APPLICATION NOTE 3497

PIC'ing the MAX5581: Interfacing a PIC Microcontroller with the MAX5581 Fast-Settling DAC


Abstract: This application note shows how to use a PIC® microcontroller with the MAX5581 DAC. Detailed schematics and source code are provided.

MAX5581 Overview

The MAX5581 is a 12-bit, fast-settling DAC featuring a 3-wire SPI™ serial interface. The MAX5581's interface can support SPI up to 20MHz with a maximum settling time of 3µs. This application note presents an application circuit and all the firmware required to interface the fastest line of PIC microcontrollers (PIC18F core) to the MAX5581 DAC. The example assembly program was written specifically for the PIC18F442 using the free assembler provided in MPLAB IDE version 6.10.0.0.

Hardware Overview

The application circuit discussed here uses the MAX5581 Evaluation (EV) Kit, which consists of the MAX5581, an ultra-high-precision voltage reference (MAX6126), two pushbutton switches, gain setting resistors, and a proven PCB layout. The PIC18F442 is not present on the MAX5581EVKIT board, but was added to the system to complete the application schematic shown in Figure 1. The /CS\, SCLK, DIN, and DOUT pads on the MAX5581EVKIT allow an easy connection for the SPI serial interface.

Figure 1. MAX5581 application schematic Sheet 1 of 2.
For Larger Image

Figure 1. MAX5581 application schematic Sheet 1 of 2.

Figure 1. MAX5581 application schematic Sheet 2 of 2.
Figure 1. MAX5581 application schematic Sheet 2 of 2.

Analog and Digital Ground Planes

It is good practice to separate the analog and digital ground planes, as shown in Figure 2. Use a ferrite bead, such as the TDK MMZ1608B601C, to connect both ground planes together through a ferrite bead. This prevents the microcontroller's system clock and its harmonics from feeding into the analog ground. Knowing that the PIC18F442's system clock is 40MHz, the MMZ1608B601C was chosen for its specific impedance vs. frequency characteristics. Figure 3 shows the impedance versus frequency curve for the MMZ1608B601C.

Figure 2. Separating analog and digital grounds.
Figure 2. Separating analog and digital grounds.

Figure 3. Impedance vs. frequency curve for the TDK MMZ1608B601C ferrite bead.
Figure 3. Impedance vs. frequency curve for the TDK MMZ1608B601C ferrite bead.

Firmware Overview

The example assembly program shown in Listing 1 initializes the MAX5581 using the PIC18F442's internal MSSP SPI peripheral. The PIC18F442's 40MHz system clock allows the MSSP to provide an SPI clock (SCLK) up to 10MHz. Table 1 shows the only configuration word required after power. Once the MAX5581 is initialized, the program constantly loads the DAC output registers with zero scale followed by full scale, as shown in Table 2. This constant loop results in a square wave, shown in Figure 4, which demonstrates the fast settling time of the MAX5581.

Figure 4. An actual scope shot of the 80kHz square wave.
Figure 4. An actual scope shot of the 80kHz square wave.

Listing 1. An Assembly Example Program that Interfaces to the MAX5581 Using the PIC18F442's Internal MSSP SPI Peripheral

Download: P18F442.INC

Listing 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



Table 1. Configuration Write Command for Setting the Settling Time to 3µs for All Four DACs.
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

Table 2. Load All DAC Output Commands.
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

In Table 2, the first command sets all the DAC outputs to zero scale. The second command sets all the DAC outputs to full scale.
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APP 3497:
APPLICATION NOTE 3497,AN3497, AN 3497, APP3497, Appnote3497, Appnote 3497