DAC implementation via VHDL within a CPLD

[AlfonsoGarcia]

Hi all,
I am curious about how a implemenation of a digital to Analog converter
could work within a CPLD. From the knowledge I gained in the last few
weeks, a CPLD can be only used for digital communications. The internal
signals are either HIGH or LOW. I saw the following vhdl code:

process (d2a_reg)
variable d2a_int : integer;
variable d2a_real : real;
begin
d2a_int := CONV_INTEGER(d2a_reg);
d2a_real := (real (d2a_int)) / scale;
-- overflow
if (d2a_real > (real (overflow)) ) then
d2a_out <= real (overflow);
else
d2a_out <= d2a_real;
end if;
end process; -- d2a conversion block

But all what it does is, to convert the digital values to a real value.
How can I create an analog voltage dependend signal out of it. I guess I
would need external circuitry around my CPLD for my application.
Any help or explanation about this issue will be very much appreciated.

Alfons

 

[Wallclimber]

Hi Alfonso,

the VHDL code below was most likely part of a testbench or part of a
behavioral model of a chip that contains a DA convertor. Maybe it's a
CPLD that also contains a DA convertor?

Tom

 

[Nicolas Matringe]

AlfonsoGarcia a écrit:
[...]

But all what it does is, to convert the digital values to a real value.
How can I create an analog voltage dependend signal out of it. I guess I
would need external circuitry around my CPLD for my application.
Any help or explanation about this issue will be very much appreciated.

Hi
Forget about this code, type 'real' is NOT synthesizable.
You can make a very simple DAC with a few resistors and an op amp as
shown here for example: http://www.allaboutcircuits.com/vol_4/chpt_13/3.html

 

[AlfonsoGarcia]

That's what I mean. I need external circuitry in order to make the DA
converter reality. Only through a CPLD or FPGA it is not possible.
Thanks for the hints and help.
Alfonso

 

[Thomas Stanka]

How can I create an analog voltage dependend signal out of it. I guess I
would need external circuitry around my CPLD for my application.
Any help or explanation about this issue will be very much appreciated.

Your right, you can't create an useful analog voltage only using
digital outputs. You need at least some resistors outside the
fpga/cpld.
A typical DAC using an fpga consists either of paralell outputs and a
resistor network, or does some PWM filtering with a lowpass.
Of course you could also use a ready DAC-IC and controll it by the
fpga.

bye Thomas