Hi guys,
I am having trouble implementing my 8 bit R-2R DAC using 10k and 20k resistors.
I connected it to PORTD of my PIC18F452, I de-activated PSP by setting
TRISE to 0
and I set TRISD to 0 to put the pins into output mode.
If I send 255 (0xFF) to PORTD I get 5v to each and everyone of the 8 pins D0-D7.
Now comes the part I really don't understand : measuring the voltages
on the output
pin of the DAC by sending different values on PORTD I get the
following results :

Sending decimal 50  = 0.89v
Sending decimal 100 = 1.70v
Sending decimal 150 = 2.24v
Sending decimal 200 = 2.30v
Sending decimal 250 = 2.35v

I think the PIC is not able to supply enough current and this is the reason I
get all the values from above.
If i try to analyse the value after sending "50" over to the DAC and I
multiply it by 5
I virtually get ~4.5v, close to 5v, my supply voltage.
This is almost the value I should get by sending "250" to the DAC but
instead I get 2.35,
almost half of what it should be. Am i right here ?


On Dec 13, 2007 12:17 AM, Robert Rolf <Robert.Rolf@ualberta.ca> wrote:
>
>
>
> Dumitru Stama wrote:
>
> > On Dec 12, 2007 8:37 PM, Hector Martin <hector@marcansoft.com> wrote:
> >
> >>Note that PIC16s and PIC18s internally divide the clock by 4, so the
> >>delay would be 625 cycles. Also, the I2C read will take some time. Your
> >>best bet for timing is to use one of the PIC's built in timers. In fact,
> >>if you're running your EEPROM at 100Khz, it will be too slow since you
> >>have to read/write 4 bytes = 40 bits approximately (after adding in
> >>start conditions and whatnot), so your maximum sampling rate would be
> >>100/40 = 2.5Khz. You should read chunks of data of a longer length (to
> >>reduce the overhead of three bytes for each I2C transaction), buffer
> >>them in memory, and have an interrupt running off a timer put them into
> >>PORTB.
> >
> >
> > In the end i will use two buffers and a timer for feeding my DAC with values
> > but this is my first work in this field and i wanted to hear some sound out
> > of my design :) Thank you for your correction, i will fix my delays now to see
> > what i get.
>
> So break the problem into parts.
> First make the D/A make sound. A running binary counter will
> get you a nice ramp (sawtooth) that sounds like an Oboe.
> That way you know your playback rate is OK and that your amplifier
> is not clipping or otherwise wonky.
>
> Then make sure you are reading/writing the EEPROM correctly.
> For example, write the compliment of the address
> (plus some offset),to each location
> and then make sure you are reliably getting it back.
>
> And as noted by others, you only need to write the start address
> to most serial EEPROM devices. Subsequent reads will get you
> the next byte in the sequence so your overhead is greatly reduced.
>
> Robert
>
>
>
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