MCP4922 Dual DAC

[dmairspotter]

Try mode 5?

DEFINE SHIFT_PAUSEUS xx?

[J_Brittian]

Originally I used mode 5 because that was the way my code ran for the 1257 DAC. I took another look at the MCP4922 datasheet though and on pg. 19 it shows the clock idles low and somewhere I think I read that the data gets latched on a low to high clock pulse. What would DEFINE SHIFT_PAUSEUS xx do ? I'm more concerned about the way I tacked the configuration bits on to the front of my variable. I have to send the DAC 16 bits. The first four are the configuration bits the next twelve bits are the output. Should I add/or a 16 bit configuration value with my output variable and then only Shiftout a single 16 bit number? Or should it work the way I've got it?

[dmairspotter]

DEFINE SHIFT_PAUSEUS xx where xx is some number will slow down the SDI transmission. Looking at the data sheet, it shouldn't be necessary but maybe worth a try? No harm in trying to make up one 16 bit word for transmission either, maybe that would help.

Like I said, I'm pretty new to PICs but I've found if I thrash around and change enough stuff I can mostly get stuff to work....mostly.

As far as I can see, your connections and logic look good. Sorry I can't find you a magic bullet.

[J_Brittian]

I tried DEFINE SHIFT_PAUSEUS 100. Still no output.

I used a logic probe to verify activity on the serial and clock pins. I can watch the CS pin go low. Activity on the LCD screen verifies the program is running. The PIC seems to be doing what it's supposed to. I changed the Shiftout command to:

Shiftout alldata,CLK,1,[%1111111111111111\16]' Shift out the 16-bit word

This should output 5V on output B.
Still no output.

The differences between this chip and the LTC1257 are: Configuration bits, hardware and program shutdowns, LDAC signal. I think I've addressed all of these. Using essentially the same code as below, just without configuration bits, I verified the PIC16F877 I'm using will drive the LTC1257. I've exhausted all my ideas for now. Any thoughts at all are welcome.

[Ruben Pena]

J.Britain:
Do you can live with 8 bits.?
Last week I tried the MAX520, with 4 DAC channels.
It uses I2C and worked in the first try. It is more expensive than the
Microchip part,but with 4 channels. I got them at Digikey.
I hope this helps...
Greetings...
Ruben de la Pena

[J_Brittian]

Ruben,
You want me to give up? I have ten of things sitting on my desk.

-Josh

[arniepj]

I have had problem with the shiftin/shiftout commands depending on the device being used.The clock pulses are too fast and using the define shift_pauseus doesn't seem to help.Writing your own works best as you can control the clock pulses.This example sends out the control bits,then reads in the data bits.


DataAcquisition:
for Indexer = 12 to 0 step -1 ' Output data,13 clock pulses
portc.2 = SerialDataOut.0[Indexer] ' Output data port
gosub Clockout ' Output clock pulse
next Indexer ' Next data bit,next clock pulse
for Indexer = 1 to 12 ' Read data,12 clock pulses
SerialDataIn = SerialDataIn * 2 ' Shift each bit to the left
SerialDataIn = SerialDataIn + portc.3 ' Data in port
gosub Clockout ' Output clock pulse
next Indexer ' Next data bit,next clock pulse
Clockout: ' Clock pulse
pulsout portc.0,5 ' Clock pulse port, 1 msec
return