multiple picaxes reading ADC

[adub]

Doesn't work.

I have 8 08Ms reading an ADC value from the same potentiometer.

The program reads correctly on a separate board and one picaxe. Vomplete range of 0 to 255 from a 10k pot and 5v.

When I line all 8 up to read the adc value from that pot it gives the high range 200+ and the low range 23- but no in between values.

Kind of expected that but was disappointed that I was right.

Anyone have a way for multiple 08Ms to read a single ADC value?

Arvin

 

[moxhamj]

This might be an impedence matching problem - the picaxes may be loading up the pot too much. Maybe try a 1K pot. Or if the pot has to be 10k, try an op amp voltage follower.

 

[hippy]

Also, if the 08M inputs are being set as digital inputs after a READADC executes ( and that seems to be the case with the 18X ), then this could have an adverse effect or make such a configuration impossible to use.

Having separate op-amp buffers for every 08M between each and the pot may resolve the problem but would add complexity and cost.

 

[BeanieBots]

This sounds like a simple wiring fault to me. Although there is scope for interaction, with a 10k source, the error should only be a few bits.
The clue is the fact that you only see values close to either Vcc or 0v. Just like you would if one the 08s was connected to an OUTPUT rather an input.
Systematically remove each of the 08s and see if the problem goes away when one in particular is removed.

 

[adub]

Thanks all. BeanieBots hit it sorta on the hear. The wiring was ok but when you said:
<i>one the 08s was connected to an OUTPUT rather an input. </i>
I twigged. I had a dirs statement that had set the pin I was using to an output. Oops! That cured it.

Now if I can just get the code to do what I want it to.

Thanks,
Arvin

 

[Dippy]

Arvin, you said &quot;Kind of expected that ...&quot; - why was that?

 

[adub]

Dippy, I thought the reading of the resistance by so many would have an effect on the resistance. I was basically being negative.

I was wrong.

Ultimately, bevans wants to set a digipot and have 30 08s read the resistance adjusted by an 18x to have the 08s know when to move their stepper motor.

Looks like it will work.
Arvin

 

[premelec]

In the worst case you'd simply have to put in a buffer amp to provide low impedance voltage source to feed the hundreds... why not? ... of 08Ms - sounds like fun.