Interfacing DAC with 8051 Microcontroller

In this section we will see how DAC (Digital to Analog Converter) using Intel 8051 Microcontroller. We will also see the sinewave generation using DAC.

The Digital to Analog converter (DAC) is a device, that is widely used for converting digital pulses to analog signals. There are two methods of converting digital signals to analog signals. These two methods are binary weighted method and R/2R ladder method. In this article we will use the MC1408 (DAC0808) Digital to Analog Converter. This chip uses R/2R ladder method. This method can achieve a much higher degree of precision. DACs are judged by its resolution. The resolution is a function of the number of binary inputs. The most common input counts are 8, 10, 12 etc. Number of data inputs decides the resolution of DAC. So if there are n digital input pin, there are 2ⁿ analog levels. So 8 input DAC has 256 discrete voltage levels.

The MC1408 DAC (or DAC0808)

In this chip the digital inputs are converted to current. The output current is known as I_out by connecting a resistor to the output to convert into voltage. The total current provided by the I_out pin is basically a function of the binary numbers at the input pins D₀ - D₇ (D₀ is the LSB and  D₇ is the MSB) of DAC0808 and the reference current I_ref. The following formula is showing the function of I_out

$$I_{Out}=I_{ref}\lgroup\frac{D7}{2}+\frac{D6}{4}+\frac{D5}{8}+\frac{D4}{16}+\frac{D3}{32}+\frac{D2}{64}+\frac{D1}{128}+\frac{D0}{256}\rgroup$$

The I_ref is the input current. This must be provided into the pin 14. Generally 2.0mA is used as I_ref

We connect the I_out pin to the resistor to convert the current to voltage. But in real life it may cause inaccuracy since the input resistance of the load will also affect the output voltage. So practically I_ref current input is isolated by connecting it to an Op-Amp with R_f = 5K? as feedback resistor. The feedback resistor value can be changed as per requirement.

Generating Sinewave using DAC and 8051 Microcontroller

For generating sinewave, at first we need a look-up table to represent the magnitude of the sine value of angles between 0° to 360°. The sine function varies from -1 to +1. In the table only integer values are applicable for DAC input. In this example we will consider 30° increments and calculate the values from degree to DAC input. We are assuming full-scale voltage of 10V for DAC output. We can follow this formula to get the voltage ranges.

Vout = 5V + (5 ×sin?)

Let us see the lookup table according to the angle and other parameters for DAC.

Circuit Diagram ?

Source Code

#include<reg51.h>
sfr DAC = 0x80; //Port P0 address
void main(){
   int sin_value[12] = {128,192,238,255,238,192,128,64,17,0,17,64};
   int i;
   while(1){
      //infinite loop for LED blinking
      for(i = 0; i<12; i++){
         DAC = sin_value[i];
      }
   }
}

Output

The output will look like this ?