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Hello,
I need to measure a 5V range but because of the voltage-regulator's "adjust" pin, the relative 0V (zéro) is already 1,2V!
Currently, the range is from 1,2V to 6,2V if measured against Vss (GND).
I already set ADCON0.6=1 (+Vref) and use a 6,2V Zener.
Now, AD reads from Vss to +6,2V.
But when I need to read "0V" (zéro), AD will still give me something like 70 (8bit res) because I can't get lower than 1,2V.
How can I pull the GND up from 0 to 1,2V or what is the way to do it right?
Hi Roger,
Are you really putting 6.2V on a PIC pin?
Sounds dangerous to me. But maybe I'm missing something.
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Hi Roger,
Pin input should generally not exceed Vcc on most ICs including the PIC. Your A/D input should be lowered by some means to 5V maximum (assuming you are supplying 5V to the PIC). Probably the simplest way to do this is with a voltage divider.
See Darrel’s helpful post:
http://www.picbasic.co.uk/forum/show...=2010#post2010
There are ways to negate the regulator’s 1.2V. All ways would add to your circuit complexity. Depends if you want to just subtract it from the PIC A/D reading, or really make the output from the regulator go all the way down to zero volts. You may want to bump the resolution up from 8 bits to 10 bits and just work around the missing 1.2V chunk with programming.
Anyway, don’t put more that the supply voltage into the pins (or negative voltage either :eek: ).
-Adam-
Voilà, this is how my circuit looks like:
<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=1820&stc=1&d=118301268 7">
Voltages I measure are:
1.- GPIO.5 = HPWM = 1,2V to 6,2V;
2.- GPIO.1 = AD = 1,2V to 5,1V.
To make it work (voltage control), I must connect R5. In the same time, voltage on GPIO.5 raises up from 0V to 1,2V.
Do I have to "protect" GPIO.5 pin against this 1,2V even if it is an Output?
If yes, how?
NB: is Vref limited to max 5V?
Hi Roger,
PIC devices have a good deal of protection built into their design.
There is the equivalent of Zener diodes on each I/O pin.
These “voltage clamps” help to protect the device from us.:)
The protection can handle a bit of “over voltage” caused, current (clamp current), but there is a limit.
The PIC12F675 has a limit of 20 mA output clamping current.
(Output clamp current, IOK (Vo < 0 or Vo >VDD) 20 mA).
Your schematic diagram shows R1 protecting the 12F675 HPWM Output pin (GPIO.5).
The protection depends upon the (over) voltage and the resistance value of R1.
-Adam-
Quote:
Originally Posted by PIC12F629/675 Data Sheet