PDF AN546 Data sheet ( Hoja de datos )

Número de pieza	AN546
Descripción	Using the Analog to Digital Converter
Fabricantes	Microchip Technology
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No Preview Available ! M AN546 Using the Analog-to-Digital (A/D) Converter Authors: Sumit Mitra, Stan D’Souza, and Russ Cooper Microchip Technology Inc. www.DataSheet4U.com INTRODUCTION This application note is intended for PIC16C7X users with some degree of familiarity with analog system design.The various sections discuss the following topics: • Commonly used A/D terminology • How to conﬁgure and use the PIC16C71 A/D • Various ways to generate external reference voltage (VREF) • Conﬁguring the RA3:RA0 pins COMMONLY USED A/D TERMINOLOGY The Ideal Transfer Function In an A/D converter, an analog voltage is mapped into an N-bit digital value. This mapping function is deﬁned as the transfer function. An ideal transfer is one in which there are no errors or non-linearity. It describes the “ideal” or intended behavior of the A/D. Figure 1 shows the ideal transfer function for the PIC16C7X A/D. FIGURE 1: PIC16C7X IDEAL TRANSFER FUNCTION FFh FEh Note that the digital output value is 00h for the analog input voltage range of 0 to 1LSb. In some converters, the ﬁrst transition point is at 0.5LSb and not at 1LSb as shown in Figure 2. Either way, by knowing the transfer function the user can appropriately interpret the data. Transition Point The analog input voltage at which the digital output switches from one code to the next is called the “Tran- sition Point.” The transition point is typically not a single threshold, but rather a small region of uncertainty (Figure 3). The transition point is therefore deﬁned as the statistical average of many conversions. Stated dif- ferently, it is the voltage input at which the uncertainty of the conversion is 50%. Code Width The distance (voltage differential) between two transition points is called the “Code Width.” Ideally the Code Width should be 1LSb (Figure 1). FIGURE 2: ALTERNATE TRANSFER FUNCTION FFh FEh Code Width (CW) 04h 03h 02h 01h 00h Analog input voltage 04h 03h 02h 01h 00h Analog input voltage © 1997 Microchip Technology Inc. DS00546E-page 1 1 page EXAMPLE 2: SEQUENTIAL CHANNEL CONVERSIONS ; ; InitializeAD, initializes and sets up the A/D hardware. ; Select ch0 to ch3 in a round robin fashion, internal RC OSC. ; Load results in 4 consecutive addresses starting at ADTABLE (10h) ; InitializeAD bsf STATUS, RP0 ; select Bank1 movlw b'00000000' ; select RA3-RA0 movwf ADCON1 ; as analog inputs bcf STATUS, RP0 ; select Bank0 movlw b'11000001' ; select: RC osc, ch0... movwf ADCON0 ; turn on A/D movlw ADTABLE ; point fsr to top of... www.DataSheet4U.com ; movwf FSR ; table new_ad call sample_delay ; provide necessary sampling time bsf ADCON0, GO ; start new A/D conversion loop btfsc ADCON0, GO ; A/D over? goto loop ; no then loop ; movf adres, w ; yes then get A/D value movwf 0 ; load indirectly movlw 4 ; select next channel addwf ADCON0 ;/ bcf ADCON0, ADIF ; reset interrupt flag bit. ; increment pointer to correct table offset. clrf temp ; clear temp register btfsc ADCON0, CH50 ; test lsb of channel select bsf temp, 0 ; set if ch1 selected btfsc ADCON0, CH51 ; test msb of channel select bsf temp, 1 ;/ movlw ADTABLE ; get table address addwf temp, w ; add with temp movwf FSR ; move into indirect goto new_ad ; A detailed code listing is provided in Appendix B. AN546 © 1997 Microchip Technology Inc. DS00546E-page 5 5 Page AN546 Please check the Microchip BBS for the latest version of the source code. Microchip’s Worldwide Web Address: www.microchip.com; Bulletin Board Support: MCHIPBBS using CompuServe® (CompuServe membership not required). APPENDIX A: SINGLE CHANNEL A/D (SAD) MPASM 01.40 Released SAD.ASM 1-16-1997 15:22:04 PAGE 1 LOC OBJECT CODE VALUE www.DataSheet4U.com 00000010 00000001 00000002 0000 0000 2810 0004 0004 281E 0010 0010 0010 3000 0011 0086 0012 1683 0013 0086 0014 1283 0015 201F 0016 0016 0809 0017 0086 0018 2027 0019 1088 001A 1508 001B 001B 1888 001C 2816 LINE SOURCE TEXT 00001 ;TITLE "Single channel A/D (SAD)" 00002 ;This program is a simple implementation of the PIC16C71's 00003 ;A/D. 1 Channel is selected (CH0). 00004 ;The A/D is configured as follows: 00005 ; Vref = +5V internal. 00006 ; A/D Osc. = internal RC 00007 ; A/D Channel = CH0 00008 ;Hardware for this program is the PICDEM1 board. 00009 ; 00010 ; 00011 ; Program: SAD.ASM 00012 ; Revision Date: 00013 ; 1-14-97 Compatibility with MPASMWIN 1.40 00014 ; 00015 ; 00016 LIST P=16C71 00017 ERRORLEVEL -302 00018 ; 00019 include "p16c71.inc" 00001 LIST 00002 ;P16C71.INC Standard Header File, Version 1.00 Microchip Technology 00142 LIST 00020 ; 00021 TEMP EQU 10h 00022 adif equ 1 00023 adgo equ 2 00024 ; 00025 ORG 0x00 00026 ; 00027 ; 00028 goto start 00029 ; 00030 org 0x04 00031 goto service_int ;interrupt vector 00032 ; 00033 ; 00034 org 0x10 00035 start 00036 movlw B'00000000' ;set port b as 00037 movwf PORTB ;all outputs 00038 ; tris PORTB ;/ 00039 BSF STATUS, RP0 ; Bank1 00040 MOVWF TRISB ; PortB as outputs 00041 BCF STATUS, RP0 ; Bank0 00042 ; 00043 call InitializeAD 00044 update 00045 movf ADRES,W ;get a/d value 00046 movwf PORTB ;output to port b 00047 call SetupDelay ;setup time >= 10uS. 00048 bcf ADCON0,adif ;clear int flag 00049 bsf ADCON0,adgo ;start new conversion 00050 loop 00051 btfsc ADCON0,adif ;a/d done? 00052 goto update ;yes then update new value. © 1997 Microchip Technology Inc. DS00546E-page 11 11 Page

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