A7.8051 Interfacing ADC: LDR & LM35
In this tutorial, we are going to discuss the interfacing of external ADC0808/9 with 8051.
We will be reading the ADC values from channel Zero and transmitted on UART at 9600 baudrate.
Contents
[hide]ADC-0808/9
8051 does not have the inbuilt ADC and we will be using the external 8-bit ADC ie. ADC0808/ADC0809.
ADC0809 is an 8-bit Successive Approximation ADC which is multiplexed among 8 input pins.
The A/D module has high and low-voltage reference input that can be set using the pins Vref+ and Vref- as shown in the below image.
With 5v as the Vref+ and 0v as Vref- the resolution of ADC0809 can be determined as below:
$$resolution of ADC = ((Vref+) - (Vref-))/(2^{8}-1) = 5/255 =0.0196 = 19.6mv$$
ADC Pins
Lets see the pins of the ADC0809
- Addres Lines(A,B,C): As mentioned earlier, ADC0809 has 8-muliplexed ADC channels. A particular input channel is selected by using these address bits as shown below.
| C | B | A | Selected ADC channel |
|---|---|---|---|
| 0 | 0 | 0 | INO |
| 0 | 0 | 1 | IN1 |
| 0 | 1 | 0 | IN2 |
| 0 | 1 | 1 | IN3 |
| 1 | 0 | 0 | IN4 |
| 1 | 0 | 1 | IN5 |
| 1 | 1 | 0 | IN6 |
| 1 | 1 | 1 | IN7 |
- Address Latch Enable (ALE): A low-to-high signal at this pin will latch the above-selected address and selected the respective channel for ADC conversion.
- START Conversion (SOC): The A/D converter’s successive approximation register (SAR) is reset on the positive edge of the start conversion (SC) pulse. Thus we need to generate a LOW-HIGH-LOW pulse for starting the ADC conversion.
- End of Conversion (EOC): Once the conversion is over, this pin is pulled HIGH by ADC0809. This pin needs to be monitored for the conversion to complete and then read the data.
- Output Enable(OE): ADC0809 does the adc conversion and holds the data in the internal registers. A HIGH signal on this pin will bring the data on the output lines.
Hardware Connections
Code
Below is the sample code to read the ADC value of channel0 and send it on UART at 9600 baud rate.
| #include<pic16f877a.h> | |
| #define SBIT_ADON 0 | |
| #define SBIT_CHS0 3 | |
| #define SBIT_ADFM 7 | |
| void delay(int cnt) | |
| { | |
| while(cnt--); | |
| } | |
| void ADC_Init() | |
| { | |
| ADCON0=0x00; // sampling freq=osc_freq/2,ADC off initially | |
| ADCON1=(1<<SBIT_ADFM); // All pins are configured as Analog pins and ADC result is right justified | |
| } | |
| int ADC_Read(int adcChannel) | |
| { | |
| ADCON0 = (1<<SBIT_ADON) | (adcChannel<SBIT_CHS0); //select required channel and turn ON adc | |
| delay(1000); //Acquisition Time(Wait for Charge Hold Capacitor to get charged ) | |
| GO=1; // Start ADC conversion | |
| while(GO_DONE==1); // Wait for the conversion to complete | |
| // GO_DONE bit will be cleared once conversion is complete | |
| return((ADRESH<<8) + ADRESL); // return right justified 10-bit result | |
| } | |
| int main() | |
| { | |
| int adcValue=0; | |
| TRISB = 0x00; // Configure PORTB and PORTD as output to display the ADC values on LEDs | |
| TRISD = 0x00; | |
| ADC_Init(); //Initialize the ADC module | |
| while(1) | |
| { | |
| adcValue = ADC_Read(0); // Read the ADC value of channel zero | |
| PORTB = (adcValue & 0xff); //Adc value displayed on LEDs connected to PORTB,PORTD | |
| PORTD = (adcValue>>8) & 0x03; // PORTB will display lower 8-bits and PORTD higher 2-bits | |
| } | |
| } |
Using ExploreEmbedded Libraries
| #include "adc.h" | |
| #include "uart.h" | |
| int main() | |
| { | |
| int adcValue; | |
| ADC_Init(); /* Initialize the ADC module */ | |
| UART_Init(9600); /* Initialize UART at 9600 baud rate */ | |
| while(1) | |
| { | |
| adcValue = ADC_GetAdcValue(0); // Read the ADC value of channel zero | |
| UART_Printf("ADC0 Value:%4d \n\r",adcValue); // Send the value on UART | |
| } | |
| return (0); | |
| } |
Downloads
Download the complete project folder from the below link:
Hardware design Files and Code Library
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