Oh not so obvious.....well...one cell to be measured = 1.5->1.6 from original post. If his 8051 is on Vcc=5 volts then tne multiply by three comes from 1.6*3 = 4.8V which will provide nearly full dynamic range utilization of the A/D converter on 5 volt system. If the 8051 is like Cygnal part on 3.3 volts then multiply by 2. That would give 1.6*2 = 3.2 volts on 3.3 volt system. This is how to provide better resolution of the battery voltage measurement. However if the resolution is not needed then a simple 1 to 1 connection can be made.

I still would not directly connect the battery cell directly to the uP input. The simplest is to evaluate how much load you can live with on the battery at times when the uP is powered down. For example lets say you decide on 1 microamp. If your circuit was:

                 +-----------> to uP Vcc
                 |
                ---
                /  Shottky Diode
                ---  (low Vf)
                 |
    +---xxxx-----+------------> to uP A/D
    |   Resistor                 input.
    |
   ---
    -
   ---  1.6V Batt
    -
    |
    | 
   ---
   ///

...then if batt is at 1.6 volts and Vf of
protection diode is 0.4 volts:

R = (1.6 - 0.4)Volts/1 uA
R = 1.2 Meg Ohm

The purpose of the protection diode is to prevent the battery current flow from forward biasing circuits on the uP when it is in the power down state. If you do this to the uP it can lead to latchup on the IC (particularly CMOS ICs). The Vf of the diode needs to be less than the forward bias voltage of the input protection diodes on the uP. These are typically 0.6->0.7 volts so Shottky protection diode works nice. With this example you also need to make sure that the input impedance of the A/D input is very high. I have seen uP inputs with 10^^12 ohminput impedance so a 1.2 M series resistor works quite well. However if your uP has lower A/D input impedance of say like 10^^6 ohms then there is a problem with using the series 1.2 M resistor. The 1.2M and the uP input impedance form a voltage divider and the A/D converter will not see the true battery cell voltage.

In most of my designs when I have a cell that is used to backup a RAM or an RTC I find that using an A/D input to monitor the battery voltage is not a good idea. It shortens the battey life too much due to the current load you put on the battery in order to safely connect it to the A/D input.

So if you really need to monitor a battery you live with shortened battery life.

Mike Karas