PicBasic Pro code to drive MCP4291 DAC?

[aajgss]

Mark_s

What speed are you looking for?
Mine was running at about 9 Hz on an 18F14K50. By setting Config1 to $F100 I now get around 70hZ which is what I was after.

aajgss

[mark_s]

aajgss,
I was getting similar frequencies as you, at 20mhz and 40mhz clock settings. My lookup table has 256
values, so it takes long time to cycle. I going to try the MSSP tonight and see what is gained. I know a dspic running at 80mhz can output about 4khz with the same dac and MSSP. So I am trying for 1 to 2khz. Maybe a pipe dream? I'll post the code if it works.

regards

[aajgss]

Mark_s

I'll be interested in how you go. I did try the MSSP and actually got slower. I know it was the code I had written, because its just logical it should be faster and was going to continue with it to see if I could find my error(s), but the change to the CONFIG1 gave me the frequency I wanted. If I could get it faster and use 2 degree steps instead of 4 degree steps it would be better.

regards
aajgss

[mark_s]

Okay I hammered out a routine to use the MSSP module. It gave me
a 6x speed increase over shiftout. I'm just a hack so there maybe a
better way to do it.
Results:
Shiftout MSSP
10mhz 10hz 66hz
20mhz 21hz 117hz
40mhz 42hz 2 63hz
80mhz 83hz 526hz
The 80mhz is 20mhz crystal 4xpll, overclocking probably not reliable? but worked!

'****************************************************************
'*  Name    : DAC_12BIT_MSSP                                    *
'*  Author  : MARK S                                            *
'*  Date    : 6/9/2011
'* SINE WAVE GENERATOR USING MCP4921 12BIT DAC                  *
'* USES MSSP MODULE vs SHIFTOUT                                 *
'* PIC18F4680, PBP2.6 MPASM                                     *
'*                                                              *
'****************************************************************
DEFINE  OSC 20
'REGISTER SET
INTCON = %00000000
INTCON2.7 = 0       'weak pullups portB
ADCON1 =%00001111   'ALL DIGITAL
CMCON  =%00000111   'COMP OFF
T3CON = 0
CCP1CON = 0 'ECCP OFF
ECCP1AS = 0
ECCP1DEL = 0
SSPSTAT = %00000000   'TX IDLE LOW,SAMPLE MIDDLE
SSPCON1 = %00100000 ' ENABLE MSSP,CLK IDLE LOW,FOSC/4
'-----------------------------------------------------
'PORTS
 TRISA = %00000000  ' PORTA ALL OUTPUT        ' 
 TRISB = %00000000  ' PORTB  
 PORTB = %00000000  ' CLEAR PORTB 
 TRISC = %00000000  ' PortC 
 TRISD = %00000000   
    TRISE = %00000000
'----------------------------------------------------
'DAC CONNECTIONS  
'VREF     PIN6 MCP4921 TIED TO +5V
'CLOCK    PIN3 MCP4921 TO PORTC.3  "SCK"
'DATA     PIN4 MCP4921 TO PORTC.5  "SDO"
'CS       PIN2 MCP4921 TO PORTD.2
'LOAD     PIN5 MCP4921 TO PORTD.3   CONNECT THIS TO GROUND TO SAVE A PORT PIN
'--------------------------------------------------
'VARIABLES 
CS_DAC      VAR PORTD.2
LOAD_DAC    VAR PORTD.3    'KEEP LOW
WAVE_INDEX  VAR WORD
DAC_OUT     VAR WORD       
DAC_OUT1    VAR WORD       
SSPIF       VAR PIR1.3     ' SPI interrupt flag
'----------------------------------------------------
'START    
SSPBUF = 0          ' CLEAR SPI BUFFER
SSPIF = 0           ' CLEAR SPI INTERUPT FLAG.
LOW LOAD_DAC
'===================================================================   
MAIN:
HIGH CS_DAC
FOR WAVE_INDEX = 0 TO 255
          
GOSUB SINE_TABLE
    LOW CS_DAC 
      DAC_OUT1 =$3000 | DAC_OUT   ' Combine table value with dac reg control
            SSPBUF = DAC_OUT1.HIGHBYTE  'SEND HIGH 
            While SSPIF = 0            ' WAIT FOR TX TO COMPLETE
                Wend 
                SSPIF = 0               'RESET INT FLAG
            SSPBUF = DAC_OUT1.LOWBYTE   'SEND LOW
            While SSPIF = 0            ' WAIT FOR TX TO COMPLETE   
                Wend
                SSPIF = 0               'RESET INT FLAG
    HIGH CS_DAC
NEXT WAVE_INDEX
GOTO MAIN
   
SINE_TABLE:
                   
lookup2 WAVE_INDEX, [0,1,2,6,10,15,22,30,39,50,61,74,88,103,120,137,156,_
        176, 197,219,242,266,291,318,345,374,403,433,465,497,_
        531,565,600,636,673,710,749,788,828,869,910,952,995,_
        1039,1083,1127,1172,1218,1264,1311,1358,1406,1453,1502,1550,1599,_
        1648,1698,1747,1797,1847,1897,1948,1998,2048,2098,2148,2199,2249,_
        2299,2349,2398,2448,2497,2546,2594,2643,2690,2738,2785,2832,2878,_
        2924,2969,3013,3057,3101,3144,3186,3227,3268,3308,3347,3386,3423,_
        3460,3496,3531,3565,3599,3631,3663,3693,3722,3751,3778,3805,3830,_
        3854,3877,3899,3920,3940,3959,3976,3993,4008,4022,4035,4046,4057,_
        4066,4074,4081,4086,4090,4094,4095,4095,4095,4094,4090,4086,4081,_
        4074,4066,4057,4046,4035,4022,4008,3993,3976,3959,3940,3920,3899,_
        3877,3854,3830,3805,3778,3751,3722,3693,3663,3631,3599,3565,3531,_
        3496,3460,3423,3386,3347,3308,3268,3227,3186,3144,3101,3057,3013,_
        2969,2924,2878,2832,2785,2738,2690,2643,2594,2546,2497,2448,2398,_
        2349,2299,2249,2199,2148,2098,2048,1998,1948,1897,1847,1797,1747,_
        1698,1648,1599,1550,1502,1453,1406,1358,1311,1264,1218,1172,1127,_
        1083,1039,995,952,910,869,828,788,749,710,673,636,600,_
        565,531,497,465,433,403,374,345,318,291,266,242,219,_
        197,176,156,137,120,103,88,74,61,50,39,30,22,15,10,6,_
        2,1],DAC_OUT
        
 RETURN
 END

[rmteo]

I don't know what the OP's application is but using a PIC and a serial DAC (MCP4921) to generate sine waves is extremely inefficient. It consumes a tremendous amount of CPU resources and gives very poor performance - limited frequency range and resolution. Using a DDS chip such as the AD8937 (which incidentally costs about the same as then MCP4921) allows you to generate sine (and triangle and square) waveforms from 0-16MHz with 0.06Hz (28-bit) resolution. For low frequency applications (such as VFDs) you can use a lower speed clock - with a 1MHz clock you get 0-1MHz and 0.004Hz resolution. It uses the same SPI interface as the MCP4921 and the slowest 4MHz PIC will still be able to generate up to a 16MHz sine wave.

[gadelhas]

I don't know what the OP's application is but using a PIC and a serial DAC (MCP4921) to generate sine waves is extremely inefficient. It consumes a tremendous amount of CPU resources and gives very poor performance - limited frequency range and resolution. Using a DDS chip such as the AD8937 (which incidentally costs about the same as then MCP4921) allows you to generate sine (and triangle and square) waveforms from 0-16MHz with 0.06Hz (28-bit) resolution. For low frequency applications (such as VFDs) you can use a lower speed clock - with a 1MHz clock you get 0-1MHz and 0.004Hz resolution. It uses the same SPI interface as the MCP4921 and the slowest 4MHz PIC will still be able to generate up to a 16MHz sine wave.

rmteo, isn't AD9837?

[rmteo]

Yes, it is AD9837. Sorry for the typo.