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.
For Larger Image Figure 1. MAX5581 application schematic Sheet 1 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 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.
Listing 1. An Assembly Example Program that Interfaces to the MAX5581 Using the PIC18F442's Internal MSSP SPI Peripheral
;******************************************************************************
;
; 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 equ06 ; 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.
CBLOCK0x080
WREG_TEMP;variable used for context saving
STATUS_TEMP;variable used for context saving
BSR_TEMP;variable used for context saving
;
ENDC
CBLOCK0x000
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.
ORG0x0000
gotoMain;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.
ORG0x0008
braHighInt;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.
ORG0x0018
movffSTATUS,STATUS_TEMP;save STATUS register
movffWREG,WREG_TEMP;save working register
movffBSR,BSR_TEMP;save BSR register
;*** low priority interrupt code goes here ***
movffBSR_TEMP,BSR;restore BSR register
movffWREG_TEMP,WREG;restore working register
movffSTATUS_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 ***
retfieFAST
;******************************************************************************
;Start of main program
; The main program code is placed here.
Main:
;*** main code goes here ***
start
;*** Port Initialization ***
movlw0x0FF
movwfPORTB
clrfPORTA
movlw0x06 ;T Configure PortA as Digital
movwf ADCON1
movlw 0x00FB ;T A2 OUTPUT, ALL OTHERS INPUT
movwf TRISA
movlw0x0001 ;T B0 INPUT, ALL OTHERS OUTPUT
movwfTRISB
movlw0x0093 ;T C7-C0 => bit7-0
;T OUTPUTs: C6(TX), C5(MOSI), C3(SCLK), C2(CS)
;T INPUTs:C4 (MISO) and all others
movwfTRISC ;T TRISC bit3 Master = 0
bsfPORTC,RC2;T RC2 = CS\ Make CS\ high
; *** SPI Initialization ***
callHWSPI_init ;T Initialize the MSSP for SPI
; *** SPI Configuration ***
movlwb'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
callHWSPI_W_configure
; *** SPI Speed ***
movlwb'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 ***
bcfPORTC,RC2;T RC2 = CS\ Make CS\ Low
movlw0xEC;T byte0 of settling time config
callHWSPI_W_spidata_W;T HW SPI WriteRead Operation
movlw0x0F;T byte1 of settling time config
callHWSPI_W_spidata_W;T HW SPI WriteRead Operation
bsfPORTC,RC2;T RC2 = CS\ Make CS\ high
; *** MAX5581 Load All DAC Outputs to Zero Scale ***
Loopforever bcfPORTC,RC2;T RC2 = CS\ Make CS\ Low
movlw0xD0;T byte0 of load all input/output to zeros
callHWSPI_W_spidata_W;T HW SPI WriteRead Operation
movlw0x00;T byte1 of load all input/output to zeros
callHWSPI_W_spidata_W;T HW SPI WriteRead Operation
bsfPORTC,RC2;T RC2 = CS\ Make CS\ high
; *** MAX5581 Load All DAC Outputs to Full Scale ***
bcfPORTC,RC2;T RC2 = CS\ Make CS\ Low
movlw0xDF;T byte0 of load all input/output to zeros
callHWSPI_W_spidata_W;T HW SPI WriteRead Operation
movlw0xFF;T byte1 of load all input/output to zeros
callHWSPI_W_spidata_W;T HW SPI WriteRead Operation
bsfPORTC,RC2;T RC2 = CS\ Make CS\ high
; movwf xmtreg;T move w to xmtreg
; call asyxmtc;T call UART routine
;
gotoLoopforever ;T loop forever
;******************************************************************************
errsrv
movlw0x65; load w with 'e' = 0x65
movwfxmtreg ; move w to xmtreg
callasyxmtc; 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
bcfSSPCON1,SSPEN;T Disable the MSSP, SSPCON-5
;
bcfTRISC,SDO;T TRISC bit5 RC5/SDO = 0 MOSI Output
bcfTRISC,SCK;T TRISC bit3 RC3/SCK = 0 SCLK Output
bsfTRISC,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
bsfINTCON,PEIE;T INTCON bit6 = PEIE = 1 = Enable periph interrupt
bsfPIE1,SSPIE ;T PIE1 bit3 = SSPIE = 1 = interrupt enable
movlw0x00;T load 0x00 into W
movwftcount;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
;
bcfSSPCON1,SSPEN;T Disable the MSSP, SSPCON1-5
movwftemp ;T move the confDATA byte to temp
btfscSSPCON1,SSPM3 ;T In SPI Mode?, skip if yes
call HWSPI_init;T MSSP is in wrong mode, Init for SPI
;
btfsctemp,1;T Is bit1 of confDATA byte clear? if so skip next
goto CPOL_1;T goto CPOL = 1 label => CPOL = 1
btfsctemp,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
bcfSSPCON1,CKP;T SSPCON1 bit4 = CKP = 0
bsfSSPSTAT,CKE;T SSPSTAT bit6 = CKE = 1
btfsc SSPCON1,CKP;T Is SSPCON1 bit4 = CKP = 0 ?
gotobadjump;T CKP bit test error
btfssSSPSTAT,CKE;T Is SSPSTAT bit6 = CKE = 1 ?
gotobadjump;T CKE bit test error
goto okjump2;OK configured!
;
CPOL_1btfsctemp,0;T Is bit0 of confDATA byte clear? if so skip next
;T CPOL = 1 , CPHA = ?
gotoCPOLCPHA_11;T goto => CPOL = 1, CPHA = 1
;Configure for CPOL = 1, CPHA = 0
bsfSSPCON1,CKP;T SSPCON1 bit4 = CKP = 1
bsfSSPSTAT,CKE;T SSPSTAT bit6 = CKE = 1
btfss SSPCON1,CKP;T Is SSPCON1 bit4 = CKP = 1 ?
gotobadjump;T CKP bit test error
btfssSSPSTAT,CKE;T Is SSPSTAT bit6 = CKE = 1 ?
gotobadjump;T CKE bit test error
goto okjump2;OK configured!
;
CPOLCPHA_01
;configure for CPOL = 0, CPHA = 1
bcfSSPCON1,CKP;T SSPCON1 bit4 = CKP = 0
bcfSSPSTAT,CKE;T SSPSTAT bit6 = CKE = 0
btfsc SSPCON1,CKP;T Is SSPCON1 bit4 = CKP = 0 ?
gotobadjump;T CKP bit test error
btfscSSPSTAT,CKE;T Is SSPSTAT bit6 = CKE = 0 ?
gotobadjump;T CKE bit test error
goto okjump2;OK configured!
;
CPOLCPHA_11
;configure for CPOL = 1, CPHA = 1
bsfSSPCON1,CKP;T SSPCON1 bit4 = CKP = 1
bcfSSPSTAT,CKE;T SSPSTAT bit6 = CKE = 0
btfss SSPCON1,CKP;T Is SSPCON1 bit4 = CKP = 1 ?
gotobadjump;T CKP bit test error
btfscSSPSTAT,CKE;T Is SSPSTAT bit6 = CKE = 0 ?
gotobadjump;T CKE bit test error
goto okjump2;OK configured!
;
okjump2bsfSSPCON1,SSPEN;T Re-enable MSSP
gotoclear_cf_ok
return
badjumpbsfSSPCON1,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
;
bcfSSPCON1,SSPEN ;T Disable MSSP
movwf speedLbyte;T move speedDATA stored in W to speedLbyte
btfscSSPCON1,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
movlw0x00;T load 0x00 into W
subwfspeedLbyte,W;T subtract 0x00 from tcount result in w
btfssSTATUS,Z;T test zero flag, skip next instr if z set
gotofdiv16;T goto Fosc/16 section
bcfSSPCON1,SSPM1;T SSPCON1-1 = 0
bcfSSPCON1,SSPM0;T SSPCON1-0 = 0
goto okjump3;T Fosc/4 was selected
;Test if speedLbyte = 0x01. If yes, SPI clock speed = Fosc/16
fdiv16movlw0x01;T load 0x01 into W
subwfspeedLbyte,W;T subtract 0x01 from tcount result in w
btfssSTATUS,Z;T test zero flag, skip next instr if z set
gotofdiv64;T goto Fosc/64 section
bcfSSPCON1,SSPM1;T SSPCON1-1 = 0
bsfSSPCON1,SSPM0;T SSPCON1-0 = 1
goto okjump3;T Fosc/16 was selected
;Test if speedLbyte = 0x02. If yes, SPI clock speed = Fosc/64
fdiv64movlw0x02;T load 0x02 into W
subwfspeedLbyte,W;T subtract 0x02 from tcount result in w
btfssSTATUS,Z;T test zero flag, skip next instr if z set
gototimer;T goto Timer section
bsfSSPCON1,SSPM1;T SSPCON1-1 = 1
bcfSSPCON1,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
timermovlw0x03;T load 0x02 into W
subwfspeedLbyte,W;T subtract 0x02 from tcount result in w
btfssSTATUS,Z;T test zero flag, skip next instr if z set
gotobadjmp2;T goto error section to return failure
gotobadjmp2;T goto error section to return failure
;bsfSSPCON1,SSPM1;T SSPCON1-1 = 1
;bsfSSPCON1,SSPM0;T SSPCON1-0 = 1
;goto okjump3;T Fosc/64 was selected
okjump3bsfSSPCON1,SSPEN;T Re-enable MSSP
bcfSTATUS,C;T clear c flag on success
return
badjmp2bsfSSPCON1,SSPEN;T Re-enable MSSP
bsfSTATUS,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
btfscSSPCON1,SSPM3 ;T In SPI Mode?, skip if yes
call HWSPI_init;T MSSP is in wrong mode, Init for SPI
movftemp2,W;T load W with original mosiDATA
;
movwfSSPBUF;T move byte to transmit to SSPBUF (transmit buffer)
movlw0x00;T load 0x00 into W
movwftcount;T initialize tcount to zero (0x00)
again1btfscSSPSTAT,BF;T receive completed? if no, skip next
gotookjump1;T no. goto again
incf tcount,F;T increment tcount
movlw0xFF;T load w with literal
subwftcount,W;T subtract 0xFF from tcount result in w
btfssSTATUS,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
gotoclear_cf_ok
return
;******************************************************************************
; UART routine
asyxmtc bcfPORTC,xmit ;T used to be portc,xmit
callfull
movlw0x08 ;TEST_T "08"
movwfbitctr
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 callfull
decfsz bitctr,f
goto asyxmt1
;
bsf PORTC,xmit ;T used to be portc,xmit
callfull
retlw0
;******************************************************************************
; UART baud rate of 115.2kbps using a 40MHz System Clock
full movlwd'3'
movwfcntrb
vdly0 movlwd'6' ; d'43' with 4MHz => 2400 baud
movwfcntra
vdly1 decfsz cntra,f
goto vdly1
decfsz cntrb,f
goto vdly0
retlw0
;******************************************************************************
;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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