1. Objectives

  • Using the Analog-to-Digital Converter (ADC) to read analog input.

2. Parts List

  • LPC1769 LPCXpresso board

  • USB A-Type to Mini-B cable

  • Breadboard

  • Light sensor and/or potentiometer

  • Seven-segment display

  • 330-Ohm Resistors

  • Jumper wires

3. Background

Many microcontroller have pins that can be used for analog input. Because the microcontroller processes digital data only, analog input must be converted to digital data. An analog-to-digital converter (ADC) is an I/O circuit often integrated into microcontrollers to allow directly connecting external analog devices, such as sensor. The ADC would convert the sensor voltage into a digital value by transforming it into a binary code with a specific number of bits.

Tip Although not critical to conducting this experiment, it would be useful to review the three steps involved in analog-to-digital conversion: sampling, quantization and bit encoding (COE 241).

3.1. Using LPC1769 Peripherals

The LPC1769 includes an integrated ADC peripheral device. In general, using any peripheral device involves three main issues:

3.1.1. Power Up

All microcontroller peripherals must be powered up before they can be used. This was not a concern in earlier experiments because we were using peripherals that are powered up by default.

Powering peripherals up and down is controlled through the Power Control for Peripherals Register (PCONP).

By referring to table 46 in chapter 4 of the LPC1769 manual, you can see that the reset value (default value) is 1 for some peripherals, meaning that they are powered on by default, whereas it is 0 (OFF by default) for others.

For example, in the timer experiment, if you use a timer other than timer 0 or timer 1, your experiment wouldn’t work without powering up the timer in your program.

Note The A/D converter (ADC) power is controlled by bit 12 of the PCONP register, which is 0 by default. You must set that bit to power up your ADC.
Tip To save power, you can turn the power OFF for any unused peripherals that are ON by default.

3.1.2. Peripheral Clock

Most of the microcontroller peripherals, including timers and the ADC, require setting a peripheral clock (PCLK) to drive the peripheral.

You have seen in Experiment 7 (Hardware Timers) that you can configure a device’s PCLK using the PCLKSEL0 and PCLKSEL1 registers.

Exercise

Refer to Chapter 4 in the LPC1769 manual to find out the two bits needed to configure the PCLK frequency for the ADC.

Exercise

What would happen if you skip this step?

3.1.3. Pin Functions

Many microcontroller pins can be configured to perform one of many functions. From Experiment 5 (Interrupts — Part II), recall that the PINSELx registers are used to configure a pin’s function. To use the ADC, you must set the function of an appropriate pin to be analog input (AD0.x in the manual).

Exercise

Refer to chapter 8 of the LPC176x manual to determine (1) which bits of (2) which PINSELx register should be set to (3) what value.

You should connect a device that generates an analog voltage signal to the selected pin. Examples of such devices are light sensors (LDR) and potentiometers.

Note It is professional to correctly address the above three issues for every peripheral you plan to use, regardless of the defaults.

3.2. ADC Configuration

The main setup register for the ADC is the A/D Control Register (AD0CR). The AD0CR Register Fields figure illustrates the fields of the AD0CR register.

AD0CR register fields
Figure 1. A/D Control Register (AD0CR) Fields

The following table explains the function of the B (Burst) and E (Edge) bits of the AD0CR register.

Bit

Label

Value

Effect

16

B

0

ADC is started by software using the START bits

1

ADC is continuously running (START should be 000)

27

E

0

Start conversion on a falling edge

1

Start conversion on a rising edge
Note The E bit is used only when the START bits are one the six combinations: 010 to 111. Each of these combinations starts the ADC when a value of a specific pin is changed. The E bit decides whether the ADC is triggered on the positive edge or the negative edge of the pin specified by the START bits.
Tip If ADC interrupt is to be used, the ADGINTEN bit in the A/D Interrupt Enable register (AD0INTEN) must be set to 0. See Table 534 in Chapter 29 of the LPC1769 manual for details.
Warning ADCR is the generic name of the AD0CR register, since there is only one ADC in the LPC1769 microcontroller. In a chip where there are additional ADC circuits, ADCR may refer to any ADxCR register. In the LPC17xx.h header file, only ADCR is defined.

3.3. Reading Digital Values

There are 8 ADC channels, each corresponding to an analog pin. The value of the digitized analog value corresponding to an input analog voltage is stored in 12 bits in one of the A/D Data Registers: ADDR0 to ADDR7, where each register corresponds to an analog pin.

The ADDR Register Fields figure illustrates the the fields of the ADDRx registers.

ADDR register fields
Figure 2. A/D Data Register (ADDR) Fields

The 12-bit digital value generated by the ADC ranges from 0 to 4095. The way to process this value depends on your application.

You may want to divide this range to a number of sub-ranges, and assign different actions for each sub-range. In this case, you can use an if-else block.

In many applications, however, you will want to map this range to a another range using a mathematical formula. For example, if you are reading from an analog temperature sensor, you would want to map the 0-to-4095 range to the range of temperatures supported by the sensor, as specified in the sensor’s data sheet. In most cases, a linear relationship is sufficient.

3.4. START vs. BURST

If you want the analog value to be repeatedly read and converted, you have two options:

  1. Set the B bit (Burst) of the AD0CR register to 1; or

  2. Set the START bits to 001 repeatedly, i.e. in a loop. The analog value is read every time such a statement is executed.

In the second case, you should make sure the A/D conversion is complete before you read the digitized value. A simple way to do that is:

while((LPC_ADC->ADDR3 & (1 << 31)) == 0);    // Check the DONE bit for ADC channel #3

4. Tasks

Design an experiment in which you use the ADC in LPC1769. The input can be the LDR (light sensor) or the potentiometer.

The output can be any thing you want. The seven-segment display is the minimum requirement. It is recommended that you use mapping rather than an if-else block.

5. Resources

[lpc1769-manual]

NXP Semiconductors. UM10360 LPC176x/5x User manual. Rev. 3.1. 2 April 2014.
http://www.nxp.com/documents/user_manual/UM10360.pdf