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。
查看大图
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;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
表1. 配置写命令，将所有四路DAC的建立时间设置为3µs
| 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 |
表2. 装载全部DAC输出命令
| 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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