This is ingenious, Roman.
-Lawrence
----- Original Message -----
From: "Roman Black" <fastvid@EZY.NET.AU>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Thursday, January 10, 2002 1:44 PM
Subject: Re: [PIC]: Low cost A to D reference


> 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.
>
> 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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