just wanted to do it right so Ill pull it down with a 10k and be done with it. Thanks all.
 
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KMoffett

Senior Member
I agree with Andrew. There is no need for a pull-down resistor. It's an input-only port on the PICAXE, so just ground it.

Ken
 
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