Analogue to Digital Conversion on an ATmega168

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1 Shopping Cart: Empty Login or Create Account About Blog Tutorials Library Contact Search... Go Home» Blog» Tutorials» Analogue to Digital Conversion on an ATmega168 Categories Boards Connectors Hardware Microcontrollers Integrated Circuits Passive Components Semiconductors LED/LCD Buttons and Switches Cables Sensors Accessories Kits & Modules Tools Other Featured... Analogue to Digital Conversion on an ATmega168 Read comments Leave a comment 13-Feb :44am Many AVR microcontrollers are capable of doing Analogue to Digital Conversion. The ATmega168 has 6 ports (8 ports on the SMD packages) that can be used for analogue input. This tutorial shows you how. The circuit We ll be building the following circuit on a breadboard. LED matrix 8x8 Red/Green 37.8 x 37.8mm 1 of 9 2/8/15, 1:44 PM

2 $4.80 Information About Us Online Reviews Shipping & Returns Contact Us The Breadboard layout is based on the Atmega8 breadboard circuit which is described in Atmega8 breadboard circuit Part 1 and Atmega8 breadboard circuit Part 2. AVCC The Atmega168 has 2 digital supply voltage pins, VCC and AVCC. AVCC supplies power to PC0-PC5. When these pins are used as analogue inputs, the AVCC power needs to be very stable. This is achieved by using a low pass filter comprising of an inductor and capacitor. 2 of 9 2/8/15, 1:44 PM

3 AREF The AREF pin is used to set the voltage that corresponds to a 100% value (1024) in the AD converter. In this example we tie it to AVCC. Analogue Input The Atmega168 has has 6 pins (8 for the SMD packages) that can be used for analogue input. These are PC0 to PC5. These are normally used for digital I/O but can be used for analogue input as well. In our example we are using a trimpot as the analog input device and connecting it to PC0. 3 of 9 2/8/15, 1:44 PM

4 LCD display We ve connected an LCD display to pins PD0 to PD5 and is used to display analogue readings. For more information on using LCD displays, please read Character LCD Displays Part 2. Registers The Atmega168 uses 6 different registers when performing analogue to digital conversion. These are: Register ADMUX ADCSRA ADCSRB Description ADC Multiplexer Selection Register ADC Control and Status Register A ADC Control and Status Register B DIDR0 Digital Input Disable Register 0 ADCL ADCH ADC Data Register Low ADC Data Register High The ADMUX register allows you to control: The Reference Voltage Left adjustment of results (used for 8 bit results) Selection of input channel ADMUX REFS1 REFS0 ADLAR - MUX3 MUX2 MUX1 MUX0 Read/Write R/W R/W R/W R R/W R/W R/W R/W The reference voltage is controlled by REFS1 and REFS0. The default is to use AREF, but other options are available. ADLAR is used for left shifting the converted data. This is useful when reading 8 bit values due to the fact that reading a 16 bit value is not an atomic operation. MUX0 to MUX3 are used to select which input channel you wish to read. The values 0000 to 0101 allow you to select ports PC0 to PC5, while the reserved values of 1110 and 1111 allow you to select 1.1V and 0V. ADCSRA and ADCSRB are used to control the AD conversion process. ADCSRA ADEN ADSC ADATE ADIF ADIE ADPS2 ADPS1 ADPS0 Read/Write R/W R/W R/W R/W R/W R/W R/W R/W 4 of 9 2/8/15, 1:44 PM

5 ADCSRB - ACME ADTS2 ADTS1 ADTS0 Read/Write R R/W R R R R/W R/W R/W These 2 registers provide for many different options and we will look at a subset in this tutorial. ADEN enables the AD converter subsystem. This bit needs to be set before any conversion takes place. ADSC is set when you want to start an AD conversion process. When the conversion is finished, the value reverts back to 0. ADIF is set when the conversion is complete and the data is written to the result registers (ADCL/ADCH). To clear it back to 0 you need to write a 1 to it. When an analog sample is read, a timeslice is used. The width of that timeslice is determined by the input clock. ADPS0 to ADPS2 sets the division factor between the system clock and the input clock. DIDR0 is used to disable the digital input buffers on PC0 to PC5. When set, the corresponding PINC value will be set to 0. DIDR0 - - ADC5D ADC4D ADC3D ADC2D ADC1D ADC0D Read/Write R R R/W R/W R/W R/W R/W R/W Lastly we have the ADC Data Registers, ADCL and ADCH. The AD converter returns a 10 bit value which is stored in these 2 registers. The structure of these registers depends on the ADLAR value. ADLAR=0 ADCH ADC9 ADC8 Read/Write R R R R R R R R ADCL ADC7 ADC6 ADC5 ADC4 ADC3 ADC2 ADC1 ADC0 Read/Write R R R R R R R R ADLAR=1 5 of 9 2/8/15, 1:44 PM

6 ADCH ADC9 ADC8 ADC7 ADC6 ADC5 ADC4 ADC3 ADC2 Read/Write R R R R R R R R ADCL ADC1 ADC Read/Write R R R R R R R R To make things easier, the AVR libc library returns the values of these registers as a single 16 bit value, ADC. Putting it all together The following code example shows how to read an analogue value and output it to the LCD panel #include <avr/io.h> #include "hd44780.h" uint16_t ReadADC(uint8_t channel) { ADMUX = (ADMUX & 0xf0) channel; // Channel selection ADCSRA = _BV(ADSC); // Start conversion while(!bit_is_set(adcsra,adif)); // Loop until conversion is complete ADCSRA = _BV(ADIF); // Clear ADIF by writing a 1 (this sets the value to 0) return(adc); } void adc_init() { ADCSRA = _BV(ADEN) _BV(ADPS2) _BV(ADPS1) _BV(ADPS0); //Enable ADC and set 128 prescale } int main (void) { 6 of 9 2/8/15, 1:44 PM

7 } lcd_init(); adc_init(); while (1) { char buffer[16]; sprintf(buffer,"%d lcd_goto(0); lcd_puts(buffer); } return(0); ", ReadADC(0)); Sharing is Caring If you like this post, please post it on twitter, Facebook or your blog. Your support is very much appreciated. Code Download Source code example More Information ATmega48/88/168/328 datasheet Related posts: 1. Reading and writing Atmega168 EEPROM 2. ATmega8 breadboard circuit Part 2 of 3 The Microcontroller 3. Atmega168 Experimenter s Kit 4. External Interrupts on an ATmega Atmega168 Development kit now with 10 extra components ATmega168, ATMEL, AVR, Breadboard 7 of 9 2/8/15, 1:44 PM

8 18 Comments Protostack! Login Sort by Best Share Favorite Join the discussion Kunal Pathak 5 months ago Erkko what you suggest for a better filter. Also input pins require filter else they go mad(i mean literally), any suggestions for designing filter when using pins as input of the microcontroller. Reply Share Erkko > Kunal Pathak 4 months ago The calculator tool I linked below can be used to design a better filter of the same type. Aiming for a Q value of 1/2 or below produces a filter which will not resonate around any frequency. Just plug in a cut-off frequency as low as possible, and see what kind of components it suggests, and make a resonable compromize for what kind of components you have available. For example: 1 Ohm, 22 uf, 10 uh will produce a filter with a 10 khz cutoff frequency and minimal overshoot or ringing. 10 khz may be too high if you intend to record clean audio, but low enough to filter out most switching noise caused by the CPU itself. Large capacitance values need to be made out of electrolytic capacitors which don't have good high frequency response, so adding a small value ceramic or polymer capacitor in parallel is a good idea. Also remember that the filter coil has some amount of resistance as well. For input debouncing for buttons, a simple RC filter kOhm and 100nF or so is usually enough. There are plenty of online calculators for that as well, and it's the same idea. Button debouncing can however be done in software as well, so you don't necessarily need to add filters - just add a short delay on the first readout and then read the pin again after it's settled down. Reply Share Erkko 5 months ago It seems that the filter you're proposing has a resonance at 159 khz and has a very low damping factor, and with realistic component 8 of 9 2/8/15, 1:44 PM

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