Revision as of 14:00, 18 March 2015

Amruta (talk) 13:28, 17 March 2015 (IST)

Basics

LPC1768 has 12 bit ADC which is multiplexed among 8 input pins. It's measurement range is typically 3V which means that ADC resolution is approximately 0.7 mV.

Registers

Here we will discuss commonly used ADC registers.

ADCR ( ADC Control Register )

Bit 7:0 – SEL : Channel Select

These bits select which of the AD0.7:0 pins is (are) to be sampled and converted. There is one bit per channel e. g. bit o for AD0, bit 7 AD7.

Write one to enable respective channel. All zeroes is equivalent to 0x01.

Bit 15:8 – CLCKDIV : Clock Divisor

The APB clock (PCLK_ADC0) is divided by (this value plus one) to produce the clock for the A/D converter, which should be less than or equal to 13 MHz.

Bit 16 – BURST

Repeated conversions can be terminated by clearing this bit.

Bit 21 – PDN : Power Down Mode

Setting this bit brings ADC out of power down mode and makes it operational.

Bit 24:26 – START

When the BURST bit is 0, these bits control whether and when an A/D conversion is started:

The remaining cases (010 to 111) are about starting conversion on occurrence of edge on a particular CAP or MAT pin.

Bit 27 - EDGE

This bit is significant only when the START field contains 010-111. It starts conversion on selected CAP or MAT input.

ADGDR ( ADC Global Data Register )

Bit 15:4 - RESULT

When DONE is 1, this field contains a digital value equivalent to the voltage on the AD0[n] pin selected by the SEL field in ADCR register.

Bit 23:16 - CHN : Channel

These bits contain the channel from which the RESULT bits were converted (e.g. 000 identifies channel 0, 011 channel 3...).

Bit 27 - OVERRUN

This bit is 1 in burst mode if the results of one or more conversions was (were) lost and overwritten before the conversion that produced the result in the RESULT bits.

Bit 31 - DONE

This bit is set to 1 when an A/D conversion completes. It is cleared when this register is read and when the ADCR is written. If the ADCR is written while a conversion is still in progress, this bit is set and a new conversion is started.

Schematic

Code

/*-----------------------------------------------------------------------------
note : Refer adc.h to enable ADC channels.
------------------------------------------------------------------------------*/
#include "lpc17xx.h"	//device specific heaader file
#include "uart.h"		//Explore Embedded UART library which conatins the lcd routines
#include "adc.h"		//Explore Embedded ADC library which conatins the adc routines
 
/* start the main program */
int main() 
{
   uint16_t adc_result;
 
  /* Setup and initialize the microcontroller system */
	SystemInit();
 
  /* Initialize the UART before displaying any thing on the lcd */
	UART_Init(UART0,9600);
 
  /* Initialize the adc before starting the conversion */
	ADC_Init();
 
  /* Display "ADC Channel zero" on first line*/
	UART_Printf("ADC Channel zero");
 
   /* Display the adc channel zero value continously */ 
   while(1)
    {
		/*Get the adc value of channel five */
		adc_result= ADC_GetAdcValue(5);
 
		/*Display the adc value on UART*/
		UART_Printf("\n %u",adc_result);
	}
}