微算機 Lab9 - A/D Converter

參考自去年教材

文章目錄

• 微算機 Lab9 - A/D Converter
  • ADC簡介
    • 什麼是ADC
    • VREF與resolution
  • ADC 流程：
    • TAD
    • Acquisition
    • Conversion
    • Discharge
    • 時間表
  • PIC18 ADC register introduction
    • ADCON0
    • ADCON1
    • ADCON2
    • ADRESH、ADRESL
  • Workflow of ADC using interrupt I/O
    • Step1. Configure the ADC module:
    • Step2. Configure the ADC interrupt:
    • Step3. Start conversion:
    • Step4. Conversion completed:
    • Step5. Next conversion(if required) :
  • Variable resistor
  • 範例code

ADC簡介

什麼是ADC

主要功能 : 把輸入的類比訊號轉成數位數值
本次Lab會把可變電阻輸入的電壓轉成數值形式
更多參考 ADC - 成大資工

VREF與resolution

VREF+ : 上界的參考電壓，若輸入電壓為此值則輸出為最大值
VREF- : 下界的參考電壓，若輸入電壓為此值則輸出為最小值
Resolution : ADC的解析度，若為 10bit 代表輸出從 0 ~ 1023

e.g., VREF- = 0V, VREF+ = 10V, Resolution : 10bits (range = [0,1023])
0V-> 0
5V -> 511
10V -> 1023

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ADC 流程：

Acquisition -> Convertion -> Discharge(wait before next acquisition) -> Idle until you set GODONE bit -> Acquisition -> …

Acquisition : 採樣輸入電壓
Conversion : 將電壓轉換成數值
Discharge : 釋放電壓

$T_{AD}$

• A/D Clock period, the time required to convert one bit
• 越小越好，但要大於0.7μs
  

$T_{AD}$ 設定

透過查表設定ADCS(ADCON2)

假設頻率

$F_{OSC}$ 是 2.86 MHz, 則周期 (

$T_{OSC}$)會是 

$\frac{1}{2.86\times {10}^6}\approx 0.35\mu s$，為了滿足最低 A/D Clock period (

$0.7\mu s$ )，要把

$T_{AD}$設成兩倍的

$T_{OSC}$，Operation欄位中的數值即為

$T_{AD}$

e.g.,假設頻率 (Fosc) 是 1 MHz，透過查表得知ADCS要設成000，
而Operation欄位是2×Tosc=2×

$\frac{1}{1MHz}=2𝜇𝑠$，因此

$T_{AD}$ 為2𝜇𝑠

Acquisition

採樣輸入電壓，需要時間

依據data sheet的推導(p. 228)，acquisition time最少會花

$2.4\mu s$

依據

$T_{AD}$的時間決定ACQT，若

$T_{AD}$為

$2\mu s$，則ACQT要設成001，也就是2

$T_{AD}$＝

$4\mu s$>

$2.4\mu s$

Conversion

將採樣電壓轉換成數值，需要時間

依據data sheet，conversion需要花11到12個

$T_{AD}$

Discharge

釋放電壓

Left/Right justified

ADC轉換的結果放在ADRES register裡，存放的方式分為left justified與right justified，可依據使用需求設置
e.g., 需要8 bits resolution，設定為left justified，取ADRESH數值 ; 需要10 bits resolution，則設定為right justified，將ADRESH前兩bits與ADRESL結合

時間表

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PIC18 ADC register introduction

ADCON0

CHS : 設定analog input 輸入腳位
GO/DONE : 設為1時(ADCON0bits.GO = 1)開始做ADC，轉換完後 GO/DONE會自動設為0
ADON : 開啟ADC功能

ADCON1

VCFG1 : 設定下界參考電壓
VCFG0 :設定上界參考電壓
PCFG : 設定ANx PORT為類比還是數位，使用 ADC 的同時若發現其他 PORT 的 input 值怪怪的也許是誤把那些 PORT 設成 analog input

ADCON2

ADFM : 設定justified
ADCS : 選擇conversion clock
ACQT : 選擇acquisition time要幾個

$T_{AD}$

ADRESH、ADRESL

result of conversion

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Workflow of ADC using interrupt I/O

Step1. Configure the ADC module:

• Select VREF (ADCON1.VCFG0, ADCON1.VCFG1)
• Select A/D port control(ADCON1.PCFG)
• Select A/D input channel (ADCON0.CHS)
• Select A/D conversion clock (ADCON2.ADCS)
• Select A/D acquisition time (ADCON2.ACQT)
• Select justified method (ADCON2.ADFM)
• Turn on A/D module (ADCON0.ADON)

Note : The port pins needed as analog inputs must have their corresponding TRIS bits set (input).

Step2. Configure the ADC interrupt:

• Enable A/D interrupt (PIE1.ADIE)
• Clear A/D interrupt flag bit (PIR1.ADIF)
• Enable peripheral interrupt (INTCON.PEIE)
• Set GIE bit (INTCON.GIE)

Step3. Start conversion:

• Set GO/DONE bit (ADCON0.GO)

Step4. Conversion completed:

• Go to ISR
• Read value of ADRES register
• Do things you want
• Clear ADC interrupt flag bit (PIR1.ADIF)

Step5. Next conversion(if required) :

• You need to have a minimum wait of 2
  $T_{AD}$ before next acquisition start, then go back to step 3.

Variable resistor

左邊接 5V，右邊接地，中間接 Analog 輸入

---

範例code

#include <xc.h>
#include<stdio.h>
#include<stdlib.h>
#include <time.h>

#pragma config OSC = INTIO67  //OSCILLATOR SELECTION BITS (INTERNAL OSCILLATOR BLOCK, PORT FUNCTION ON RA6 AND RA7)
#pragma config WDT = OFF      //Watchdog Timer Enable bit (WDT disabled (control is placed on the SWDTEN bit))
#pragma config PWRT = OFF     //Power-up Timer Enable bit (PWRT disabled)
#pragma config BOREN = ON     //Brown-out Reset Enable bits (Brown-out Reset enabled in hardware only (SBOREN is disabled))
#pragma config PBADEN = OFF   //PORTB A/D Enable bit (PORTB<4:0> pins are configured as digital I/O on Reset)
#pragma config LVP = OFF      //Single-Supply ICSP Enable bit (Single-Supply ICSP disabled)
#pragma config CPD = OFF      //Data EEPROM Code Protection bit (Data EEPROM not code-protected)

void __interrupt(high_priority)H_ISR(){
    
    //step4
    int value = ADRESH;
    
    
    //do things
    
    
    //clear flag bit
    PIR1bits.ADIF = 0;
    
    
    //step5 & go back step3
    /*
    delay at least 2tad
    ADCON0bits.GO = 1;
    */
    
    return;
}

void main(void) 
{
    //configure OSC and port
    OSCCONbits.IRCF = 0b100; //1MHz
    TRISAbits.RA0 = 1;       //analog input port
    
    //step1
    ADCON1bits.VCFG0 = 0;
    ADCON1bits.VCFG1 = 0;
    ADCON1bits.PCFG = 0b1110; //AN0 為analog input,其他則是 digital
    ADCON0bits.CHS = 0b0000;  //AN0 當作 analog input
    ADCON2bits.ADCS = 0b000;  //查表後設000(1Mhz < 2.86Mhz)
    ADCON2bits.ACQT = 0b001;  //Tad = 2 us acquisition time設2Tad = 4 > 2.4
    ADCON0bits.ADON = 1;
    ADCON2bits.ADFM = 0;    //left justified 
    
    
    //step2
    PIE1bits.ADIE = 1;
    PIR1bits.ADIF = 0;
    INTCONbits.PEIE = 1;
    INTCONbits.GIE = 1;


    //step3
    ADCON0bits.GO = 1;
    
    while(1);
    
    return;
}