Vref - a few questions?

[JimB]

The compiler has the function SETUP_VREF() for the internal reference for the A/D. Using the two expressions given in that section the following table in Excel shows the multiple choices for Vdd = to 5 Volts. Do the values look correct?

VDD=> 5

Value........VREF_LOW......VREF_HIGH
0..............0.000.............1.250
1..............0.208.............1.406
2..............0.417.............1.563
3..............0.625.............1.719
4..............0.833.............1.875
5..............1.042.............2.031
6..............1.250.............2.188
7..............1.458.............2.344
8..............1.667.............2.500
9..............1.875.............2.656
10............2.083.............2.813
11............2.292.............2.969
12............2.500.............3.125
13............2.708.............3.281
14............2.917.............3.438
15............3.125.............3.594

In the 16F877A.h file there are the expressions shown below. Are these the same expressions used to calculate the above values. If not how are these used?

// Constants used in setup_vref() are:
#define VREF_LOW 0xa0
#define VREF_HIGH 0x80
// Or (with |) the above with a number 0-15
#define VREF_A2 0x40

Questions:
1. What is VREF_A2 and what does it mean.
2. I assume that the lack of any statement sets VREF to VDD.
3. How does one enable an external Vref and I assume that this would
be at pin AN2/Vref+.
4. How does one setup Vref- and is it Vss or ground
when no statement is made.

[PCM programmer]

Most of your questions can be answered by looking in this
section of the 16F877A data sheet:

[Guest]

I clearly did not understand the difference. Thanks for the clarification.