[Coco] Cassette Program Loading Help Needed

[RETRO Innovations]

On 3/18/2017 10:36 PM, Dave Philipsen wrote:
> What does the output of the AVR look like going to the CoCo?  If the 
> signal is coming out with a high frequency component 
Not really a HF component, but the decay on the cap is making the sine 
wave very sawtooth shaped.

I tried doing a square wave, but the Coco3 did not like it at all (the S 
does not even change to inverse), and so I decided to try the Coco 
method of a R DAC.  I have it looking good on the scope, using the exact 
values in the Coco1 schematic, but when I connect it to the Coco input, 
it just drags the entire thing down to ground.  I am using the output 
pins on the AVR, which can source quite a bit of current, so I am not 
sure what is going on.
> due to the way the PWM works it will trigger undesirable 
> zero-crossings in the comparator input on the CoCo. You could try 
> increasing the value of the resistor.
I will try that.
>
> You could go about it in a completely different manner and not 
> generate the audio using PWM.  Just generate a straight square wave 
> and feed that to the CoCo.
As noted above, I thought of that, but early results were not encouraging.

I thought for sure the resistor DAC would work, and I could get back to 
laying out the board, but I am back to diagnosing again.

Jim