> Hi All,
>
> I have to measure my PIC16C711's own battery supply (2 x AA cells) so need to
> generate a reference for the A to D, cost is everything on this one.
>
> I'm tempted to use two diodes in series to ground fed via a resistor from a
> port pin so I can enable the reference only when needed, this should produce a
> nominal 1.4V reference.
> The input volts to the A to D will be divided down from the same port pin so no
> current is consumed while the PIC is sleeping.
>
> The project will operated at room temperature so thermal drift should not be an
> issue.
>
> Has anyone come unstuck doing this in the past?
>
> Thanks in advance.
>
> Nigel
>
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>
> Hi All,
>
> I have to measure my PIC16C711's own battery supply (2 x AA cells) so need to
> generate a reference for the A to D, cost is everything on this one.
>
> I'm tempted to use two diodes in series to ground fed via a resistor from a
> port pin so I can enable the reference only when needed, this should produce a
> nominal 1.4V reference.
> The input volts to the A to D will be divided down from the same port pin so no
> current is consumed while the PIC is sleeping.
>
> The project will operated at room temperature so thermal drift should not be an
> issue.
>
> Has anyone come unstuck doing this in the past?
>
> Thanks in advance.
>
> Nigel
>
> --
> http://www.piclist.com#nomail Going offline? Don't AutoReply us!
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> > generate a reference for the A to D, cost is everything on this one.
>
> I just took a stab at the "cost is everything"
> approach and came up with a 3 resistor one
> 1N4148 diode and one cap design. It doesn't need
> an A to D input, just 3 normal PIC inputs.
>
> * will work on any PIC, even 12-core PICs
> * very low parts cost
> * can be turned on/off by the PIC
> * very low power waste even when turned on
> * should give ok accuracy with a lookup table
>
> Unfortunately needs 3 PIC pins total, but you
> were already using 2. :o)
>
> Turning a PIC output pin hi activates the circuit
> and it starts to draw a tiny amount of power, from
> the PIC pin through a 100k resistor into a 1N4148
> diode. This resistor lets about 20uA through, which
> is the max power draw of the circuit and occurs at
> battery=3.4v. At battery =2.4v, a typical low limit,
> there is still about 10uA into the diode so diode
> current is within the 10uA to 20uA range. Ideal for us,
> the diode forward voltage will change very little
> on this part of the curve, AND it uses the tinyest
> of power. 2 birds. ;o)
>
> This charges a small cap (a low leakage one), up to
> the diode forward voltage, through a 56k series resistor.
> The cap is connected to a PIC "sense" pin and another
> PIC I/O pin through a 10k resistor. These are both set
> as inputs and their port data bit is set to low.
> Total; 1 diode, 1 cap, 3 resistors, 3 PIC pins
>
> We wait the minimum time for cap to be fully charged,
> then the PIC I/O pin with the 10k resistor goes from
> hi-impedance input to a hi output.
>
> The sense pin measures the time to go from low to hi,
> and this will be determined by the battery voltage so
> a simple lookup table will give decent precision to
> measure a few battery levels.
>
> To do a battery voltage measurement should not take
> much more energy than that needed to charge the cap
> once to 5v, and with a small cap (and obviously; high
> resistor values) each measurement will only cost a
> tiny bit of the battery.
>
> There will be some spread between the Vin hi voltages
> of different PICs, giving slightly different readings
> from PIC to PIC. But with most products battery level
> sensing is only needed in 8 or less levels, to drive
> a bar indicator etc. You said minimum cost? This is a
> cheap way to do it. :o)
> -Roman
>
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>At 10:54 AM 1/10/02 +0000, Nigel Duckworth wrote:
>>Hi All,
>>
>>I have to measure my PIC16C711's own battery supply (2 x AA cells) so need to
>>generate a reference for the A to D, cost is everything on this one.
>
>What supply voltage does the PIC run from?  Is there a boost convertor to
>5V or does the PIC run directly from the battery?
>
>Since this is such a non-precise requirement, I'd be tempted to just use
>the logic 1 threshold of a PIC pin - I belive it stays relatively constant
>at about 1.4 Vdc regardless of supply voltage.  You would need 1 i/o line,
>2 resistors, 1 cap.
>
>100R resistor from PIC pin to 100n cap (other side of cap to gnd), 10K from
>battery to same PIC pin.  Turn PIC pin into output and LO, wait a few dozen
>cycles until cap is discharged, turn pin into input, time how long it takes
>for pin to go HI.  Time is dependant on battery voltage.
>
>Give it a try - should work fine.
>
>dwayne
>
>
>
>Dwayne Reid   <EraseMEdwaynerplanet.eon.net>
>Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
>(780) 489-3199 voice          (780) 487-6397 fax
>
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>
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>
> Hi,
>
> Regular PIC inputs are standard CMOS inputs and the threshold DOES change
> linearly with supply voltage. The threshold is close to 1/2 Vdd, so if your
> Vdd falls by 1 volt, the threshold will fall by 1/2 volt, so it won't work
> to use it as a reference. I'm not sure about the Schmitt Trigger inputs,
> but I'd suspect that their threshold is highly Vdd dependent, too.

> >>I have to measure my PIC16C711's own battery supply (2 x AA cells) so need to
> >>generate a reference for the A to D, cost is everything on this one.

> >Since this is such a non-precise requirement, I'd be tempted to just use
> >the logic 1 threshold of a PIC pin - I belive it stays relatively constant
> >at about 1.4 Vdc regardless of supply voltage.  You would need 1 i/o line,
> >2 resistors, 1 cap.
> >
> >100R resistor from PIC pin to 100n cap (other side of cap to gnd), 10K from
> >battery to same PIC pin.  Turn PIC pin into output and LO, wait a few dozen
> >cycles until cap is discharged, turn pin into input, time how long it takes
> >for pin to go HI.  Time is dependant on battery voltage.
> >
> >Give it a try - should work fine.
> >
> >dwayne

>Well, according to one graph, VinH typically varies from about 0.9Vdc
>@Vdd=2.5V through about 1.4Vdc @Vdd=5.5Vdc (DS30561B Pg 112).  Thats for a
>non-schmitt trigger input.
>
>Because the change in VinH varies less than the change in Vdd, there should
>be enough accuracy available to at least give an indication of battery
>voltage.  And if he *IS* using a boost convertor to give a regulated Vdd,
>there is no problem at all.
>
>dwayne
>
>
>
>
>
>Dwayne Reid   <RemoveMEdwaynerspam_OUTKILLspamplanet.eon.net>
>Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
>(780) 489-3199 voice          (780) 487-6397 fax
>
>Celebrating 17 years of Engineering Innovation (1984 - 2001)
>
>* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
>Do NOT send unsolicited commercial email to this email address.
>This message neither grants consent to receive unsolicited
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> How about a JFET selected for Vp ~= 2..2.5V and a resistor. You would
> switch it by grounding or not grounding the resistor. There are FETs like
> this with IDss of 1..3 mA. Just what the doctor prescribed for your
> problem ;-).
>
> Peter
>
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> I don't know what you think about this solution for an acurate reference
> voltage- but many old  measuring devices which use wheatstone bridges use a
> battery ( mercury cell 1.4V.. I can't remember exactly) They last for years
> because you're not really drawing much current from it -and their voltage
> remains extremely stable over time.
>
> Probably not the ideal solution- but one of the quickest and cheapest.
>
> --
> Jon Baker
>