[avr-gcc-list] Re:[avr-gcc-list]AVR and external ADC

[Marlin Unruh]

You could use a r/2r resistor network and pass its output thru an opamp voltage 
follower. Then present the voltage to the 
voltage comparator in the MCU. Do a search on Goggle with R/2R ladder and check 
out the  links. Its a cheap D/A convertor.

Then in your code do the higher/lower scheme. This will speed up the time it 
takes to locate the unknown analog value. If 
using 8 bit resolution, first apply 128 to the r/2r ladder if it is lower then 
divide 128 in half and check if higher or 
lower, etc. You should be able to find the value in eight or less samples.

Example: unknown analog = 125

        128 (lower)
128-64=  64 (higher)
64+32=   96 (higher)
96+16=  112 (higher)
112+8=  120 (higher)
120+4=  124 (higher)
124+2=  126 (lower)
126-1=  125 (answer)

code example: 

void get_unknown_analog(unsigned char result)
{
        unsigned char v1, v2; // define variables
        v1 = v2 = 128; // load vars
        do {
                v1 /= 2; // adjust v1 each time around
                portx = v2; // value to place on the r/2r ladder
                delay(x); // settle time
                if(comparator_is_set) // test higher/lower condition
                        v2 -= v1; // if above trip point then sub v1 from v2
                else
                        v2 += v1; // if below trip point then add v1 to v2
        } while(v1);
        return v2;
}

This is not perfect, but you get the idea.

Marlin Unruh



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