Compte rendu Technique de Projet

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Workshop Craiova Cachan Robotic Cup
Project pedagogy approach of microcontrollers
Craiova University
IUT Cachan
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Analog to Digital Conversion
Craiova University
IUT Cachan
3
Analog to Digital Conversion
1 From the analog signal to the digital signal 2
The analog to digital converter of pic18f4320 3
The Conversion Time 4 The Acquisition Time 5
Programming analog to digital conversion
Bibliography microcontroller PIC 4320
documentation (PDF file) MPLAB_C18 librairies
documentation (PDF files) MPLAB_C18 header file
adc.h MPLAB_C18 c file sources (mcc18\src\traditio
nnal\pmc)
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1 From the analog signal to the digital signal
Analog Signal
001 100 110 111 110 101 100 010 010 001 010 010
011 100 101
5
2 The analog to digital converter of pic18f4320

N (10 bits digital value)
microcontroller
0x3FF 1023
ADC 10 bits
analog input
10 bits value N
internal digital number
Vin
Vin
0x000 0
5 Volts
0 Volts
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2 The analog to digital converter of pic18f4320
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2 The analog to digital converter of pic18f4320
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2 The analog to digital converter of pic18f4320
Configuration / initialization function
Conversion Time
OpenADC( ADC_FOSC_32 ADC_RIGHT_JUST
ADC_12_TAD, ADC_CH0 ADC_INT_OFF
ADC_VREFPLUS_VDD ADC_VREFMINUS_VSS,
15 )
Right or left justification
Acquisition Time
Channel selection
Interrupt deactivation
Voltage reference configuration
Mode selection for each Analog or Digital Inputs
9
3 The conversion time

microcontroller
ADC 10 bits
ST
analog input
10 bits value N
internal digital number
Vin
ST DONE
DONE
10
3 The conversion time
AD conversion time per bit is defined as TAD.
The conversion needs 11 TAD for 10-bit
conversion. TAD is selectable by software.
TAD 16 / 8 MHz 2 ms and Tcon 2 x 11
22 ms
11
4 The acquisition time
Multiplexer and analog acquisition schematic
AN0
AN1
AN2
analog mux
Vin
13 analog input channels
AN10
AN11
AN12
input selection
12
4 The acquisition time

• 1 select an analog input
• 2 - the switch is closed CHOLD is charged
• Vin VDD (1 e t/?) with ? (RsRicRss)CHOLD
  
• Maximum charging time reached when Vin VDD 2047
  / 2048 (1/2 LSB)
• Tc ? ln(2048) 1.05 ms
• 3 - the switch is opened, Vin is read and
  converted (Tcon)
• 4 - CHOLD is discharged

Before conversion, the minimum required sample
time (worst case) 12,8 ms
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4 The acquisition time
TACQ is configurable between 0 TAD and 20 TAD
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5 Programming analog to digital conversion
Flow chart for Analog to Digital Conversion
GO/DONE 1
GO/DONE 0 ?
N is read
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5 Programming analog to digital conversion
An example reading the potentiometer (plugged
on AN0)
void main(void) int i short N //
Configure ADC OpenADC( ADC_FOSC_xx
ADC_RIGHT_JUST ADC_xx_TAD,
ADC_CH0 ADC_INT_OFF, 0x0E)
only AN0, is analog input
while(1) // infinite loop
ConvertADC() // Start sampling and conversion
while (BusyADC() 1) // Wait until
conversion is done N ReadADC()
// Read N