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 VREFN to VREFP, or commonly from 0V to ~3V. Maximum of 8 multiplexed inputs can be used for ADC.
Pins relating to ADC Module of LPC1768/LPC1769 :
Pin | Description |
---|---|
AD0.0 to AD0.7 (P0.23/24/25/26, P1.30/31, P0.3/2) | Analog input pins. Note from Datasheet: “If ADC is used, signal levels on analog input pins must not be above the level of VDDA at any time. Otherwise, A/D converter readings will be invalid. If the A/D converter is not used in an application then the pins associated with A/D inputs can be used as 5V tolerant digital IO pins.” |
VREFP, VREFN | These reference voltage pins used for ADC and DAC. |
VDDA, VSSA | VDDA is Analog Power pin and VSSA is Ground pin used to power the ADC module. |
LPC176x ADC Registers
Lets see registers which are used for ADC programming.
- Bits[7:0] – SEL: Bit ‘x'(in this group) is used to select pin A0.x in case of AD0.
- Bits[15:8] – CLKDIV: ADC Peripheral clock i.e. PCLK_ADC0 is divided by CLKDIV+1 to get the ADC clock. Note that ADC clock speed must be <= 13Mhz! As per datasheet user must program the smallest value in this field which yields a clock speed of 4.5 MHz or a bit less.
- Bit[16] – BURST: Set to 1 for doing repeated conversions, else 0 for software controlled conversions. Note: START bits must be set to 000 when BURST=1 or conversions will not start. Refer user manual for detailed info.
- Bit[21] – PDN : Set it to 1 for powering up the ADC and making it operational. Set it to 0 for bringing it in powerdown mode.
- Bits[26:24] – START: These bits are used to control the start of ADC conversion when BURST (bit 16) is set to 0. 000 = No start , 001 = Start the conversion now, for other values refer LPC17xx User Manual.
- Bit[27] – EDGE: Set this bit to 1 to start the conversion on falling edge of the selected CAP/MAT signal and set this bit to 0 to start the conversion on rising edge of the selected signal. (Note: This bit is of used only in the case when the START contains a value between 010 to 111 as shown above.)
- Other bits are reserved.
2) ADGDR – A/D Global Data Register : Contains the ADC’s flags and the result of the most recent A/D conversion.
- Bits[15:4] – RESULT: When DONE bit = 1, these bits give a binary fraction which represents the voltage on the pin AD0.x, in range VREFP & VREFN. Value of 0x0 indicates that voltage on the given pin was less than, equal to or greater than VREFN. And a value of 0xFFF means that the input voltage was close to, equal to or greater than the VREFP.
- Bits[26:24] – CHN: Represents the channel from which RESULT bits were converted. 000= channel 0, 001= channel 1 and so on.
- Bit[30] – OVERRUN: In burst mode this bit is 1 in case of an Overrun i.e. the result of previous conversion being lost(overwritten). This bit will be cleared after reading ADGDR.
- Bit[31] – DONE: When ADC conversion completes this bit is 1. When this register(ADGDR) is read and ADCR is written, this bit gets cleared i.e. set to 0. If ADCR is written while a conversion is in progress then this bit is set and a new conversion is started.
- Other bits are reserved.
4) ADDR0 to ADDR7 – A/D Data registers : This register contains the result of the most recent conversion completed on the corresponding channel [0 to 7]. Its structure is same as ADGDR except Bits[26:24] are not used/reserved.
5) ADSTAT – A/D Status register : This register contains DONE and OVERRUN flags for all of the A/D channels along with A/D interrupt flag.
- Bits[7:0] – DONE[7 to 0]: Here xth bit mirrors DONEx status flag from the result register for A/D channel x.
- Bits[15:8] – OVERRUN[7 to 0]: Even here the xth bit mirrors OVERRUNx status flag from the result register for A/D channel x
- Bit 16 – ADINT: This bit represents the A/D interrupt flag. It is 1 when any of the individual A/D channel DONE flags is asserted and enabled to contribute to the A/D interrupt via the ADINTEN(given below) register.
- Other bits are reserved.
LPC1768 ADC Modes, Setup and Programming
ADC modes in LPC1768 & LPC1769:
- 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.
- 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 VREFP, VREFN, VDDA & VSSA connections. These connections will be generally present on most LPC176x development boards. If not you can hook it up to VCC 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 :
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