# 微算機 Lab9 - A/D Converter
> 參考自去年教材

## 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](http://wiki.csie.ncku.edu.tw/embedded/ADC)

### VREF與resolution

VREF+ : 上界的參考電壓
VREF- : 下界的參考電壓
Resolution : ADC的解析度

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

### $T_{AD}$
$T_{AD}$ : A/D Clock period, the time required to convert one bit
$T_{AD}$ 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

### 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
[Source](https://techetrx.com/pic-microcontroller/adc-in-pic16f887-microcontroller/)

## ADC流程
1. Acquisition : 採樣輸入電壓
2. Conversion : 將電壓轉換成數值
3. Discharge : 釋放電壓
### 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 and Discharge
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$T_{AD}$
### 時間表

## 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
[Source](https://techetrx.com/pic-microcontroller/adc-in-pic16f887-microcontroller)

## workflow of ADC using interrupt I/O
### step-1
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).

### step-2
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)

### step-3
Start conversion:
* Set GO/DONE bit (ADCON0.GO)

### step-4
Conversion completed:
* Go to ISR
* Read value of ADRES register
* Do things you want
* Clear ADC interrupt flag bit (PIR1.ADIF)

### step-5
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

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;
}