LPC1768 ADC Programming Tutorial

Hi folks, in this tutorial we will go through ARM Cortex-M3 LPC1768 ADC programming tutorial. Basically we convert an Analog signal to its Digital version. We will also see LPC176x ADC Interfacing Example. This also applies to LPC1769 Microcontroller.

ADC in ARM LPC176x

Analog to Digital Conversion is used when want to interface an external analog signal or when interfacing analog sensors, like for example a temperature sensor. The ADC block in ARM Cortex-M3 LPC1768 Microcontroller is based on Successive Approximation(SAR) conversion method. LPC1768 ADC Module uses 12-bit SAR having a conversion rate of 200 kHz. The Measurement range if from V_REFN to V_REFP, or commonly from 0V to ~3V. Maximum of 8 multiplexed inputs can be used for ADC.

Pins relating to ADC Module of LPC1768/LPC1769 :

LPC176x ADC Registers

Lets see registers which are used for ADC programming.

LPC1768 ADC Modes, Setup and Programming

ADC modes in LPC1768 & LPC1769:

1. Software controlled mode : In Software mode only one conversion will be done at a time. To perform another conversion you will need to re-initiate the process. In software mode, only 1 bit in the SEL field of ADCR can be 1 i.e. only 1 Channel(i.e. Pin) can be selected for conversion at a time. Hence conversions can be done only any channel but one at a time.
2. Hardware or Burst mode : In Hardware or Burst mode, conversions are performed continuously on the selected channels in round-robin fashion. Since the conversions cannot be controlled by software, Overrun may occur in this mode. Overrun is the case when a previous conversion result is replaced by new conversion result without previous result being read i.e. the conversion is lost. Usually an interrupt is used in Burst mode to get the latest conversion results. This interrupt is triggered when conversion in one of the selected channel ends.

1A. Setting up and configuring ADC Module for software controlled mode

First, lets define some values which will help us in setting up ADCR register to configure & Initialize ADC block.

#define ADC_CLK_EN (1<<12)

#define SEL_AD0_0  (1<<0) //Select Channel AD0.0 

#define CLKDIV     1 // ADC clock-divider (ADC_CLOCK=PCLK/CLKDIV+1)  
 
#define PWRUP      (1<<21) //setting it to 0 will power it down
 
#define START_CNV  (1<<24) //001 for starting the conversion immediately
 
#define ADC_DONE   (1U<<31) //define it as unsigned value or compiler will throw #61-D warning

#define ADCR_SETUP_SCM ((CLKDIV<<8) | PowerUP)
//SCM = Software Controlled Mode

Now we assign ADCR_SETUP to ADCR along with channel selection information to select channels as required. Finally we assign(by ORing) START_NOW to ADCR to start the conversion process as shown:

LPC_SC->PCONP |= ADC_CLK_EN; //Enable ADC clock
LPC_ADC->ADCR =  ADCR_SETUP_SCM | SEL_AD0_0; 
LPC_ADC->ADCR |= START_CNV;

//==OR==
LPC_SC->PCONP |= ADC_CLK_EN; //Enable ADC clock
LPC_ADC->ADCR =  ADCR_SETUP | SEL_AD0_0 | START_CNV; 

1B. Fetching the conversion result in software controlled mode :

In software controlled mode we continuously monitor bit 31 in the corresponding channel data register ADDR. If bit 31 changes to 1 from 0, it means that current conversion has been completed and the result is ready. For example, if we are using channel 0 of AD0 then we monitor for changes in bit 31 as follows :

while((LPC_ADC->ADDR0 & ADC_DONE) == 0); //this loop will end when bit 31 of AD0DR6 changes to 1.

After this we extract the result which is stored in ADDR bits 4 to 15. Here we right shift ADDR by 6 places and force all other unrelated bits to 0 by using a 12-bit mask value of 0xFFF. 0xFFF is a mask containing 1's in bit locations 0 to 11 and rest 0's. In our case with ADDR0 being used, it can be done as follows :

result = (LPC_ADC->ADDR0>>4) & 0xFFF;

2A. Setting up and configuring ADC Module for Burst mode

Configuring ADC Module is similar to what was done in software controlled mode except here we use the CLKS bits and don't use the START bits in ADCR. ADC_DONE is also not applicable since we are using an ISR which gets triggered when a conversion completes on any of the enabled channels. Additionally, we define the following constants:

 
#define SEL_AD0_1 (0x2) //Select Channel AD0.1

#define BURST_ON  (1<<16) // 1 for on and 0 for off

#define ADCR_SETUP_BURST ((CLKDIV<<8) | BURST_ON | PWRUP)

We configure and setup the ADC module in a similar manner(as shown above) as follows :

LPC_SC->PCONP |= ADC_CLK_EN; //Enable ADC clock
LPC_ADC->ADCR =  ADCR_SETUP_BURST | SEL_AD0_0 | SEL_AD0_1; 

Note that in this case we can select multiple channels for conversion when setting up AD0CR. START bits are not applicable here since the conversions starts as soon we setup AD0CR register.

2B. Fetching the conversion result in Burst mode :

In Burst mode we use an ISR which triggers at the completion of a conversion in any one of the channel. Now, we just need to find the Channel for which the conversion was done. For this we fetch the channel number from ADGDR which also stores the conversion result. Bits [26:24] in ADGDR contain the channel number. Hence, we shift it 24 places and use a 3-bit mask value of 0x7 as shown below:

unsigned long ADGDR_Read = LPC_ADC->ADGDR;
int channel = (ADGDR_Read>>24) & 0x7; //Extract Channel Number

After knowing the Channel number, we have 2 options to fetch the conversion result from. Either we can fetch it from ADGDR or from ADDRx of the corresponding channel. Lets use ADGDR for extracting the conversion result as follows:

int currentResult = (ADGDR_Read>>4) & 0xFFF; //Extract Conversion Result

ARM Cortex-M3 LPC1768/LPC1769 ADC Example

Note that in this case no input protection nor filtering was required. But, when Interfacing external analog signals it is recommended to use some form of input protection.

Interfacing Potentiometer using ADC on LPC176x

This example performs Analog to Digital conversion in Software Controlled Mode. Here we use P0.23 as analog input for measuring the voltage. P0.23 corresponds to Channel 0 of AD0 i.e. AD0.0. For testing I had used a 10K potentiometer for ADC Interfacing and connected the middle leg to P0.23 of my LPC1768 development board. You can also use a 5K potentiometer for this ADC Example. Make sure your board V_REFP, V_REFN, V_DDA & V_SSA connections. These connections will be generally present on most LPC176x development boards. If not you can hook it up to V_CC and GND respectively using decoupling capacitors and ferrite beads to reduce noise. Schematic for this example is as shown below:

C/C++ Source Code :

/*(C) Umang Gajera - www.ocfreaks.com
LPC1768/LPC1769 ADC Interfacing Example 1 Source Code using KEIL ARM
More Embedded tutorials @ www.ocfreaks.com/cat/embedded/
License : GPL.*/

#include <lpc17xx.h>
#include <stdio.h>
#include "ocf_lpc176x_lib.h" //contains uart, timer & fputc to retarget printf  

#define VREF       3.3 //Reference Voltage at VREFP pin, given VREFN = 0V(GND)
#define ADC_CLK_EN (1<<12)
#define SEL_AD0_0  (1<<0) //Select Channel AD0.0 
#define CLKDIV     1 //ADC clock-divider (ADC_CLOCK=PCLK/CLKDIV+1) = 12.5Mhz @ 25Mhz PCLK
#define PWRUP      (1<<21) //setting it to 0 will power it down
#define START_CNV  (1<<24) //001 for starting the conversion immediately
#define ADC_DONE   (1U<<31) //define it as unsigned value or compiler will throw #61-D warning
#define ADCR_SETUP_SCM ((CLKDIV<<8) | PWRUP)

int main(void)
{
	//SystemInit(); //Gets called by Startup code, sets CCLK=100Mhz, PCLK=25Mhz
	initUART0(); //Initialize UART0 for uart_printf() - both defined in tmr_uart_printf.cpp
	initTimer0(); //For delayMS() - both defined in tmr_uart_printf.cpp

	LPC_SC->PCONP |= ADC_CLK_EN; //Enable ADC clock
	LPC_ADC->ADCR =  ADCR_SETUP_SCM | SEL_AD0_0;
	LPC_PINCON->PINSEL1 |= (1<<14) ; //select AD0.0 for P0.23
	int result = 0;
	float volts = 0;
	
	printf("OCFreaks.com LPC176x ADC Tutorial Example 1.\nSoftware Controlled ADC Mode on AD0.0 Channel.\n");
	
	while(1)
	{
		LPC_ADC->ADCR |= START_CNV; //Start new Conversion

		while((LPC_ADC->ADDR0 & ADC_DONE) == 0); //Wait untill conversion is finished
		
		result = (LPC_ADC->ADDR0>>4) & 0xFFF; //12 bit Mask to extract result
		
		volts = (result*VREF)/4096.0; //Convert result to Voltage
		
		printf("AD0.0 = %dmV\n" , (int)(volts*1000)); //Display milli-volts
		
		delayMS(500); //Slowing down Updates to 2 Updates per second
	}
	
	//return 0; //This won't execute
}

Serial Output :