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'[PIC] 16F877A A/D Weirdness'
2005\10\06@022408
by
Padu
|
Maybe I'm doing something wrong, but here's what I'm trying to do.
I'm trying to measure voltage of a battery that can go as high as 16V. I'm
using RA0 set as analog input for the A/D and I'm using Vdd as a reference.
In order to reduce the voltage to compatible levels, I've connected the
following voltage divider: from the + side of the battery, I have a 4.7K and
then a 2.0K in series going to the - side. Let's say that the battery has
12V across poles. I put my multimeter + probe in between the two resistors
and the - probe on the - of the battery. It reads about 3.6V, perfect.
Now I connect my circuit's ground to the ground on the battery to be
measured, still 3.6V. The problem happens when I connect my RA0 pin to the
divider (same spot where my probe is). Immediately the voltage drops to
2.65V.... why is that? 1V doesn't seem a lot but when I do the math to
output the real battery voltage, of course it doesn't match anymore.
What am I doing wrong?
I measured the voltage accross RA0 and ground, it is not zero. I figured
that perhaps it is because the pin is set as input and is floating?
Help is very much appreciated
Padu
2005\10\06@032544
by
Padu
|
|
From: "Palmed GmbH"
> Padu,
>
> give us a code snippet of your port init. This sounds, that your input
isn't
> an input...
>
> A simple test is, to connect a relative high resistor (let's say 100K)
from
> RA0 to GND, if RA0 is really an input the voltage should be 0V. Then
connect
> same R to +5V and RA0 must be +5V.
>
> Peter
Ok, I didn't have an 100k around, so I did it with a 68k. When I measure it
from RA0 to GND, it is zero allright, but when I connect it to +5V, measured
voltage is 1.27V. If I connect RA0 directly to VCC then it goes to 5V, but I
think that would happen no matter what right?
I'm using mikroPascal, which sometimes simplifies lots of things because of
their libraries, but sometimes it screws me up because I have to debug their
libraries for them....
Here's a snippet of pascal:
////////select Vref and analog inputs, in order to use ADC_read
///////////////
ADCON1 := 0; // all porta pins as analog, VDD as Vref
TRISA := $FF;
////////////////////////////////////////////////////////////////////////////
////
and to read I use this command:
Ein := ADC_read(0);
Do I need to do something other than TRISA := FF in order to set RA0 as
input?
I'm gonna try to examine the assembly code generate by mP to see what how
they are initializing the ADC.
2005\10\06@051255
by
Tom Sefranek
Padu wrote:
>Ok, I didn't have an 100k around, so I did it with a 68k. When I measure it
>from RA0 to GND, it is zero allright, but when I connect it to +5V, measured
>voltage is 1.27V. If I connect RA0 directly to VCC then it goes to 5V, but I
>think that would happen no matter what right?
>
Forcing an OUTPUT to +5 quite likely exceeded the current specification
of the pin driver.
It is likely you have damaged the device.
Why it is an output is still a mystery to me.
{Quote hidden}>I'm using mikroPascal, which sometimes simplifies lots of things because of
>their libraries, but sometimes it screws me up because I have to debug their
>libraries for them....
>
>Here's a snippet of pascal:
>
>////////select Vref and analog inputs, in order to use ADC_read
>///////////////
> ADCON1 := 0; // all porta pins as analog, VDD as Vref
> TRISA := $FF;
>////////////////////////////////////////////////////////////////////////////
>////
>
>and to read I use this command:
>
> Ein := ADC_read(0);
>
>Do I need to do something other than TRISA := FF in order to set RA0 as
>input?
>
>
>I'm gonna try to examine the assembly code generate by mP to see what how
>they are initializing the ADC.
>
>
A good idea!
BTW: What did you assign to ADCON0?
--
*
| __O Thomas C. Sefranek spam_OUTWA1RHPTakeThisOuT
ARRL.net
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2005\10\06@112555
by
Bob Blick
On 5 Oct 2005 at 23:27, Padu wrote:
> The problem happens when I connect my RA0 pin to the
> divider (same spot where my probe is). Immediately the voltage drops to
> 2.65V.... why is that?
You must apply power to the PIC before putting input signal to it.
Cheerful regards,
Bob
2005\10\07@103823
by
Padu
Peter wrote:
>>
Padu,
your inits sound good (even if don't know anything about µPascal, so I hope
:= means a LET or "=" in Basic). 68K is also perfect for this test.
Two things come into my mind:
1. May be you already killed your PIC by hardwiring RA0 to +5V.
2. Are you sure, your PIC is running? Is any LED blinking? If not, there
also would be no init, which could explain.
Peter
<<
Yes, := is assignment in pascal. The pic is working. My program involves LED's, compactflash and USART, everything works fine. Right now everything is on a development board (mikroelektronika easypic2).
On RA1 I have an ACS750 (current sensing) that outputs VCC/2 to VCC levels and I'm reading it perfectly. One test I didn't perform but you reminds me of doing is trying to read the ACS750 on RA0 and see if I get the same results.
I cannot do any tests until next Monday. Today I'm taking off to Vegas to watch the Darpa Grand Challenge :)
Cheers
Padu
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