Analog to Digital Converts
By Ibrahim Kamal
By Ibrahim Kamal
The Software
The software to be loaded on a microcontroller to perform the task of converting analog signals to digital ones is very simple; The pins P3.0 and P3.1 are both set to Logic 0, and the value of port 2 (which is connected to the first 8 bits of the DAC) is gradually incremented. After each increment the pin P3.7 is checked. Whenever P3.7 is low (logic 0) that means that the generated Analog signal corresponds (with one bit accuracy) to the measured analog signal, and consequently corresponds to the last counted digital value in the micro-controller. In order to extend the resolution from 8 bits to 10 bits, the value of the pins P3.0 and P3.1 is increased each time Port 2 overflows. This way, the precision of this converter is 5 / 1024 = 0.005V, and you can freely increase the precision by increasing the number of bits.
Here is an example C code for the 8051 micro-controller to read the voltage from the above schematic.
While (1){
done = 0;
P3_0 = 0;
P3_1 = 0;
P3_7 = 1; //set P3_7 as input
P2 = 0; //Start counting from 0
delay(100);
while (P2 < 255){
P2++;
delay(100); //Slow down the process, to be compatible with
if (P3_7 == 1){ //the response time of the Op-Amp.
done = 1;
break;
}
}
if (done == 0){
P3_0 = 1;
P3_1 = 0;
P3_7 = 1;
P2 = 0;
while (P2 < 255){
P2++;
delay(100);
if (P3_7 == 1){
done = 1;
break;
}
}
}
if (done == 0){
P3_0 = 0;
P3_1 = 1;
P3_7 = 1;
P2 = 0;
while (P2 < 255){
P2++;
delay(100);
if (P3_7 == 1){
done = 1;
break;
}
}
}
if (done == 0){
P3_0 = 1;
P3_1 = 1;
P3_7 = 1;
P2 = 0;
while (P2 < 255){
P2++;
delay(100);
if (P3_7 == 1){
done = 1;
break;
}
}
}
if (done == 1){
bit8 = P3_0;
bit9 = P3_1;
voltage = ((P2 + (bit8 * 256) + (bit9 * 512))*conversion_factor);
}
The variable 'conversion_factor' in the code represent a factor that relates the counted value to the corresponding voltage, and is easily obtained using some trials and errors. Noting that the relation between the counted value and the corresponding voltage is linear, it's easy to find this factor based on only 2 readings.
References:
- Counting Type ADC, Analog to Digital Converter, http://www.ikalogic.com/
- http://en.wikipedia.org
