摘要
本应用笔记介绍MAX5581 DAC与PIC®微控制器的配合使用，给出了详细的电路图和源代码。
MAX5581简介
MAX5581是一款12位、快速建立DAC，采用3线SPI™串行接口。MAX5581的接口支持高达20MHz的SPI，最快建立时间为3µs。本应用笔记给出了连接高速PIC微控制器(PIC18F核)和MAX5581 DAC的应用电路以及所需要的全部固件。汇编程序利用MPLAB IDE、6.10.0.0版本中免费提供的编译器编写，用于PIC18F442。
硬件说明
这里讨论的应用电路采用了MAX5581评估板，包括：MAX5581、超高精度电压基准(MAX6126)、两个按键开关、增益设置电阻和PCB。PIC18F442没有安装在MAX5581EVKIT板上，但已添加到系统中，图1所示是一个完整的应用电路。MAX5581EVKIT上的/CS、SCLK、DIN和DOUT焊盘便于SPI串行接口的连接。
图1. MAX5581应用电路第一部分
图1. MAX5581应用电路第二部分
模拟与数字地平面
将模拟地与数字地分离开(如图2所示)可以得到较好的实际效果，利用铁氧体磁珠，如TDK MMZ1608B601C，连接两个地平面。这种布局可以避免微控制器系统时钟及其谐波成份馈入模拟地。已知PIC18F442的系统时钟为40MHz，考虑到MMZ1608B601C特殊的阻抗与频率特性，我们选择了这款铁氧体磁珠。图3给出了MMZ1608B601C阻抗随频率的变化曲线。
图2. 分离的模拟地和数字地
图3. TDK MMZ1608B601C铁氧体磁珠阻抗随频率的变化曲线
固件说明
列表1给出的汇编程序通过PIC18F442的内部MSSP SPI接口对MAX5581进行初始化，PIC18F442的40MHz系统时钟允许MSSP提供高达10MHz的SPI时钟(SCLK)。表1所示为上电后的配置字。一旦完成了对MAX5581的初始化，程序便将DAC输出寄存器装载为零，然后装载满量程输出，如表2所示。该固定循环程序可产生方波输出，如图4所示，可以用来演示MAX5581的快速建立时间。
图4. 一个输出为80kHz方波的实际示波器测试图
列表1. 用于连接MAX5581和PIC18F442内部MSSP SPI接口的汇编程序
下载: P18F442.INC
列表1.asm
;******************************************************************************
;
; Filename: Listing 1 (Absolute Code Version)
; Date: 2/25/05
; File Version: 1.0
;
; Author: Ted Salazar
; Company: Maxim
;
;******************************************************************************
;
; Program Description:
;
; This program interfaces the internal SPI MSSP
; (Peripheral) of the PIC18F442 to the MAX5581 SPI
; Quad DAC. The program initializes the MAX5581
; and dynamically generates a 50% duty cycle square
; wave with a frequency of 80KHz.
;
;
;******************************************************************************
;
; History:
; 2/25/05: Tested SPI DAC format
; 2/25/05: Initialized MAX5591
; 12/14/04: Cleared tcount timer in HWSPI_W_spidata_W
;******************************************************************************
;******************************************************************************
;
;******************************************************************************
;
; Files required: P18F442.INC
;
;******************************************************************************
radix hex ;Default to HEX
LIST P=18F442, F=INHX32 ;Directive to define processor and file format
#include <p18f442.inc> ;Microchip's Include File
;******************************************************************************
;******************************************************************************
xmit equ 06 ; Asynchronous TX is at C6
;
;******************************************************************************
;Configuration bits
; The __CONFIG directive defines configuration data within the .ASM file.
; The labels following the directive are defined in the P18F442.INC file.
; The PIC18FXX2 Data Sheet explains the functions of the configuration bits.
; Change the following lines to suit your application.
;T __CONFIG _CONFIG1H, _OSCS_OFF_1H & _RCIO_OSC_1H
;T __CONFIG _CONFIG2L, _BOR_ON_2L & _BORV_20_2L & _PWRT_OFF_2L
;T __CONFIG _CONFIG2H, _WDT_ON_2H & _WDTPS_128_2H
;T __CONFIG _CONFIG3H, _CCP2MX_ON_3H
;T __CONFIG _CONFIG4L, _STVR_ON_4L & _LVP_OFF_4L & _DEBUG_OFF_4L
;T __CONFIG _CONFIG5L, _CP0_OFF_5L & _CP1_OFF_5L & _CP2_OFF_5L & _CP3_OFF_5L
;T __CONFIG _CONFIG5H, _CPB_ON_5H & _CPD_OFF_5H
;T __CONFIG _CONFIG6L, _WRT0_OFF_6L & _WRT1_OFF_6L & _WRT2_OFF_6L & _WRT3_OFF_6L
;T __CONFIG _CONFIG6H, _WRTC_OFF_6H & _WRTB_OFF_6H & _WRTD_OFF_6H
;T __CONFIG _CONFIG7L, _EBTR0_OFF_7L & _EBTR1_OFF_7L & _EBTR2_OFF_7L & _EBTR3_OFF_7L
;T __CONFIG _CONFIG7H, _EBTRB_OFF_7H
;******************************************************************************
;Variable definitions
; These variables are only needed if low priority interrupts are used.
; More variables may be needed to store other special function registers used
; in the interrupt routines.
CBLOCK 0x080
WREG_TEMP ;variable used for context saving
STATUS_TEMP ;variable used for context saving
BSR_TEMP ;variable used for context saving
;
ENDC
CBLOCK 0x000
EXAMPLE ;example of a variable in access RAM
;
temp ;
temp2
;
xmtreg ;
cntrb ;
cntra ;
bitctr ;
tcount ;
speedLbyte ;T Being used in HWSPI_speed
;
ENDC
;******************************************************************************
;Reset vector
; This code will start executing when a reset occurs.
ORG 0x0000
goto Main ;go to start of main code
;******************************************************************************
;High priority interrupt vector
; This code will start executing when a high priority interrupt occurs or
; when any interrupt occurs if interrupt priorities are not enabled.
ORG 0x0008
bra HighInt ;go to high priority interrupt routine
;******************************************************************************
;Low priority interrupt vector and routine
; This code will start executing when a low priority interrupt occurs.
; This code can be removed if low priority interrupts are not used.
ORG 0x0018
movff STATUS,STATUS_TEMP ;save STATUS register
movff WREG,WREG_TEMP ;save working register
movff BSR,BSR_TEMP ;save BSR register
; *** low priority interrupt code goes here ***
movff BSR_TEMP,BSR ;restore BSR register
movff WREG_TEMP,WREG ;restore working register
movff STATUS_TEMP,STATUS ;restore STATUS register
retfie
;******************************************************************************
;High priority interrupt routine
; The high priority interrupt code is placed here to avoid conflicting with
; the low priority interrupt vector.
HighInt:
; *** high priority interrupt code goes here ***
retfie FAST
;******************************************************************************
;Start of main program
; The main program code is placed here.
Main:
; *** main code goes here ***
start
; *** Port Initialization ***
movlw 0x0FF
movwf PORTB
clrf PORTA
movlw 0x06 ;T Configure PortA as Digital
movwf ADCON1
movlw 0x00FB ;T A2 OUTPUT, ALL OTHERS INPUT
movwf TRISA
movlw 0x0001 ;T B0 INPUT, ALL OTHERS OUTPUT
movwf TRISB
movlw 0x0093 ;T C7-C0 => bit7-0
;T OUTPUTs: C6(TX), C5(MOSI), C3(SCLK), C2(CS)
;T INPUTs:C4 (MISO) and all others
movwf TRISC ;T TRISC bit3 Master = 0
bsf PORTC,RC2 ;T RC2 = CS\ Make CS\ high
; *** SPI Initialization ***
call HWSPI_init ;T Initialize the MSSP for SPI
; *** SPI Configuration ***
movlw b'00000000' ;T load W with test byte for CPOLCPHA 0,0
;T b'00000000' => CPOLCPHA 0,0
;T b'00000001' => CPOLCPHA 0,1
;T b'00000010' => CPOLCPHA 1,0
;T b'00000011' => CPOLCPHA 1,1
call HWSPI_W_configure
; *** SPI Speed ***
movlw b'00000000' ;T load W with test byte for SPI Freq
;T b'00000000' => Fosc/4 = 10MHz
;T b'00000001' => Fosc/16 = 2.5Mhz
;T b'00000010' => Fosc/64 = 625kHz
;T b'00000011' => Reserved.
call HWSPI_W_speed
;******************************************************************************
; *** MAX5581 Initialization ***
bcf PORTC,RC2 ;T RC2 = CS\ Make CS\ Low
movlw 0xEC ;T byte0 of settling time config
call HWSPI_W_spidata_W ;T HW SPI WriteRead Operation
movlw 0x0F ;T byte1 of settling time config
call HWSPI_W_spidata_W ;T HW SPI WriteRead Operation
bsf PORTC,RC2 ;T RC2 = CS\ Make CS\ high
; *** MAX5581 Load All DAC Outputs to Zero Scale ***
Loopforever bcf PORTC,RC2 ;T RC2 = CS\ Make CS\ Low
movlw 0xD0 ;T byte0 of load all input/output to zeros
call HWSPI_W_spidata_W ;T HW SPI WriteRead Operation
movlw 0x00 ;T byte1 of load all input/output to zeros
call HWSPI_W_spidata_W ;T HW SPI WriteRead Operation
bsf PORTC,RC2 ;T RC2 = CS\ Make CS\ high
; *** MAX5581 Load All DAC Outputs to Full Scale ***
bcf PORTC,RC2 ;T RC2 = CS\ Make CS\ Low
movlw 0xDF ;T byte0 of load all input/output to zeros
call HWSPI_W_spidata_W ;T HW SPI WriteRead Operation
movlw 0xFF ;T byte1 of load all input/output to zeros
call HWSPI_W_spidata_W ;T HW SPI WriteRead Operation
bsf PORTC,RC2 ;T RC2 = CS\ Make CS\ high
; movwf xmtreg ;T move w to xmtreg
; call asyxmtc ;T call UART routine
;
goto Loopforever ;T loop forever
;******************************************************************************
errsrv
movlw 0x65 ; load w with 'e' = 0x65
movwf xmtreg ; move w to xmtreg
call asyxmtc ; call UART routine
dead goto dead ; goto endless loop
;******************************************************************************
set_cf_error
movlw 0x00 ; 0x00 into W
sublw 0x00 ; Subtract W-0x00: If W<=N C set; If W>N C clear.
return ; error=> cf=set
;******************************************************************************
clear_cf_ok
movlw 0x01 ; 0x00 into W
sublw 0x00 ; Subtract W-0x00: If W<=N C set; If W>N C clear.
return ; success=> cf=clear
;******************************************************************************
HWSPI_init ;T SPI MSSP Initialization for M2EAM schematic
;T CPOL,CPHA = 0,0 => CKP = 0 & CKE = 1
bcf SSPCON1,SSPEN ;T Disable the MSSP, SSPCON-5
;
bcf TRISC,SDO ;T TRISC bit5 RC5/SDO = 0 MOSI Output
bcf TRISC,SCK ;T TRISC bit3 RC3/SCK = 0 SCLK Output
bsf TRISC,SDI ;T TRISC bit4 RC4/SDI = 1 MISO Input
movlw 0x0040 ;T SSPSTAT bit8 = 0 sampled in middle
;T SSPSTAT bit6 = CKE = 1
movwf SSPSTAT ;T Used to be sspstat on older PICs
movlw 0x0020 ;T SSPCON1 bit5 SSPEN = 1 Enables sycn serial port
;T SSPCON1 bit4 = CKP = 0
;T SSPCON1 bit3= 0 = Turn MSSP ON for SPI
;T SSPCON1 bit2-0 = 000b = SCLK = Fosc/4
;T SSPCON1 bit2 = 0 = Master
movwf SSPCON1 ;T Used to be sspcon on older PICs
bsf INTCON,PEIE ;T INTCON bit6 = PEIE = 1 = Enable periph interrupt
bsf PIE1,SSPIE ;T PIE1 bit3 = SSPIE = 1 = interrupt enable
movlw 0x00 ;T load 0x00 into W
movwf tcount ;T initialize tcount to zero (0x00)
;******************************************************************************
HWSPI_W_configure
;Configure SPI Mode
;
;On Entry: WREG = confDATA
;On Exit:
;On Success: return with C flag clear
;On Failure: return with C flag set
;
bcf SSPCON1,SSPEN ;T Disable the MSSP, SSPCON1-5
movwf temp ;T move the confDATA byte to temp
btfsc SSPCON1,SSPM3 ;T In SPI Mode?, skip if yes
call HWSPI_init ;T MSSP is in wrong mode, Init for SPI
;
btfsc temp,1 ;T Is bit1 of confDATA byte clear? if so skip next
goto CPOL_1 ;T goto CPOL = 1 label => CPOL = 1
btfsc temp,0 ;T Is bit0 of confDATA byte clear? if so skip next
;T => CPOL = 0 , CPHA = ?
goto CPOLCPHA_01 ;T goto => CPOL = 0 CPHA = 1
;Configure for CPOL = 0, CPHA = 0
bcf SSPCON1,CKP ;T SSPCON1 bit4 = CKP = 0
bsf SSPSTAT,CKE ;T SSPSTAT bit6 = CKE = 1
btfsc SSPCON1,CKP ;T Is SSPCON1 bit4 = CKP = 0 ?
goto badjump ;T CKP bit test error
btfss SSPSTAT,CKE ;T Is SSPSTAT bit6 = CKE = 1 ?
goto badjump ;T CKE bit test error
goto okjump2 ;OK configured!
;
CPOL_1 btfsc temp,0 ;T Is bit0 of confDATA byte clear? if so skip next
;T CPOL = 1 , CPHA = ?
goto CPOLCPHA_11 ;T goto => CPOL = 1, CPHA = 1
;Configure for CPOL = 1, CPHA = 0
bsf SSPCON1,CKP ;T SSPCON1 bit4 = CKP = 1
bsf SSPSTAT,CKE ;T SSPSTAT bit6 = CKE = 1
btfss SSPCON1,CKP ;T Is SSPCON1 bit4 = CKP = 1 ?
goto badjump ;T CKP bit test error
btfss SSPSTAT,CKE ;T Is SSPSTAT bit6 = CKE = 1 ?
goto badjump ;T CKE bit test error
goto okjump2 ;OK configured!
;
CPOLCPHA_01
;configure for CPOL = 0, CPHA = 1
bcf SSPCON1,CKP ;T SSPCON1 bit4 = CKP = 0
bcf SSPSTAT,CKE ;T SSPSTAT bit6 = CKE = 0
btfsc SSPCON1,CKP ;T Is SSPCON1 bit4 = CKP = 0 ?
goto badjump ;T CKP bit test error
btfsc SSPSTAT,CKE ;T Is SSPSTAT bit6 = CKE = 0 ?
goto badjump ;T CKE bit test error
goto okjump2 ;OK configured!
;
CPOLCPHA_11
;configure for CPOL = 1, CPHA = 1
bsf SSPCON1,CKP ;T SSPCON1 bit4 = CKP = 1
bcf SSPSTAT,CKE ;T SSPSTAT bit6 = CKE = 0
btfss SSPCON1,CKP ;T Is SSPCON1 bit4 = CKP = 1 ?
goto badjump ;T CKP bit test error
btfsc SSPSTAT,CKE ;T Is SSPSTAT bit6 = CKE = 0 ?
goto badjump ;T CKE bit test error
goto okjump2 ;OK configured!
;
okjump2 bsf SSPCON1,SSPEN ;T Re-enable MSSP
goto clear_cf_ok
return
badjump bsf SSPCON1,SSPEN ;T Re-enable MSSP
goto set_cf_error ;T configuration error
return
;******************************************************************************
HWSPI_W_speed
;On Entry: WREG = speedDATA & checks SSPCON1-3 for SPI mode
; speedDATA = 0x00 => Fosc/4
; speedDATA = 0x01 => Fosc/16
; speedDATA = 0x02 => Fosc/64
; speedDATA = 0x03 => Timer Divisor (Not working yet)
;
;On Exit:
;On Success: return with C flag clear
;On Failure: return with C flag set
;
bcf SSPCON1,SSPEN ;T Disable MSSP
movwf speedLbyte ;T move speedDATA stored in W to speedLbyte
btfsc SSPCON1,SSPM3 ;T In SPI Mode?, skip if yes
call HWSPI_init ;T MSSP is in wrong mode, Init for SPI
;
;Test if speedLbyte = 0x00. If yes, SPI clock speed = Fosc/4
movlw 0x00 ;T load 0x00 into W
subwf speedLbyte,W ;T subtract 0x00 from tcount result in w
btfss STATUS,Z ;T test zero flag, skip next instr if z set
goto fdiv16 ;T goto Fosc/16 section
bcf SSPCON1,SSPM1 ;T SSPCON1-1 = 0
bcf SSPCON1,SSPM0 ;T SSPCON1-0 = 0
goto okjump3 ;T Fosc/4 was selected
;Test if speedLbyte = 0x01. If yes, SPI clock speed = Fosc/16
fdiv16 movlw 0x01 ;T load 0x01 into W
subwf speedLbyte,W ;T subtract 0x01 from tcount result in w
btfss STATUS,Z ;T test zero flag, skip next instr if z set
goto fdiv64 ;T goto Fosc/64 section
bcf SSPCON1,SSPM1 ;T SSPCON1-1 = 0
bsf SSPCON1,SSPM0 ;T SSPCON1-0 = 1
goto okjump3 ;T Fosc/16 was selected
;Test if speedLbyte = 0x02. If yes, SPI clock speed = Fosc/64
fdiv64 movlw 0x02 ;T load 0x02 into W
subwf speedLbyte,W ;T subtract 0x02 from tcount result in w
btfss STATUS,Z ;T test zero flag, skip next instr if z set
goto timer ;T goto Timer section
bsf SSPCON1,SSPM1 ;T SSPCON1-1 = 1
bcf SSPCON1,SSPM0 ;T SSPCON1-0 = 0
goto okjump3 ;T Fosc/64 was selected
;Test if speedLbyte >= 0x03. If yes, SPI clock speed will be set by the timer
;SETTING THE SPI CLOCK WITH THE TIMER WILL RETURN A FAILURE AT THIS TIME.
;Future To do: Implement the TIMER section
timer movlw 0x03 ;T load 0x02 into W
subwf speedLbyte,W ;T subtract 0x02 from tcount result in w
btfss STATUS,Z ;T test zero flag, skip next instr if z set
goto badjmp2 ;T goto error section to return failure
goto badjmp2 ;T goto error section to return failure
; bsf SSPCON1,SSPM1 ;T SSPCON1-1 = 1
; bsf SSPCON1,SSPM0 ;T SSPCON1-0 = 1
; goto okjump3 ;T Fosc/64 was selected
okjump3 bsf SSPCON1,SSPEN ;T Re-enable MSSP
bcf STATUS,C ;T clear c flag on success
return
badjmp2 bsf SSPCON1,SSPEN ;T Re-enable MSSP
bsf STATUS,C ;T set c flag on failure
return
;******************************************************************************
HWSPI_W_spidata_W
;Simultaneously write SPI data on MOSI and read SPI data on MISO
;
;on Entry: WREG = mosiDATA & checks bit3 of SSPCON1 for SPI mode
;On Exit: WREG = misoDATA
;On Success: return with C flag clear
;On Failure: return with C flag set
;
movwf temp2 ;T move mosiDATA stored in W to WREG_TEMP
btfsc SSPCON1,SSPM3 ;T In SPI Mode?, skip if yes
call HWSPI_init ;T MSSP is in wrong mode, Init for SPI
movf temp2,W ;T load W with original mosiDATA
;
movwf SSPBUF ;T move byte to transmit to SSPBUF (transmit buffer)
movlw 0x00 ;T load 0x00 into W
movwf tcount ;T initialize tcount to zero (0x00)
again1 btfsc SSPSTAT,BF ;T receive completed? if no, skip next
goto okjump1 ;T no. goto again
incf tcount,F ;T increment tcount
movlw 0xFF ;T load w with literal
subwf tcount,W ;T subtract 0xFF from tcount result in w
btfss STATUS,Z ;T test zero flag, skip next instr if z set
goto again1 ;T loop until timeout
goto set_cf_error ;T receive timeout error
return
okjump1 movf SSPBUF,W ;T put received data in W
goto clear_cf_ok
return
;******************************************************************************
; UART routine
asyxmtc bcf PORTC,xmit ;T used to be portc,xmit
call full
movlw 0x08 ;TEST_T "08"
movwf bitctr
asyxmt1 rrcf xmtreg,f
btfsc STATUS,C
goto asyxmt2
bcf PORTC,xmit ;T used to be portc,xmit
goto asyxmt3
asyxmt2 bsf PORTC,xmit ;T used to be portc,xmit
;
asyxmt3 call full
decfsz bitctr,f
goto asyxmt1
;
bsf PORTC,xmit ;T used to be portc,xmit
call full
retlw 0
;******************************************************************************
; UART baud rate of 115.2kbps using a 40MHz System Clock
full movlw d'3'
movwf cntrb
vdly0 movlw d'6' ; d'43' with 4MHz => 2400 baud
movwf cntra
vdly1 decfsz cntra,f
goto vdly1
decfsz cntrb,f
goto vdly0
retlw 0
;******************************************************************************
;End of program
END
</p18f442.inc>
| SPI Line | C7 | C6 | C5 | C4 | C3 | C2 | C1 | C0 | D7 | D6 | D5 | D4 | D3 | D2 | D1 | D0 |
| DIN | 1 | 1 | 1 | 0 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 1 | 1 | 1 |
| SPI Line | C3 | C2 | C1 | C0 | D11 | D10 | D9 | D8 | D7 | D6 | D5 | D4 | D3 | D2 | D1 | D0 |
| DIN(1st) | 1 | 1 | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| DIN(2nd) | 1 | 1 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
表2中，第一个命令将所有DAC输出设置为零，第二个命令将所有DAC设置为满量程输出。
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