SPI - The basics

[rnuke77]

Greetings to all and thanks for a very thorough user forum. I am very new to pics, picbasic and microcontrollers. I won't pretend to understand 1/2 of what I have read but some of it must have sunk in as I am 1/2 way through a project. I am building a PID controller and can successfully read in 2 AI channels, display their values on an LCD and create the output value for the PID.
Now for control I realize I can use PWM or a DAC chip. I have (regretfully??) chosen the later of the two and am now stuck. I cant really find sufficient documentation and code examples. I am using a 16F877A and need to get the DAC conversion through a MCP4812.
So before a I beg for any direct answers - I would love some pointers as to where to get the relevant code/ pinout/ examples to learn from.

I do have one other anomaly that has no caused me too much grief yet, but I fear it is coming as I might need it to get through this DAC section. When I try to use the ASM ... ENDASM within my code - it will not compile!!!!

Thanks to any whom might be able to give some guidance

[rnuke77]

Okay, maybe I can focus this on where exactly I am getting lost. The MCP4812 needs the data brought in a specific format. That is bits 15:12 are formatting information for the data and bits 11:0 are the 12 bit digital (or 10 bit ...) information that is being transfered.
As I understand it, the SEROUT command will send 8 bit packets and I am not sure how to get the cofiguration data combined with the actual data??????? and, combine this as 1 - 16 bit packet???????

[Bruce]

For SPI use SHIFTOUT. Something like this should work;

A VAR WORD
B VAR WORD
 
CS VAR PORTB.0  ' chip select
SCK VAR PORTB.1 ' clock
SDI VAR PORTB.2 ' data
LDAC VAR PORTB.3 ' load DAC
 
HIGH CS   
LOW SCK
HIGH LDAC
 
A = %0001111111111100 ' channel A, 2x, active, max output
B = %1001111111111100 ' channel B, 2x, active, max output
 
Main:
  LOW CS ' enable writes
  SHIFTOUT SDI,SCK,0,[A\16]
  HIGH CS 
  PAUSE 5
 
  LOW CS
  SHIFTOUT SDI,SCK,0,[B\16]
  HIGH CS
 
  LOW LDAC ' load both DAC outputs
  PAUSE 5
  HIGH LDAC
 
  PAUSE 5000
  GOTO Main
  END

[rnuke77]

Thanks Bruce. I will give this a go in the next day or two and let you know if I had some success!

[rnuke77]

Thanks Bruce - I now have some level of success !??!

I was stuck for a while with only getting a Vref output from the 4812 - no matter what 10 bit information I loaded it was the same. I just had to change the mode on the SHIFTOUT command to a 1 or MSB first and it fell in line.

For this example, you are predefining the 10 bits for conversion in A and B. How would I load up 'A' with the same first 4 bits but then the next 12 (10) bits varying, based on my PID loop output?

Thanks again ...

[Bruce]

There are several ways to do it, but I would probably go for something like this;

The upper 4-bits for DAC channel A, with 2x gain, with A active, will remain constant. So create a constant like ASel CON 4096. This would be the following in 16-bit binary: %0001000000000000.

The lower 12-bits, with the 2 least significant bits ignored, + ASel would be your value to send to the DAC to adjust the channel A output.

Just use a word variable. Load your word variable with any value from 0 to 1023, then shift this left x 2 so the 2 LSBs are 0, then add the ASel constant for the value to shiftout.

Say your word var is ChanA, and it's = 501. ChanA = (ChanA << 2) + ASel.

501d = %0000000111110101. Shifted left x 2 it's %0000011111010100. When you add the ASel constant, it ends up being %0001011111010100.