Lecture 3a analog to digital converter
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This document discusses analog to digital conversion using a microcontroller. It describes configuring the microcontroller's analog to digital converter to take readings from sensors connected to analog input pins. It explains concepts like reference voltages, conversion time, and reading the digital output. The document provides an example of reading an analog potentiometer input using Microchip's ADC functions in C. It also outlines some workshops involving reading sensors and controlling a robot using the analog to digital converter.
Lecture 3a analog to digital converter
- 1. Microcontroller Lab. Eng.Khaled Tamizi Project Pedagogy approach of Microcontroller – Palestinian Robotic Cup ١ PPU IUT Cachan Mechanical Department Mechatronic
- 2. Analog to Digital Conversion Bibliography microcontroller PIC 4550 documentation (PDF file) Project Pedagogy approach of Microcontroller – Palestinian Robotic Cup ٢ PPU IUT Cachan microcontroller PIC 4550 documentation (PDF file) optical sensor CNY 70 documentation (PDF file) MPLAB_C18 librairies documentation (PDF files) MPLAB_C18 header file adc.h MPLAB_C18 c file sources (mccsrctraditionnalpmc)
- 3. ADC 10 bits Vin N N 0x000 0x3FF dVin analog input internal digital number Vin Introduction microcontroller Project Pedagogy approach of Microcontroller – Palestinian Robotic Cup ٣ 0x000 Vref- Vref+ 210 N = Vin (Vref+ - Vref-) precision : Vref+ - Vref- if N=1 dVin = 1024 Vin
- 4. Consequence Example : Vreff- = 0 and Vref+ = 5 Volt 0x000 ≤ N ≤ 0x3ff 0 ≤ N ≤ 1023 precision = 5V / 1024 = 5 mV In our microcontroller, Vref+ and Vref- are configurable : -1st configuration Vref- = 0 and Vref+ = VDD = 5 Volt we’ll use this one Project Pedagogy approach of Microcontroller – Palestinian Robotic Cup ٤ -1st configuration Vref- = 0 and Vref+ = VDD = 5 Volt -2nd configuration Vref- and Vref+ are dedicated external pins (PORT A) 0 < Vref+ < 5V 0<Vref-<5V
- 5. Conversion time ADC 10 bits Vin N ST starts conversion conversion in progress Tcon Project Pedagogy approach of Microcontroller – Palestinian Robotic Cup ٥ ST DONE DONE To whom may be concerned …. our AD converter is a « successive approximation » one conversion done, N is avalaible
- 6. Some software In our microcontroller : - ST is set when the internal bit GO/DONE is set by software - When the conversion is done, this bit is automatically cleared - N is then available in a 16 bits register (ADRES right or left justified) GO/DONE = 1 Project Pedagogy approach of Microcontroller – Palestinian Robotic Cup ٦ GO/DONE = 0 ? N is read AD flow chart
- 7. Some software In our microcontroller the AD conversion time per bit is defined as TAD. The conversion needs 11 TAD for 10-bit conversion. TAD is selectable by software. internal configuration bits Project Pedagogy approach of Microcontroller – Palestinian Robotic Cup ٧ TAD = 64 / 48 MHz = 1.3 µµµµs and Tcon = 1.3 x 11 = 14.3 µµµµs
- 8. 13 analog input channels AN0 AN1 AN2 Vin An internal analog multiplexor allows to choose 1 analog input channel between 13 AN0, AN1, AN2, AN3, AN4 are connected to PORT A analog mux Project Pedagogy approach of Microcontroller – Palestinian Robotic Cup ٨ AN10 AN11 AN12 input selection AN0, AN1, AN2, AN3, AN4 are connected to PORT A AN8, AN9, AN10, AN11, AN12 to PORT B AN5, AN6, AN7 to PORT E Tmux = 0.2 µµµµs + (temp – 25°C) 0.02 µµµµs/°C As temp max = 85°C Tmux max = 0.2 µµµµs + 1.2 µµµµs = 1.4 µµµµs
- 9. Rs : source impedance, must be < 2,5 kΩΩΩΩ Rss = 2 kΩΩΩΩ Electrical analog model for one input When the analog input is selected, the channel must be sampled and hold : Vin Project Pedagogy approach of Microcontroller – Palestinian Robotic Cup ٩ -1st step : the switch is closed, CHOLD is charged Vin = VDD (1 – e –t/ττττ) with ττττ= (Rs+Ric+Rss)CHOLD Maximum charging time reached when Vin = VDD 1023 / 1024 (1/2 LSB) Tc = ττττ ln(2048) = 1.05 µµµµs - 2nd step : the switch is opened, Vin is read and converted (Tcon) - 3rd step : CHOLD is discharged Before conversion, the minimum required sample time (worst case) = 2,45 µµµµs
- 10. Some software The PIC microcontroller provides 2 ways to manage with AD conversion 1st method : « manual » delay set by software Project Pedagogy approach of Microcontroller – Palestinian Robotic Cup ١٠ The user must ensure that the required time (2.45 µµµµs) as passed between selecting the desired channel and setting GO/DONE
- 11. Some software The PIC microcontroller provides 2 ways to manage with AD conversion 2nd method : « automatic » Project Pedagogy approach of Microcontroller – Palestinian Robotic Cup ١١ When GO/DONE is set, the AD module continues to sample the selected input for the selected acquisition time (TACQ). The conversion then automatically begins. TACQ is configurable between 0 TAD and 20 TAD We’ll choose TACQ = 2 TAD = 2.6 µµµµs < 2.45 µµµµs
- 12. Our configuration We use AN0, AN1, AN2, AN3, AN4 (PORT A RA0, RA1, RA2, RA3, RA5) Vdd AN0 potentiometer 10K AN1, AN2, AN3, AN4 are all connected to a reflective optical sensor Vdd AN1 or 2, 3, 4 Vdd Rd Rt Project Pedagogy approach of Microcontroller – Palestinian Robotic Cup ١٢ AN0 AN0 is connected to a potentiometer LED Phototransistor
- 13. Microchip C functions Project Pedagogy approach of Microcontroller – Palestinian Robotic Cup ١٣ More in the documentations ….. header file : adc.h
- 14. An example : reading the potentiometer void main(void) { int i; short N; // Configure ADC OpenADC(ADC_FOSC_64 & ADC_RIGHT_JUST & ADC_2_TAD, ADC_CH0 & ADC_INT_OFF & ADC_VREFPLUS_VDD & ADC_VREFMINUS_VSS ,0x0A); only AN0, AN1, AN2, AN3 are analog inputs Project Pedagogy approach of Microcontroller – Palestinian Robotic Cup ١٤ for(i=0;i<5;i++) // performs 5 acquisitions { } CloseADC(); } while (BusyADC() == 1); // Wait until conversion is done N = ReadADC(); // Read N ConvertADC(); // Start sampling and conversion
- 15. Let’s go back to the OpenADC function OpenADC ( ADC_FOSC_64 & ADC_RIGHT_JUST & ADC_2_TAD , ADC_CH0 & ADC_INT_OFF // TAD = 64 TOSC // N (10 bits) is right justified in a 16 bits register // TACQ = 2 TAD performs « automatic » conversion // multiplexor on Channel 0 Project Pedagogy approach of Microcontroller – Palestinian Robotic Cup ١٥ ADC_INT_OFF & ADC_VREFPLUS_VDD & ADC_VREFMINUS_VSS , 0x0A ); //0x0A = (00001010)2 only AN0, AN1, AN2, AN3 are used as analog inputs Prototype : void OpenADC (unsigned char config1, unsigned char config2, unsigned char portconfig); // ADC interrupt OFF // Vref+ = VDD = 5 Volt // Vref- = VSS = 0 Volt
- 16. void main(void) { int i; short N; OpenADC(ADC_FOSC_64 & ADC_RIGHT_JUST & ADC_2_TAD, ADC_INT_OFF & ADC_VREFPLUS_VDD & ADC_VREFMINUS_VSS ,10); for(i=0;i<5;i++) { SetChanADC(ADC_CH0); // Channel 0 An example : to change of channel no more ADC_CH0 Project Pedagogy approach of Microcontroller – Palestinian Robotic Cup ١٦ SetChanADC(ADC_CH0); // Channel 0 ConvertADC(); while (BusyADC() == 1); N = ReadADC(); SetChanADC(ADC_CH1); // Channel 1 ConvertADC(); while (BusyADC() == 1); N = ReadADC(); } CloseADC(); }
- 17. Workshops 1 - Displaying the value of the potentiometer on the LCD display 2 - Calibrating the optical sensors with the help of the LCD display 3 – The robot goes straight when the jack is off and stops when the floor is white Project Pedagogy approach of Microcontroller – Palestinian Robotic Cup ١٧ 3 – The robot goes straight when the jack is off and stops when the floor is white 5 – The robot follows the white line 4 – The potentiometer is used to control the speed of the robot