[sdiy] AVR based module and ADC questions

[Paul Maddox]

Ryan,

> in the above diagram, I have two 10K pull up resistors on the ADC CS
> lines. I don't think they are required but I haven't been able to find
> any info about these inputs (I checked the ADC0820 data sheets also).

I see one for the two ADC0820's and one for the 74hc595 OE pin.

> the AVR has the I/O pins in high impedance mode on power up so if those
> CS don't have an internal pull up then I think there would be problems?

yes, you could have problems, for example, when you drop into programming
mode (using the ISP connector) the pins of the AVR float, so there's no
guarantee of what the CS/RD/WR pins will be doing.

> are they needed?

I would say include them, its better to be safe than to have a design that
keeps blowing chips.

> also, I wonder if anyone has used the ADCs of an AVR at speeds of about
> 75Ksps without needing much precision? From the data sheet, the ADC0820
> should easily handle this and my code looks like it'll be maybe even a
> little faster with a 16MHz clock.  the code doesn't do much except check
> both ADCs, then the shift register, then output a new DAC value.  The
> AVR supports only 15Ksps i think it was.

the ADCs on the AVR aren't very quick, I think the faster you go, the lower
the accuracy.
I can't think of anyone who reads them faster than 15k.

> also if you see any obviously stupid things let me know.

where's the progrmaming interface?
Also, be aware, unless you tie the CS line of the DAC high, you'll most
likely get 'noise' from the DACs output when you go into programming mode.

Paul