Vi điều khiển và ứng dụng ADC

Chuyên đề II
Vi điều khiển và ứng dụng
ADC


Ví dụ về ADC

2


Conversion time

3


Mạch ứng dụng

To help designing analog hardware filters, Microchip offers a small
application named FilterLab

4


A/D
 12-bit/10bit Resolution with +/- 1-bit accuracy
 100 K Samples / Sec conversion rate
 1 Sample/Hold Amplifier

 Up to 16 Analog Inputs
 Analog Input Range: (VREF-) to
 Multiple conversion trigger sources

5

(VREF+)


Hardware

6


A/D block

7


Các bước khởi động ADC
 Configure theA/D module
 configure the port pins as analogue inputs, voltage

reference, and digital I/O pins,
 select A/D converter input channel,
 select A/D conversion clock,
 select A/D conversion trigger source,
 turn on A/D module;

8


Tiếp

 ConfigureA/D interrupt (if required)
 clear ADIF bit (IFS0,11>),
 select A/D interrupt priority,
 set ADIE bit (IEC0<11>);

 Start sampling
 Wait the required acquisition time;
 Trigger acquisition end, start conversion;

9


Tiếp
 Wait for A/D to complete, by either
 waiting for the A/D interrupt, or
 waiting for the DONE bit to get set;

 Read A/D result buffer, clear ADIF bit if required.

10


Voltage reference

11


Acquisition / Conversion Timing

12

Chọn kênh

13


Manual Acquisition / Conversion

14


Setting A/D Conversion Time
 ADCS sets conversion clock period



ADCS value limited by maximum SPS of A/D
Related to clock used by processor

 Tcy =1 / MIPS
 Tconv =Desired conversion time
 Clocks per conversion =14 for 12-bit A/D


15

ADCS =((2 * Tconv) / (14 * Tcy)) - 1

Ví dụ
 ADCS =((2 * Tconv) / (CConv * Tcy)) - 1

Example: 30 MIPS device, 12-bit A/D, max rate
 Tcy =1 / 30 MIPS =33.3 nsec

Tconv =10 mSec (min) by specification
 ADCS =((2 * 10*10-6) / (14 * 33.3*10-9)) – 1 =41.8
 ADCS =42 =0x24 =0b101010
 Tad =(ADCS+1) * (Tcy/2) =716 nsec
 Tconv =14 * Tad=10.03 mSec

16


Internal AD RC clock
 ADRC enables internal AD RC clock
 ADCS bits are ignored



17

For 12-bit A/D
Nominal Rate =47 KSPS
Nominal Tconv =21 mSec


Chu kỳ ADC

18


Ví dụ lấy mẫu và chuyển đổi thủ
công

19


Ví dụ về khởi động bằng timer ngoài

20


C sample

21


A/D Buffer
 Module has a 16-word

buffer
 Module writes results
into buffer in completion
 Buffer will reset to
ADCBUF0 after each
Interrupt
 User software has read
only access to buffer

22


Buffer Data Formats
 Results presented in one of four formats
 Integer (DOUT =0000 dddd dddd dddd)
 Fractional
(DOUT =dddd dddd dddd 0000)
 Signed Integer (DOUT =ssss sddd dddd dddd)
 Signed fractional (DOUT =sddd dddd dddd 0000)

23


Channel Scanning
 When (CSCNA =1) group A will scan analog inputs

24


C examples
void initADC( int amask)
{
AD1PCFG =amask; // select analog input pins
AD1CON1 =0; // manual conversion sequence control
AD1CSSL =0; // no scanning required
AD1CON2 =0; // use MUXA, AVss and AVdd are used as Vref+/AD1CON3 =0x1F02; // Tad =2 x Tcy =125ns >75ns
AD1CON1bits.ADON =1; // turn on theADC

} //initADC

int readADC( int ch)
{
AD1CHS =ch; // 1. select analog input channel
AD1CON1bits.SAMP =1; // 2. start sampling
TMR1 =0; // 3. wait for sampling time
while (TMR1<100); // 6.25 us
AD1CON1bits.DONE =1; // 4. start the conversion
while (!AD1CON1bits.DONE); // 5. wait for the conversion to complete
return ADC1BUF0; // 6. read the conversion result

} // readADC
25