configuring AD converter in a 12F615

[dhouston]

So what I'm trying to do is read the battery level of the 2 coin batteries that power the PIC so I can have a low battery warning.

I recently lost a NAS HDD so I don't have the code but 4-5 years back I designed a generic, battery powered ADC module. My battery monitor used a couple of pins so it probably is not applicable here but it may be useful for others. In my case, I needed only a single ADC input pin so I had a surplus of pins.

I used two BR2032 coin cells. BR type cells maintain their rated charge until slightly before their end of life while CR type cells fall off gradually throughout their life.

The batteries connected to Vdd and I fed an LP2980 LDO from a PIC pin. Vdd was 6V but the PIC12F675 (and PIC12F615) can handle 6.5V. The output of the LDO was used to power the attached analog sensor and also connected to Vref. A second pin can also supply the LDO if more current is needed.

To monitor the battery, measure the output of the LDO using Vdd as reference. Initially, the reading will be something less than maximum but will approach maximum as the batteries start to drop off just before end of life.

This wasn't my original idea but I cannot recall where I came across it.

[Aussie Barry]

Hi All,

I am having difficulty understanding a line from Darrel's no-pin battery monitor code snippet. Can someone please explain the line

Vref_AD CON 117 ;A/D reading for 0.6V Vref @ 5V VDD

Is this a constant or does it vary? I am confused!

I would like to implement this no-pin battery monitor into a project using a 12F683. The two devices appear to be pin compatible so I am hopint this will work. Am I sadly mistake?

Cheers
Barry

[mackrackit]

Vref_AD CON 117 ;A/D reading for 0.6V Vref @ 5V VDD

Is this a constant or does it vary? I am confused!

From the manual:

4.6. Constants

Named constants may be created in a similar manner to variables. It can be more convenient to use a constant name instead of a constant number. If the number needs to be changed, it may be changed in only one place in the program; where the constant is defined. Variable data cannot be stored in a constant.

Label CON Constant expression

Some examples of constants are:

mice con 3
traps con mice * 1000

[Darrel Taylor]

I would like to implement this no-pin battery monitor into a project using a 12F683. The two devices appear to be pin compatible so I am hopint this will work. Am I sadly mistake?

The 12F683 does not have an internal 0.6V fixed reference.
So you won't be able to do it with that chip.

[Aussie Barry]

Thanks for the input Darrel and Dave.

It is amazing what you can find in the manual when you know where to look

Darrel, can you please explain how you arrived at the value 117 for the Vref_AD constant? I would have expected it to be 0.6/5*1023 = 122

For my project I was able to use another ADC pin to monitor the supply voltage so all is good (for the time being).

Cheers
Barry

[Darrel Taylor]

Darrel, can you please explain how you arrived at the value 117 for the Vref_AD constant? I would have expected it to be 0.6/5*1023 = 122

The internal fixed references are not very accurate, so you have to read the actual value to determine the number to use.

For my project I was able to use another ADC pin to monitor the supply voltage so all is good (for the time being).

Thats a bit like lifting yourself up by your own bootstraps.

[Aussie Barry]

Thanks Darrel.
I am using the other ADC pin to sense the pre-regulator voltage (via an appropriate voltage divider).
If the pre-reg voltage drops below 5.3V then the circuit will flash a "Low Battery" indicator and disable any future water pump operation.
I am trying to save the 6V SLA battery from a harmful depletion condition.

Cheers
Barry